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Why are some fitness trackers so inaccurate?

Find out why your activity tracker might mislead you, which we rate best for accuracy and what you can do to improve things

If you buy a fitness tracker you expect it – at the very least – to track your fitness. But some are so inaccurate they’ll leave you completely in the dark about the calories you’ve burned and the distance you’ve travelled.

A new study by the University of British Columbia concluded that Fitbits and other fitness trackers are often hugely inaccurate. Researchers found that the devices are prone to overestimating activity in high-intensity workouts, underestimating activity during fast walking and registering steps while you’re carrying about other tasks.

That’s no surprise to us, unfortunately. Our tests have exposed some shockingly inaccurate devices that leave you with no clear idea of how much activity you’ve done. We’ve found trackers that:

  • Underestimated steps taken by as much as 40%. So you might think you’ve walked 10,000 steps when you’ve only actually walked 5,900.
  • Overstated calories burned by 105%. So you might think you’ve earned a whole Big Mac meal when you’ve only actually earned the burger and a sip or two of coke.
  • Overestimated distance covered by 32%. So you might think you’re up to running the marathon, when you’re actually still eight miles short of the finish line.

Learn more, and browse in-depth accuracy data for all the trackers we’ve tested.

Why are fitness and activity trackers inaccurate?

Fitness watches and activity trackers calculate your distance travelled in a couple of ways. Models with built-in GPS can track your distance all the time, even when your smartphone isn’t on you. These are generally the most accurate at distance tracking – but not always. Our tests uncovered one model that overstated distance by as much as 20%.

The ones that don’t have built-in GPS, and don’t work by using the GPS on your smartphone either, will use the number of steps you’ve taken to calculate the distance you’ve travelled. That’s usually based on your estimated stride length multiplied by the number of footfalls. However, some trackers don’t allow you to add your measured stride length – and this inevitably means they’ll be less accurate.

How to improve your fitness tracker’s accuracy

  1. Enter your biometric data on the smartphone app or, on more advanced trackers, on the device itself. Logging your weight can help your tracker to more accurately calculate with calories burned. And logging height and stride length will help it to accurately track step count or distance calculated using steps.
  2. Make sure you’re wearing it properly. That’s especially important if it has a built-in heart-rate monitor. Follow the steps in the manual or during the set-up on your smartphone for positioning your device, and choose whether to wear it on your right or left wrist at the outset. You should wear your tracker just above the wrist bone, and it should be tight enough not to move around.
  3. Run updates on your fitness watch or activity tracker, and on the companion app. This will fix bugs, introduce new features and make sure you get any privacy or data updates.

Not sure if you’re setting up your fitness watch or activity tracker properly? Use our step by step guide to setting up your fitness tracker.

What fitness tracker sensors do I need?

Fitness or activity trackers can capture lots of different metrics. These include:

  • Step tracking – every fitness tracker sensor will have a pedometer
  • Built-in GPS – this is useful if you want to track your distance while running or cycling and don’t want to carry your phone on you
  • Heart-rate monitoring – not an essential, but worth considering. This will collect data on your average heart rate, as well as peaks and troughs throughout the day
  • Calorie burn – the majority of trackers will record your calorie burn. If heart-rate data is being collected, this will feed into it
  • Sleep monitoring – most trackers can interpret your sleep data for you, including REM, light and deep sleep. This could be useful if you often feel that you haven’t had a good night’s sleep. Some have a smart alarm feature, meaning that when the time you’ve set into your alarm is approaching, your tracker will pick the optimal time in your sleep cycle to wake you up.

More sophisticated devices will be able to track things like your oxygen consumption (VO2 Max) and the light levels where you are. These might be interesting to you, but they’re not necessary if all you want is to track your fitness.

Collecting all this data can help you build a bigger picture of your general health and progress – but that depends how accurate the sensors are.

Bear in mind that having more sensors will drain the battery sooner, and could make your device more of a headache to use. That’s why we check each device for ease of use as well as accuracy.

New fitness trackers for running, gym, swimming and more

Garmin Forerunner 645 Music – £400

The Garmin Forerunner 645 Music is designed for keen athletes who like listening to music while exercising. You can store your own music on it and use it to listen to playlists from certain streaming services.

In addition to having built-in GPS and sensors for steps, distance, calories burned, intensity minutes and sleep, it captures advanced running data, including ground contact time balance, stride length, cadence and vertical ratio. That should help you develop a good running style. It can also estimate your oxygen consumption and evaluate your training status, improvements in your training and your recovery time.

There are pre-set training cardio and strength training workouts for the gym – such as indoor rowing, elliptical training and yoga – and for outdoor sports – such as skiing, snowboarding, paddleboarding and rowing. And you can use it for swimming, diving and snorkelling.

Read our full Garmin Forerunner 645 Music review to find out if it’s worth its high price tag.

Suunto 3 Fitness – £170

Like the Garmin Forerunner, the Suunto 3 Fitness goes beyond steps taken, calories burned, heart rate and distance travelled. It tracks speed, pace and cadence when you’re out running, and you can set it to monitor treadmill, track or trail runs. It even has profiles for activities such as hiking, trekking, weight training, basketball, boxing, dancing, kayaking and roller skating. It’s waterproof to 30 metres too.

Other interesting features include an ‘adaptive training guidance’ feature which records your fitness level, training history and even your mood after exercise and then creates a personalised training plan for you. It also has an interesting Maps feature that uses other users’ training sessions to build a map of the most popular routes in your area.

Surprisingly, given that Suunto specialises in making precision navigation instruments, there’s no built-in GPS, so it relies on your smartphone GPS for accurate distance or route tracking. That means you can’t leave your smartphone at home during a workout.

Read our full Suunto 3 Fitness review to find out if it’s accurate, as well as feature-packed. And come back soon to check our review of the Suunto 9, which Suunto claims combines GPS and motion sensor data for even more accurate track and distance accuracy.

Xiaomi Mi Band 3 – £25

If you don’t want to spend too much on a tracker, but don’t want to miss out on some key features, the Mi Band 3 looks like a great choice.

It’s a simple, cheap, wrist-worn activity tracker that looks remarkably similar to the Mi Band 2 with a few updates to the design.

The OLED touchscreen has increased in size from 0.40 inches to 0.78 inches, and it has a 128 x 80 pixel resolution. This is low, but given the size of the screen and the information you’ll view on it, it should be sufficient. It weighs just 20g, meaning it’s lightweight, and Xiaomi claims it will last an impressive 20 days per charge.

For fitness tracking, there’s a built-in heart-rate monitor, and it tracks steps, distance travelled, calories burned and sleep. It has a water-resistance rating of IP67, which means it’s splash and dust-proof, but shouldn’t be worn swimming. You can set up step and weight goals via the app.

It sounds good, but too good to be true? We challenged the fitness tracking accuracy and battery-life claims, as well as the durability of this cheap device, in our test lab. Read our full Xiaomi Mi Band 3 review for more. 

How Which? tests accuracy of fitness watches and activity trackers

To find out which devices are best for tracking steps, calories and heart-rate, we wear them while carrying out a range of activities. Then we assign them an accuracy score.

We get participants in our tests to walk on a calibrated treadmill at 4.8km/h for 10 minutes and run at 9-10km per hour for 10 minutes, and we compare how well the trackers log the number of steps taken and the distance travelled. We also have participants wear them while going about daily tasks such as carrying shopping, loading and unloading the dishwasher, sweeping the floor and relaxing reading a magazine.

Our testers put on a face mask linked to a gas-analysis system which logs calorie burn by measuring oxygen intake and carbon dioxide production. We compare the number of calories that the tracker thinks you’ve burnt with the amount logged by our gas-analysis system.

We test all the fitness trackers that have built-in GPS on a 1km route – which includes areas where GPS signal might be patchy. And we test them on an uphill climb, and compare the data they collect with Ordnance Survey topographical data.

There are plenty more models that impressed our fitness experts in our list of Best Buy fitness trackers.

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