Skip to main content

The best antivirus software for 2020

best antivirus software free premium and business
(Image credit: Future)

Luckily, internet security companies are fighting the good fight - constantly updating their software packages to protect against these threats to your online life. The best antivirus choices out there combine watertight virus protection, extra features to keep you safe from scams, and are an absolute doddle to install and use.

If you’re looking for all-round protection against the latest cyber threats, then investing in the strongest software is the best solution you can hope for - and we also have the best business antivirus plans if you're looking to take care of your company computers. That doesn't mean you have to start spending big money either, as our guide also gives you the cheapest available prices for the top software.

And if it's free antivirus that you're after, we have some top recommendations for you, too. But considering some of the best antivirus providers out there cost less than $10/£10 per year, we’d always recommend playing it safe and going for a premium package.

Today's top 3 best antivirus software

1. Bitdefender is 2020's best antivirus
You may be more familiar with names such as Norton, McAfee and AVG – but at the moment, we think Bitdefender is the best antivirus available. It combines watertight security tools with an array of other excellent security features. It’s also really easy to use and, best of all, offers extremely good value for money.

Click here if you're in the US (or anywhere else)
Click here if you're in the UK
Click here if you're in Australia
View Deal

2. Norton - great protection that won't slow you down
It's a name almost synonymous with the word 'antivirus', and Norton didn't disappoint when it came to our testing. It was notable how little an affect it had on the performance of our test computer. And it has a feature-list as long as your arm.
View Deal

3. Kaspersky - no-nonsense suite with top-notch security
It may not have quite the feature set that the above providers offer, but Kaspersky really does the basics brilliantly. Its outright virus defence is airtight and we like that even its entry-level plan offers protection covering multiple devices.
View Deal

The best antivirus 2020 in full:

Bitdefender offers the most accurate and reliable protection around and has received several awards. (Image credit: Future)

1. Bitdefender Antivirus

Loaded with features, giving you rock-solid protection - 2020's best antivirus

Operating system: Windows and Mac options | Maximum devices covered: 10 | Stand out features: Safepay banking protection, privacy tools, web protection, VPN

Excellent antivirus engine
Tons of features
Good value for money
Not the best in terms of resource usage

TechRadar's #1 rated antivirus in a world packed with loads of virus protection providers, and doesn't even have to cost that much more than free antivirus downloads.

Techradar Editor's Choice

If you peruse the rankings of the major independent testing labs, you’ll doubtless notice that Bitdefender inevitably appears towards the top. And for good reason – Bitdefender’s core antivirus engine is excellent, and we found in our testing that it delivers top-notch protection. Which is, of course, the main point of any antivirus product.

Although not the only consideration, by any means, and Bitdefender’s other main strength is that it positively bristles with features. Exactly how many features depends on which product you purchase, although even the entry-level Bitdefender Antivirus Plus 2020 packs in a lot for the money.

As you'll see in our full Bitdefender review, that includes a very thorough web protection module (which offers class-leading protection from dangerous links), Safepay online banking protection, a password manager, as well as a vulnerability scanner which can alert you to missing security updates and patches.

As for downsides, about the only one really worth mentioning is that Bitdefender uses more system resources than some rival products.

If you want more features, Bitdefender Internet Security 2020 delivers exactly that, adding a firewall, spam filter, parental controls to keep your kids safe online, with measures to bolster privacy such as an anti-webcam hijacking feature.

Bitdefender Total Security 2020 is the ultimate package which further provides a suite of PC maintenance tools, and is capable of covering all types of devices, delivering apps for Android and iOS hardware, as well as Mac (and Windows) computers.

This makes Total Security a superb overall value proposition, although that doesn’t take away from what Bitdefender Antivirus Plus 2020 gives you – which is still a great deal for the money...

Bitdefender Antivirus Plus 2020 | 1 year | $59.99 $23.99 | 60% off
Not only is this one of the cheapest antivirus deals, it's coming from the world's best provider. Bitdefender Antivirus Plus comes with the added bonus of a VPN and Safepay as well as boasting multi-layer ransomware protection. This constantly tracks any malware and records the cyberthreat's pattern to keep you secure online.

Click here if you're in the US (or anywhere else)
Click here if you're in the UK | £17.99
Click here if you're in Australia | AU$35
View Deal

Norton AntiVirus will protect your PC without being a system hog. (Image credit: Future)

2. Norton AntiVirus

Protection that won't slow you down

Operating system: Windows and Mac | Maximum devices covered: 1 | Stand out features: Intelligent firewall, PC maintenance features, bundled backup tool with online storage

Minimal impact on PC resources
Useful bundled backup tool
Highly impressive browsing protection
Anti-ransomware works – but could be improved

Norton AntiVirus Plus was previously known as AntiVirus Basic, but was revamped to AntiVirus Plus back in 2019. It offers top-quality virus definitions, and a very light touch when it comes to resource usage. In fact, according to PassMark’s most recent rankings, Norton has the least impact on the performance of a PC compared to rival security suites.

Features are present in spades, too, with some of the highlights being top-notch browsing protection, an intelligent firewall, PC maintenance utilities (including an informative Startup Manager that could streamline your PC’s boot times) and a bundled backup tool with 2GB of online storage.

It’s a suitably easy to use app, but Norton also offers neat touches for experts, allowing plenty of control over configuring custom virus scans, and offering elements like a tool that evaluates the reputation of various files on your PC.

Downsides? The interface could be better thought out, and Norton didn’t perform quite as well as some competitors in our anti-ransomware testing – while it blocked our custom-made threat, we lost a few files before that happened.

NortonLifeLock (the new brand name for Symantec) also provides some beefier security offerings than AntiVirus Plus.

Norton 360 Standard adds some impressive extras, including a fully integrated VPN, dark web monitoring (for US users), more backup storage (10GB) and mobile apps for Android and iOS. If you want to cover more than one device, though, you’ll need Norton 360 Deluxe, which allows for five devices and throws parental controls into the mix. You’re getting a lot of protection for the money here, and it’s likely to be worth the upgrade over the 360 Standard subscription.

Finally, the top package, Norton 360 with LifeLock Select, gives those in the US a full range of identity protection features as well as all the above.

Kaspersky Anti-Virus is comprehensive and easy to use without alienating more expert users (Image credit: Future)

3. Kaspersky Anti-Virus

A no-nonsense suite that provides top-notch protection

Operating system: Windows only | Maximum devices covered: 5 | Stand out features: Highly configurable virus scans, self-protection defense, drive-by cryptomining infection prevention

Excellent antivirus engine
Impressive against undiscovered threats
Fast scans
Not strong on extra features

Kaspersky Anti-Virus is the entry-level package from this well-respected security vendor. It provides coverage for up to five Windows PCs, focusing on the fundamental security essentials, so while it hasn’t got the longest list of features – particularly compared to some of the other products highlighted here – what Kaspersky does, it does very well.

And that includes smart self-protection routines, fast and highly configurable virus scans, not to mention great overall virus protection (we were impressed with its ability to deal with fresh and undiscovered threats). Its anti-malware capabilities are up there with the best of them.

Weaker points are the aforementioned lack of features, and the fact that the extra trimmings which are present with Kaspersky Anti-Virus feel a bit shaky. For example, some of the troubleshooting wizards provided seem of limited value, like the Browser Configuration Wizard which only covers Internet Explorer (hardly the most popular browser these days).

You get more with the higher-tier packages, though, with Kaspersky Internet Security providing an intelligent firewall (which hardly ever hassles you with any queries) and secure browser (to keep your online financial dealings safe). Possibly the biggest benefit here, however, is that it covers not just Windows PCs, but also Mac computers and mobile devices.

Kaspersky Total Security is the flagship product, giving you the most comprehensive coverage with the addition of a password manager, parental controls, and automated local backups. Those are useful extra features, and when you consider that you don’t pay much more for Total Security – indeed, the suite is actually the same price as Internet Security at the time of writing – it’s almost certainly worth going for the top package.

Trend Micro Antivirus offers solid protection but has strong demands (Image credit: Future)

4. Trend Micro Antivirus

Accomplished fire-and-forget antivirus solution

Operating system: Windows only | Maximum devices covered: 1 | Stand out features: Ransomware monitoring, Pay Guard banking protection, high-quality URL blocking

Simple jargon-free menus and interface
Impressive antivirus defenses
Folder Shield anti-ransomware monitoring
Not massively configurable
Somewhat draining on your PC resources

Trend Micro Antivirus+ Security offers simple but effective protection, with a user-friendly interface and a refreshing lack of jargon when it comes to describing the program’s functions - you can see a full list of prices below.

The antivirus engine is a quality affair – while there’s a little disagreement over exactly how good it is, Trend Micro is generally highly rated by the major independent testing labs. It offers some tough defenses beyond real-time behavior monitoring, and that includes an anti-ransomware system called Folder Shield.

In the past, we’ve observed that one of Trend Micro’s major weaknesses is that it’s a bit of a resource hog. The good news is that this situation has been improving of late, and we found the latest version of Antivirus+ Security only had an average performance impact on our PC (not great, but not as bad as it’s been in the past).

The next tier product, Trend Micro Internet Security, adds some interesting extras like social networking protection. However, for most folks, it’ll be worth stepping up to the flagship Trend Micro Maximum Security, which delivers protection for Mac and mobile devices (up to 10 of them) as well as Windows (plus it adds a password manager and secure file vault).

Avira is a strong antivirus  - and our exclusive price makes it even more attractive (Image credit: Future)

5. Avira antivirus

Solid security with a remarkable exclusive price

Operating system: Windows and Mac | Maximum devices covered: Unlimited | Stand out features: Proactive anti-ransomware, banking and shopping protection, file repair tool

Our exclusive price makes it so cheap
Protection scores high at test labs
Configure as you like it
Limited on features

Avira is probably best known for its free antivirus option (which is among the best out there), but its premium packages should not be sniffed at – especially when you consider the incredible price you can now pay for its Antivirus Pro package.

That’s the name of Avira’s entry-level antivirus and it includes everything you’d expect of a modern-day security program. So there’s real-time scanning on everything that you download, torrent, pull from the cloud and upload from a USB; dedicated defences against ransomware; and a file shredder for extra confidentiality.

There isn’t a huge amount outside those basics - you’d have to upgrade to either its Internet Security or Prime plan to start adding extra functions like a password manager, VPN, PC cleaning tools and mobile app support. But Avira has really impressed independent test labs likes AV-Comparatives, which gave it a 100% score for vanquishing threats.

And for the super cheap price that you can now get Antivirus Plus, it makes your decision very easy on whether to go for this instead of its free option…

EXCLUSIVE: Avira Antivirus Pro | 1 year | $44.99 $9.99 | 78% off
Simply put, premium antivirus doesn’t come cheaper than this. TechRadar readers (and TechRadar readers alone) now have the chance to pick up a year of award-winning internet security for less than $10. What’s more, if you decide that you don’t like it within the first 30 days, you can get a refund thanks to Avira’s money-back guarantee. A real bargain.

Click here if you're in the US (or anywhere else)
Click here if you're in the UK | £29.99 £7.99
View Deal

Webroot SecureAnywhere AntiVirus is extremely undemanding without compromising on protection  (Image credit: Future)

6. Webroot SecureAnywhere AntiVirus

A tempting lightweight antivirus option

Operating system: Windows and Mac | Maximum devices covered: 3 | Stand out features: Always-on security, identity protection, real-time anti-phishing, firewall monitor

Very well-featured
Incredibly streamlined app
Superfast virus scans
Limited testing data from independent labs

Just about every antivirus tool claims to be ‘lightweight’, but Webroot SecureAnywhere AntiVirus really delivers on this front. Installation takes seconds, and it only uses 15MB (yes, megabytes) of hard drive space – plus the app’s memory footprint is just as tiny, with virus definitions stored in the cloud.

Considering this, there’s no compromise on features, which makes Webroot all the more impressive. SecureAnywhere AntiVirus gives you solid malware protection – although it isn’t ranked by the big-name testing labs – and extremely quick virus scans, along with a smart ‘firewall’ monitoring system, accurate real-time anti-phishing, identity theft protection and much more.

Further up the Webroot range, SecureAnywhere Internet Security Plus expands coverage from Windows and Mac PCs to mobile devices, also providing a password manager (courtesy of LastPass) and covering up to five devices (rather than three).

And at the top of the tree is Internet Security Complete which introduces an automatic backup tool (with 25GB of cloud storage) and additional online privacy features.

SecureAnywhere AntiVirus gives you everything you need, really, although Internet Security Plus is the top pick for the most rounded package. Another boon worth bearing in mind here is Webroot’s confidence-instilling lengthy 70-day 100% money-back guarantee.

The famous free antivirus has a fantastic paid-of version in Avast Premium (Image credit: Shutterstock)

7. Avast antivirus

Heartily well-featured security suite

Operating system: Windows, Mac, Android, iOS | Maximum devices covered: 10 | Stand out features: Ransomware Shield, Real Site DNS hijack prevention, Wi-Fi Inspector

Plenty of smart features
Highly configurable
May slow your PC down a little
Mixed results from antivirus testing labs

Avast's Premium Security adds a hefty load of features on top of the firm’s famous free antivirus offering. That includes – but isn’t limited to – an effective Ransomware Shield, very thorough anti-phishing protection, a secure browser, password manager, file shredder, and even a Wi-Fi Inspector to look for vulnerabilities on your network. It’s a crowd pleasing selection, for sure.

Premium Security is also highly configurable, even down to the installation process, which is a major boon. And while its antivirus capabilities get somewhat mixed ratings from the independent testing labs, they should prove solid enough.

There is one issue you should definitely be aware of, though; namely that we observed some slowdown on our system at times when running Avast.

Avast Ultimate is another option, which gives you Premium Security bundled with Avast’s SecureLine VPN and some other extras. If you want the VPN – which is a decent enough offering – then the Ultimate package is certainly worth considering.

(Image credit: Shutterstock)

8. Sophos Home

A great choice for multi-device protection

Operating system: Windows and Mac | Maximum devices covered: 10 | Stand out features: Central management console, AI-powered threat detection, advanced ransomware protection

Capable antivirus engine
Great value for protecting a lot of PCs
Lack of options
Clunky interface in some respects

Sophos Home Premium covers (up to) an impressive 10 devices (Windows and Mac computers). It’s designed with novices in mind, which is no bad thing, although experts may find the interface oversimplified, and be disappointed that there are few options or low-level tweaks on offer.

The UI is also rather clunky in some ways – although that said, the web management console could be great for those who wish to take control and manage their non-tech-savvy family members’ devices.

Sophos provides capable antivirus protection, and worked well in passing our own anti-ransomware tests. For the protection on offer – and given the 10 device ceiling – Sophos Home Premium could work out as a great value proposition for those who want to protect multiple computers.

ESET NOD32 Antivirus might lack in features but it offers power antivirus protection (Image credit: Future)

9. ESET Antivirus

Expert-level antivirus for the more experienced user

Operating system: Windows only | Maximum devices covered: 5 | Stand out features: Ransomware Shield, UEFI Scanner, Exploit Blocker, cloud powered scanning

Supremely configurable, tons of options
Light on system resources
Some shaky results from independent AV labs
Not for novices

While ESET Antivirus and, in particular, its NOD32 plan doesn’t have all that many features, it’s highly configurable. Beginners will find it a tricky program to work with in some ways, but experts will love the level of control available here.

Furthermore, the streamlined program’s slender system footprint is another major plus point too. It also boasts some of best heuristic detection around, as well as some quality URL filtering.

Do bear in mind, however, that some of the independent testing labs have produced slightly worrying reports showing that ESET’s protection is a bit shaky, but others have come to more positive conclusions.

ESET does have higher-end packages in the form of ESET Internet Security and Smart Security Premium, and they add a good mix of features, but some of those extras are underpowered – and on balance, NOD32 Antivirus represents the pick of the bunch.

Best free antivirus downloads

Paying for a premium service means you get more features like spam filters, parental controls, system scans and advanced firewalls. This makes them more suitable for power users, those who want a bit of extra security and anyone who needs their system to run as smoothly as possible.

It also means you avoid annoying pop-ups that try and tempt you to upgrade to the full version. And, as you've discovered above, they won’t cost you an arm and a leg.

In fact, these days you can get top quality protection absolutely free, with just about every single major vendor out there offering a free antivirus. By doing so, developers gain access to more devices from which they can collect data, and therefore improve their knowledge and security platforms so it’s really not in their interest to reduce the amount of protection provided by the free version.

Windows has come a long way in terms of security, but it's still fair to say that Windows 10's built-in solution, Windows Defender, is the last free AV app you should trust to keep your system clean.

Avira Free Antivirus is today's best free security download
If you're determined to get free antivirus, we strongly suggest that you go for our pick of best security downloads - Avira Free Antivirus. We've highlighted its long list of features below, together with information on how well it performs at independent testing labs. A very impressive freebie.
View Deal

Best free antivirus 2020 at a glance

  1. Avira Free Antivirus
  2. Bitdefender Antivirus Free Edition
  3. Kaspersky Free
  4. Avast Free Antivirus
  5. Sophos Home

Avira Free Antivirus

Avira Free Antivirus remains a superb free antivirus suite

1. Avira Free Antivirus

The best free antivirus in 2020

Operating system: Windows, Mac, Android, iOS | Features: Virus detection, Phishing protection, Built-in VPN, System optimization tools

Little impact on system performance
Great detection rates
Unexpected long list of features
Quite a few popups when running

Avira Free Antivirus for Windows continues to score highly on AV-TEST's stringent testing program, quashing 99.7% of tested threats, and it generally doesn't put too big a burden on your hardware. That's pretty good for a freebie, and why we have no qualms at all in naming it as our #1 best free antivirus download.

As well as the essential security smarts, we also love the clean, friendly interface and the fact it has a track record of throwing up minimal false positives. The introduction of a free security suite to optionally run alongside it – with anti-ransomware and a 500MB/month free VPN and various speedup promises – just sweetens the deal.

In fact, the list of features reads more like a fully paid premium package. It throws in identity protection, a password manager, anti-scam protection to prevent phishing attempts, and even a PC cleaner to keep your system spick and span.

If there's one thing we'd change then it's the tad overeager badgering - although popups and advertisements are very common to free products, and that doesn;t prevent us from wholeheartedly recommending Avira Free Antivirus.

Review and where to download: Avira Free Antivirus

Bitdefender Antivirus Free

Bitdefender Antivirus Free edition has taken the top spot in our free antivirus roundup. It's the best free security software you can download today

2. Bitdefender Antivirus Free Edition

Brilliant free antivirus from one of the best in the business

Operating system: Windows, Mac, Android | Features: Phishing protection, Behavioural analysis, Automatic scanning

Fast scanning
Excellent virus detection
Advanced users may want more control
Scans cannot be scheduled

Essentially consisting of the AV section of the full product, Bitdefender Antivirus Free Edition's clean, light, well constructed scanning mechanisms – which are almost universally faster than the industry average – and unrivalled virus definitions mean Bitdefender is a free antivirus force to be reckoned with.

The free version's insistence on running in autopilot mode is a double-edged sword, making the process of virus management easy but taking it out of the user's hands; while we're inclined to trust it, there have been cases in the past of automatic systems going slightly rogue, such as the incident a few years back when Panda Antivirus identified itself as a virus and locked up a large number of Windows systems. 

AV-TEST's testing did pick out a few flaws in its handling of zero-day threats, however. And it's worth pointing out that for you can currently get our top rated paid-for solution - Bitdefender Antivirus Plus - for the equivalent of around $2 per month. That's a tiny amount to pay for a tonne of extra features, such as online banking protection, integrated password manager, permanent file shredder and and free online 24/7 support. 

Review and where to download: Bitdefender Antivirus Free Edition

Kaspersky Free

The free version of Kaspersky's extremely successful antivirus software

3. Kaspersky Free

Supremely easy to use

Operating system: Windows | Features: Real-time scanning, Anti-phishing, Email scanning, Spyware protection

Accurate, reliable antivirus engine
Basics-only feature set
Limited technical support

The paid-for version of Kaspersky's Windows security software frequently tops charts of best virus prevention solutions and AV-TEST once again gave it full marks across the board in its latest round of testing.

As you would expect, Kaspersky Free is a scaled down version of the full program - a tool that we really like (more on which below). The free version ditches additional features such as privacy protection and safe payment sandboxes to concentrate on powerful, no-frills and - most importantly - free protection for your PC. The result is a tremendously easy to use and navigate bit of software that does an effective job of keeping you protected from viruses.

Using the nuts and bolts real-world scanning engine of the paid-for suite, Kaspersky Free is fiendishly hard for viruses and trojan horses to break down. And we love that it doesn't constantly pester you with nudges to upgrade to the full version.

But, ironically, we will just for a moment, because Kasperksy Anti-Virus isn't expensive to buy and yet affords much better protection to your PC (or PCs). The standard price is $29.99 to protect three computers and includes more muscle when it comes to preventing ransomware, spyware and Trojans.

Review and where to download: Kaspersky Free

Avast AntiVirus Free

Following its takeover of AVG, Avast now benefits from a much larger userbase, all reporting details of new security threats

4. Avast Free Antivirus

Lots of freebie extras

Operating system: Windows, Mac, Android | Features: Virus detection, Gaming mode, Password manager, Malware scanner

Doesn’t slow your computer
Great virus protection
Irritating privacy settings
Includes links to paid-for components

Avast and AVG haven't yet fully merged, despite the former formally acquiring the latter in mid-2016. The newly combined company says the two free antivirus products will remain separate, although there's apparently a joint AV package on the way soon. Obviously, though, Avast now has a lot more data to work with, having expanded its effective userbase (and, therefore, its threat detection network) to a whopping 400 million users.

The latest edition of Avast Free Antivirus adds an automatic gaming mode to mute popups and reduce system load when you're firing up a processor-hungry game, which is very handy indeed, and the interface has been given a clean new overhaul. There's a password manager, too, which is an undeniably good addition to your security portfolio.

It scores well on AV-TEST's widespread malware benchmark and continues a clean sweep against 0-day attacks – presumably that expanded detection network is really helping. Less impressive is the slight negative effect Avast has on software launch times, and its slightly popup-heavy attitude.

Review and where to download: Avast Free Antivirus

Sophos Home

Sophos Home delivers enterprise-level antivirus protection for home users

5. Sophos Home

A great choice for a home full of PCs

Operating system: Windows, Mac | Features: Virus detection, Phishing protection, Parental controls, Protects up to 10 PCs

Simple and non-intrusive
Good cloud-based control of protected devices
No scan scheduling
Limited controls for advanced users

Marketing itself as "business-grade security", Sophos Home does a little more than most free antivirus software, and actually seems better suited to families.

You get standard virus protection and anti-malware protection, along with browser tools like anti-phishing and, most importantly, content control. Combined with central management of up to 10 PCs, this means you can effectively lock down your kids' browsing options.

Although AV-TEST hasn't formally assessed Sophos' skills, fellow lab AV-Comparatives offers a decent rating of its abilities. 

Although Sophos Home has always had desktop notifications, there was no premium version of the software for it to encourage an upgrade to. That changed in February with the launch of Sophos Home Premium.

Review and where to download: Sophos Home

Head back up to read about our number one recommended best antivirus ^

Best business antivirus 2020

Each entry in this list has been carefully chosen for its business features, such as remote installation and central management. The best business antivirus software will have enterprise-level protection against threats, but that doesn't mean they have to cost a lot of money, either. The price per installation can sometimes be cheaper than even our recommended best antivirus packages.

1. Avast Business Antivirus Pro
An excellent business antivirus suite, giving you various tools such as virus protection,  Firewall, email protection, anti-spam and the ability to sandbox applications for complete security. Avast Business Antivirus Pro (as opposed to the standard Avast Business Antivirus) also includes Sharepoint and Exchange protection, as well as a number of tools for your servers.

2. Bitdefender GravityZone Business Security
This package makes reliable protection its top priority. Bitdefender products are loved by the independent testing labs, highly rated for malware detection, removal, performance and usability. Most of the features work automatically – anti-malware, firewall, web advisor, URL filtering – but you can also customise the product to control user actions.  

3. Symantec Endpoint Protection
Symantec's first benefit is the company's Insight file reputation technology, an effective way to detect and block even the very latest undiscovered threats. Other layers of protection include virus protection, behaviour monitoring, intrusion protection, a firewall, and the 'Power Eraser' to remove stubborn threats and repair your system.  

4. Avira Antivirus for Endpoint
This is Avira's main small business product. It takes all the core features you'd expect – antivirus, baseline network protection, web filtering – and extends them with file server protection and optimisation, along with application whitelisting and blacklisting.  

5. Kaspersky Endpoint Security Cloud
A small business product, which comes ready to protect Windows PCs, file servers, Android and iOS devices. The desktop side of the package has all the usual quality Kaspersky modules – antivirus, antispam, a firewall, more – but the mobile security technologies are even more impressive.   

Head back up to read about our number one recommended best antivirus ^

  • Even if you own the best computer in the world, you're still at risk