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How we test fitness watches and activity trackers

By Hannah Walsh

Slogging it out on the treadmill or wearing a gas mask while emptying a dishwasher - we go the extra mile to find the best and most accurate fitness watches and activity trackers.

Put us to the test

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We only recommend fitness watches and activity trackers that are comfortable to wear, easy to use, and good at accurately tracking your fitness day in, day out. Because if a tracker digs into your wrist or wildly overestimates how much exercise you’ve done, there’s simply no point in wearing it.

Fitness watches and activity trackers from all the big brands, including Fitbit, Garmin, Polar and TomTom, go through a raft of tests in and out of our lab. The best will provide in-depth and accurate data on the device or an easy-to-use app, as well as helpful tools for motivation. They'll be comfortable to wear, and have a range of features. The worst we've found will over or understate steps, calories and distances, and lack useful features.

Each fitness watch or tracker is tested in different scenarios to find out how they perform - from day-to-day activities to exercise, both in and out of the lab. We put them through several rounds of testing, too, so we can be sure our fitness band reviews can reliably answer your key questions, including:

  • How accurately does the fitness watch or activity tracker capture steps?  
  • Can I rely on it to tell me exactly how many calories I’ve burned?
  • How well does it track sports and exercise?
  • Can it help me exercise more and stay motivated?
  • Is it comfortable and will I want to wear it every day?  
  • Should I buy it?  

Find out which models ace our tough tests by reading our fitness watch and activity tracker reviews.

How accurately does the fitness watch or activity tracker capture steps?  

Our test participants walk on a calibrated treadmill at 4.8km/h for 10 minutes and run at 9.0-10km/h for 10 minutes. This means we can compare the trackers’ abilities to log steps taken and distance travelled.

Consistency is just as important as accuracy. If a tracker is accurate on some days but not on others, you won’t know whether you’re improving. So we repeat our walking test to see if the trackers consistently overstate, understate or hit the accuracy mark.  

28%The amount by which measurements varied on the least consistent tracker, making it almost impossible to track day on day progress.

And it doesn’t stop there. Our guinea pigs also go through a routine of daily tasks, including loading and unloading a dishwasher, carrying the shopping, sweeping the floor, and even sitting reading a magazine. We've found activity trackers that are flummoxed into clocking up steps when you’re just moving your arms. So while you may think you're hitting your step targets, your body will know differently - so your overall fitness won't improve.  

Can I rely on it to tell me the calories I’ve burned?

Most trackers claim to do this, but our tests show they aren’t always accurate. Calorie burn relates directly to the oxygen you consume. So our testers don a face mask linked to a gas-analysis system. This measures oxygen intake and carbon dioxide production breath-by-breath, so we can log calorie burn as accurately as possible. Our testers hit the treadmill in their kit - running and walking. Then they leave the treadmill to carry out routine tasks.

We compare the tracker results with our gas-analysis data to find the most accurate calorie-counting devices. Our tests have uncovered trackers that overstate how many calories you've burned. If you are on a calorie-controlled diet, this could inadvertently lead to you overeating.

105%The biggest walking calorie burn overstatement by a fitness watch or tracker. The difference between thinking you’ve burned off a Big Mac meal when you’ve really only burned off the burger and a few sips of coke.

How well does it track sports and exercise?

We check what different kinds of activities the fitness watch can track, both on the device itself and in the accompanying app.

If you're a runner looking for a tracker with GPS, we've got you covered. All fitness watches with built-in GPS are tested for accuracy on a 1km route. The route includes dense trees and an underpass to add the challenge of potential loss of GPS signal. We even add a hill climb and compare the elevation data to Ordnance Survey topographical data to get an idea of the accuracy of the distance calculator. Fitness watches with advanced running capabilities are put to the test on a longer 5km run, too.

Although many watches claim to be suitable to track swimming, we’ve discovered that the swim metric data they return can vary wildly. Some do a great job of capturing swim time, distance, swim pace and stroke rate. Others can only offer the overall time spent swimming, which won’t be helpful if you’re looking to monitor any improvement in your swimming technique.

Can it help me exercise more and stay motivated?

The very best fitness watches and activity trackers from our tests offer appealing and fun motivational tools to help keep you engaged. Some of the best we’ve seen include customisable goals, competing against mates online, earning ‘badges’ and receiving weekly emails and workout summaries. Some even offer a choice of downloadable fitness programmes, which you can follow from your watch or smartphone. 

Our testers spend weeks getting stuck into using not only the tracker, but any accompanying phone apps and computer software. They rate each tracker for the quality of the app and its ability to keep you interested - sorting the fun words of encouragement from the annoying nags. 

Is it comfortable and will I want to wear it?

A watch or tracker could be super-accurate, but if it rubs your skin, digs into your arm or is just plain heavy and ugly, you’re not going to wear it.

You can't tell from looking at an activity tracker or fitness watch in the box what it will be like to wear. So our male and female testers rate each tracker for how comfy it is to wear for sports and daily life, how well it sits on different-sized wrists and if you’re likely to end up with dirty marks from wearing it. 

70%The score a fitness watch or activity tracker needs to earn to be considered a Which? Best Buy

Should I buy it?

Each of our tests makes an impact on our overall test score – the overall percentage score we award to each activity tracker or fitness watch on test.

The scoring break down is different for activity trackers and fitness watches. Activity trackers are aimed at those looking to track daily activity, such as steps, while fitness watches tend to be more expensive and are aimed at keen runners or sports people.

Activity trackers

  • 45% fitness functions, including accuracy and reproducibility
  • 25% ease of use
  • 15% smart functions
  • 10% battery 
  • 5% features

Fitness watches 

  • 50% ease of use
  • 40% fitness functions
  • 10% features

An activity tracker or fitness watch needs to score at least 70% in our tests to become a Best Buy. Those that score 45% or less are Don't Buys.

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