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Five reasons not to trust your fitness tracker

Sales of fitness trackers continue to increase but our testing shows that not all can be trusted

Fitness trackers are useful for monitoring your activity and lifestyle, but our rigorous tests show that you can’t rely on every tracker to provide accurate data. 

We buy six of every fitness tracker and put them through several rounds of testing in various scenarios, while also checking the data they produce against measurements we take using specialist equipment.

Read on to find out the five areas where your fitness tracker could be failing to accurately record your exercise and daily activity, and how you can ensure you choose a tracker that won’t let you down.

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1. GPS/distance

Most fitness trackers are paired with your smartphone to take advantage of your phone’s GPS. But a tracker with built-in GPS is useful if you want to jog or cycle without carrying your phone and still accurately record distance. We’ve tested the Microsoft Band 2, Fitbit Surge and Samsung Gear Fit 2, which all have in-built GPS. On the whole we’ve found this to be pretty accurate, but be sure to read our reviews to find out how well it works for each individual tracker.

2. Steps

Accelerometers in fitness trackers pick up your arm movements, and during our tests we’ve found trackers that are confused into clocking up steps when you’re just moving your arms. So you may think you’re hitting your step targets, but your overall fitness won’t improve. Some fitness trackers are right on the money in terms of correctly calculating steps, but others have been surprisingly bad and we’ve seen some overstate steps by as much as 35%. Make sure you don’t pick up a tracker that leads you to believe you’ve covered more distance than you actually have.

3. Calorie burn

Our tests have unearthed significant discrepancies in terms of calorie burn. Our testers wear a face mask connected to a gas analysis system to precisely measure the oxygen consumed when running, walking and simulating household tasks. This enables us to check if the calorie burn data on your tracker is correct because calorie burn relates directly to the oxygen you consume. We’ve seen a particular Fitbit tracker overstate calorie burn by 105%, which is not ideal if you’re on a strict calorie-controlled diet and trying to watch your weight, as the wrong data could then lead you to potentially overeat.

4. Heart-rate monitor

Being able to see at a glance how your resting heart rate changes in response to sleep, exercise or nutrition is a really useful tool. Many athletes use heart-rate information and heart-rate zones to calculate whether they’re pushing themselves enough. The Garmin Vivosmart HR+, TomTom Touch and Xiaomi MiBand 2 all have heart-rate monitors so you can chart your fitness levels. But you’ll need to read our reviews to find out just how accurate they are at recording your heart rate at rest, as well as during low-intensity and high-intensity exercise.

5. Reproducibility

Each fitness tracker review will show you if a particular device has a tendency to understate or exaggerate your measurements. But we also test how it fares in terms of reproducibility – whether the tracker is consistent in its error margins, so you can generally trust it to tell you if you’ve improved on the previous day, or if it’s all over the place and wide of the mark by a different amount every time. Generally reproducibility is pretty good with all the fitness trackers we’ve tested, but in a few cases the same distance recorded by the same tracker varied by as much as 10%.

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