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Technology.

12 July 2021

How accurate is your fitness tracker or smartwatch?

We pound the pavements to reveal the most accurate fitness trackers and smartwatches for steps, distance and heart-rate.
CW
Christina Woodger
Fitness watch header
If you're using a smartwatch or fitness tracker to help you achieve a specific goal - say, reaching a daily step count target or running a certain distance - you'll want to ensure it's accurate. We put our wearables through a range of scenarios, including an outdoors run, to see just how accurate each one really is. 

We've found fitness trackers and smartwatches 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, and will make it impossible to track any improvements in fitness over time.

Below you can see the accuracy scores for steps, distance and (for those with a heart rate monitor) heart rate for every wearable we've tested, plus how each one fares overall for fitness tracking accuracy. Enter a model you're interested in into the search bar at the top of the table to filter the results.

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

Find out which Best Buy models aced all of our accuracy tests, by heading over to our results for the best fitness trackers and best smartwatches

Fitness tracker and smartwatch metric ratings

Only logged-in Which? members can view the exclusive ratings in the table below. Join Which? to get instant access.

Table notes
Ratings for steps are based on accuracy and reproducibility of data captured during a ten minute walk, a ten minute run and a test of five different 5-minute each household activities. Ratings for distance are based on accuracy and reproducibility of data captured during a walk and run of known distances. Ratings for heart rate monitoring are based on accuracy of data captured at rest, at low-intensity exercise and at high-intensity exercise.

If you're not logged in to Which?, you'll see a sample of our results.

How we uncover the most accurate step trackers

Our test participants walk for 10 minutes, run for 10 minutes and carry out five 5-minute household tasks, including flipping the pages of a magazine, packing and unpacking a dishwasher and going up and down stairs. 

Reproducibility, or consistency, is just as important as accuracy. If a tracker is accurate on some days but not on others, you won’t ever be able to trust it, and so you still won't know if you're improving. So we repeat our walking and running test to see if the trackers consistently overstate your stats, understate them, or hit the accuracy mark.

An accurate tracker or smartwatch can be a big help in the home if you're thinking of setting up a gym. Read our exercise equipment buying guide for more on how to get started.

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 is important. Our testers strap on a chest belt heart-rate monitor - which are known to be more accurate than wrist worn heart rate monitors - before taking part in low intensity exercise, high intensity running and cycling and a well-deserved rest. 

How does my smartwatch or fitness 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 tracker or smartwatch 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 miscalculated distance travelled based on step data by as much as 66%  - meaning you could end up walking or running further than you need to, or  believing you've hit your goal when you haven't. Those that base distance on GPS are usually better but that isn't always the case. 

Our guide to the top fitness trackers for 2021 reveals the best.