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Updated: 5 Jul 2021

How we test VPNs

Which?'s VPN tests get to the bottom of whether they really keep you private and connected with fast download speeds.
Michael Passingham

We test every aspect of a VPN to not only make sure that it’s fast and easy to use, but also to see whether it’s actually improving your security and privacy or opening you up to even more threats than if you didn’t have one at all.

Want to know which VPN is best? See our guide to the best VPNs for privacy and streaming.

Why our VPN tests are different

Unlike some other sites that list VPNs you might see around the web, our reviews are first and foremost published to inform our members on which products are the best, and which are the worst. Other sites provide overwhelmingly positive reviews for all the products they feature, making it difficult to assess which products are genuinely the very best.

All of our security, privacy and speed tests are conducted by an accredited cyber security lab based in Europe, plus our assessment of the VPN purchasing experience is assessed by an expert lab in the UK.

The Which? overall percentage score is calculated purely on the measurements and ratings supplied by the lab, meaning there is no room for unconscious bias when giving an overall rating. We also buy every single product we test, and do not accept free samples for any of our full lab tests.

What are Which? Best Buys and Don’t Buys?

Best Buys are awarded to the VPNs that excel over their rivals. As of January 2021 a VPN must score more than 75% to receive Best Buy status. VPNs that score less than 45% will be rated as a Don’t Buy and should be avoided.

Best Buy VPNs are secure, keep your data private, are fast and are easy to use, while the products to avoid put your data at risk and perform poorly when it comes to speed tests.

How is the Which? score calculated?

The Which? overall score is a percentage. This score only takes into account the results of our tests and ignores price completely. This means that all VPNs are tested on exactly the same scale, so you can compare any VPN at any price and know how it measures up against its rivals in key areas. All VPNs are tested in the same way, regardless of the company’s claims.

A Which? overall score is made up of dozens of individual tests and checks including privacy, security, speed and ease of use.

Behind each of those ratings could be more than 50 individual tests and checks. This means that the most important things – such as how secure a VPN is, will more greatly impact a itss score than whether or not the VPN has an app for Amazon Fire sticks. To keep things simple, the most important scores are shown as star ratings out of five on each VPN’s Test Results page as an easy-to-compare list of strengths and weaknesses, so you can quickly work out whether a VPN is right for you.

  • Speed (25%) 
  • Features (15%)
  • Ease of use (15%)
  • Computer speed (10%)
  • Security (15%) 
  • Privacy (20%)

Our key testing criteria

Privacy and security 

One of the most important parts of a VPN is its ability to keep your personal data safe. While it isn’t possible to see what happens inside a VPN company, our lab can perform a number of key checks to ensure privacy and security is up to scratch when it comes to the things you, the user, can control.

To ensure a VPN is keeping the data travelling between your computer and its own servers anonymous, our lab checks for leaks. A leak can be caused when VPN software is unable to catch all the data leaving your device, which means anybody monitoring your activity will be able to see what you’re up to. It only takes a small leak for the privacy of your VPN to be completely compromised.

In addition, the lab also performs checks on how the VPN service encrypts the data that is travelling between its servers and your device, and it also checks how connections are made and whether these can leave you open to so-called ‘man-in-the-middle’ (MITM) attacks. In this context, a MITM attack could lead to a hacker being able to impersonate your VPN service, thereby intercepting your private data without you knowing.

We also put apps under the microscope. We check for any excessive use of personal data in VPN apps on smartphones, such as asking to access too many ‘permissions’ (such as the camera, microphone or your contacts) and perform a full audit on what data is sent to third-party companies that are not the VPN company itself. Almost every VPN company does this, and while it does not necessarily present a major privacy risk, only VPNs that don’t send data to any other companies will score maximum points in our tests.

Streaming and speeds

Many people use VPNs to access content only available in certain countries. As such, our lab tests include checks on eight popular streaming services in various regions, including the US, the UK, Canada and France. 

Because it is against the terms and conditions of most streaming services to use a VPN to access its content, they often block people using them. As such, if you’re using a VPN for this purpose you’ll want one that can reliably access this content without being blocked. Our tests check for this. Read our guide to the best VPNs to see which scored best in this regard. 

We also perform speed tests on seven separate days to different servers offered by the VPN. These include servers located in the USA, Japan, the UK and France. From this we take an average speed to give you an overall speed score. We go into detail how fast each VPN is in the review so you know whether it’s likely to slow your connection down enough for you to notice the difference.

We also check the software won’t slow down your computer while you’re using it. This is done by checking the speed of the computer with and without the VPN installed to see if it has any impact. 

Ease of use

Our lab testers go through the full process of setting up all the available apps for each VPN service we test. This includes MacOS, Windows, iOS and Android. We make sure that the everyday experience of activating the VPN is as simple as it can be, but also that changing more advanced settings is also clear and easy. 

All of this adds up to the overall ease of use star rating you see on each VPN’s Test Results page, and our reviews will point out any issues that could impact how your experience of using the VPN.

Read all our VPN reviews to see which software is right for you