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How accurate is your fitness watch or activity tracker?

By Hannah Walsh

We pound the pavements and treadmill to reveal the best fitness watches and activity trackers for tracking steps, calories and your heart-rate. Use our interactive table to find the most accurate.

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All fitness watches and activity trackers will capture your steps or calories burned, but if you're using one to meet a daily step goal or lose weight then you'll want to ensure it's accurate. We pound the pavements and take a turn on the treadmill to challenge the accuracy of every wearable we test.

We've found fitness watches and trackers that capture data with no error, but others that over or understate data, or that are unreliable and inconsistent day to day. These devices will leave you wondering how much activity you've actually done, or how many calories you've burned, and will make it impossible to track any improvements in fitness over time.

Below you can see the accuracy scores for every wearable we've tested basic on metrics such as steps, distance, calories and heart rate, plus how they fare overall. Enter a model you're interested in into the search bar at the top of the table to filter the results.

We also reveal the lengths we go to challenge the fitness tracking of every fitness watch or activity tracker on test, as well as how your device calculates important metrics, such as distance travelled. 

Find out which models aced all of our tough accuracy tests, by heading over to our guide to the best fitness watches and activity trackers guide. 

Only logged-in Which? members can view the exclusive ratings in the table below. If you're not yet a member, sign up for a Which? trial to get instant access. If you're already a member then search the table below by brand or name, or sort it by star rating or score by clicking on the table headings.

How we uncover the most accurate step trackers

Our test participants walk on a calibrated treadmill at 4.8km/h for 10 minutes and run at 9-10km/h for 10 minutes. This means we can compare the trackers’ abilities to log steps taken, as well as distance travelled. We also put test participants through a series of daily tasks, such as loading and unloading the dishwasher and carrying shopping.

40%the amount one device understated steps taken while walking, meaning you'd only have taken 5,900 steps when it says you've marched 10,000 steps.

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.

Wearables with the most accurate heart-rate tracking 

Tracking your heart rate is one of the best ways to monitor improvements in health and fitness levels, so accuracy and consistency is important. Our testers strap on a heart-rate monitor before hitting the treadmill walking and running, and then taking a well-deserved rest. We compare the readings with the data from the fitness watch or activity tracker, and calculate the percentage difference between them.

Two of the activity trackers on test score just one star for heart-rate tracking.

Why does accurate calorie tracking matter?

If burning calories is your main reason for strapping on your running shoes or picking up that heavier weight, then a fitness watch or tracker that captures calories burned accurately is key. It's important for those on calorie controlled diets, too. 

Our tests show that they aren't always accurate, though, and we've uncovered trackers that wildly over or understate how many calories you've burned. And just because we rate a device highly for its step tracking ability, doesn't guarantee it's the best choice for those keeping an eye on calories burned - we've found trackers and watches that counted steps with no error while walking or running, but overstated calories burned by 105%.

105%The biggest calories burned overstatement we've found in any of our testing. 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 does my fitness watch or tracker calculate distance?

While many of us will be aiming to beat our daily step goal, it's also interesting to know just how far you've travelled. And for those training for a particularly long hike, or perhaps a marathon, distance is an important metric.

There are two main ways that a fitness watch or activity tracker will calculate the distance you've travelled. 

There are two main ways that a fitness watch or activity tracker will calculate the distance you've travelled. 

  • GPS Models that have built-in GPS can track your distance at all times, even without your smartphone by your side - great for those that like to run without their mobile. Some devices that don't have built-in GPS will use the sensor on your phone to track distance. If this isn't available, distance travelled will be based on steps.
  • Steps Trackers that don't have built-in GPS or use the GPS on your smartphone will calculate the distance you've travelled by the number of steps you've taken. Usually this will be based on an estimated stride length multiplied by the number of footfalls, but some trackers allow you to add your measured stride length to improve accuracy of distance tracking.

We've found trackers with built-in GPS and those without that did a great job of tracking distance travelled. But, we've also uncovered models that understated distance travelled based on step data by 30% or more - meaning you could end up walking or running further than you need to. 

Accurate step tracking isn't always a guarantee of spot on distance tracking either. Read our guide to the top activity trackers and fitness watches to find out which are best for monitoring steps, heart-rate, running and more. 


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