Fitness slackers: Which? reveals fitness trackers that fail to go the distance

New analysis from Which? has exposed the least reliable fitness trackers for measuring running distance – with one tracker underestimating the distance of a full marathon by 11 miles.

Ahead of this weekend’s London Marathon, Which? tests have uncovered the most inaccurate fitness trackers from big brands including Apple, Fitbit and Garmin – with the least reliable tracker underestimating distance covered by as much as 41.5 per cent.

The worst offender was the Garmin Vivosmart 4, which underestimated the distance of a full marathon by 10.8 miles. Which?’s tests found that when the Vivosmart 4 tells you that you’ve run 26.2 miles, you’ve actually gone almost 11 miles beyond that – meaning those training for a marathon would be actually be running around 37 miles in total.

To illustrate the difference in distance covered using the London Marathon route, runners relying on this model would cross the finish line at Buckingham Palace, carry on running south and continue all the way to South Croydon.

On the other side of the finish line, a runner using the Huawei Watch 2 Sport could find themselves around seven miles short of the finish line of the marathon, with Which? finding this model overestimating the distance covered by 28 per cent.

Relying on the Watch 2 Sport would leave a runner finishing around the 19 mile mark, or near Canary Wharf in this weekend’s marathon – the largest overestimation of distance of any wearable in the study.

Apple fared relatively well in the Which? tests, with the smallest difference between its most and least accurate models. Of the eight Apple models we tested, the most inaccurate was the Apple Watch Series 3 GPS, which overestimated distance covered by 13 per cent, or around 3 miles under the finish line of a marathon. The best Apple model – the Apple Watch Series 1 – only overestimated distance while running by 1 per cent.

Which? testing has also shown considerable variations for tracking running distance within brands too, suggesting that buying a fitness tracker from a well-known or reliable brand doesn’t guarantee accuracy.

For example, Garmin has produced several good models – the Vivoactive 3, for example, was 100 per cent accurate at tracking running distance in our tests. Yet the Garmin Vivosmart 4 underestimated distance covered by 41.5 per cent, and the Garmin Vivosmart HR overestimated by 30 per cent.

See below for the least reliable tracker, based on distance while running, from each of the eight biggest brands:


Brand Model % Inaccuracy Actual distance covered by marathon runners
Garmin Vivosmart 4 -41.5% 37 miles
Samsung Samsung Gear S2 -38% 36.2 miles
Misfit Misfit Ray -32% 34.6 miles
Xiaomi Xiaomi Amazfit Bip -30% 34 miles
Fitbit Fitbit Zip -18% 30.9 miles
Polar Polar A370 -18% 30.9 miles
Apple Apple Watch Series 3 (GPS) +13% 22.8 miles
Huawei Huawei Watch 2 Sport +28% 18.9 miles


Natalie Hitchins, Which? Head of Home Products and Services, said:

“Running a marathon is no mean feat, so runners who are putting in the months of training beforehand will want to know their fitness tracker is trustworthy, and not jeopardising their finish times.

“Our tests have found a number of models from big-name brands that can’t be trusted when it comes to measuring distance, so before you buy, make sure you do your research to find a model that you can rely on.”

Rights of reply:

A spokesperson from Garmin said:

“The Vivosmart 4 is an all-round smart fitness activity tracker used to monitor wellness, health and fitness for a range of activities. As the Vivosmart 4 does not incorporate GPS, our recommendation for someone who is running long distances such as a marathon, would be to choose a tracking device such as our Forerunner range which is dedicated to running and incorporates GPS”

A spokesperson from Huawei said:

“Thank you for your comprehensive testing. The test results may vary depending on testing conditions such as indoor and outdoor environments and individual runner variances.

“With regards to running indoors, as this particular test was carried out on a treadmill, the algorithm of HUAWEI WATCH 2 Sport calculates the user’s stride length from the acceleration sensor data while running at different speed. WATCH 2 Sport tracks the distance by evaluating the data of user’s stride length and steps. The results may deviate owing to individual runner variances. Huawei is fully committed to providing accurate and better running experience for users, and we will continuously work to optimize our existing and new generation products.”


Notes to editors:

  1. Which? tested 118 fitness trackers and smartwatches between May 2016 and February 2019 for accuracy using a calibrated treadmill to compare the trackers’ abilities to log steps taken and distance travelled.
  2. For more on accuracy, read our guide to the most and least accurate fitness trackers.
  3. For the full London Marathon route, click here.
  4. Visit our full range of fitness trackers and watches reviews here.

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