Marathon training? We reveal the least accurate fitness trackers from popular brands

With the London Marathon 2019 around the corner, we've laced up our trainers and put more than 100 activity trackers and smartwatches to the test. The least accurate based on distance while running underestimated by more than 40% - in a marathon (26.2 miles) that equates to almost 11 miles.

After crunching the numbers in the Which? test lab, we've highlighted some of the worst wearables across popular brands for under or overestimating distance while running. If you're using one of these while training for a marathon, you could well struggle to pace yourself on the big day.

Browse our best fitness watches and activity trackers to see models that are both accurate and easy to use.

The least accurate wearables for running

Our graphic shows that there was a significant difference in accuracy between models. For each of the eight activity tracker brands we looked at, we've highlighted the worst product for underestimating or overestimating distance while running.

Apple did relatively well in our tests, but the Apple Watch Series 3 (GPS) was its worst performing wearable for accuracy here, overestimating distance covered by 13%. If the Series 3 says you've run 26.2 miles, you've actually run 22.8 miles. Perhaps surprisingly, the Apple Watch Series 1 only overestimated distance while running by 1%.

Other models didn't fare so well: Samsung's popular Gear S2 smart watch was 38% out, and even household fitness name Fitbit missed the mark by nearly five miles.

Underestimating distance: Garmin Vivosmart 4 (-41.5%)

Brace yourself, because the Garmin Vivosmart 4 will have you running further than you need to. When the £100 Vivosmart 4 thinks you've travelled 26.2 miles, you've actually gone 11 grueling miles beyond that - around 37. You deserve a rest.

Fortunately Garmin has produced several very good models - the Vivoactive 3, for example, was 100 per cent accurate at tracking distance while running in our tests.

Garmin told us that it recommends choosing a tracking device from its dedicated runner Forerunner range, whichhas in-built GPS, rather than theVivosmart 4, which doesn't, and which is aimed at those wanting to track a range of activities. If running is your main form of exercise, a Forerunner could come in handy - check out our Garmin fitness watch reviews to see how the Forerunners perform.

Overestimating distance: Huawei Watch 2 Sport (+28%)

If you're using the Huawei Watch 2 Sport to track your long-distance runs, you'll find the ground covered has been overestimated. If this fitness watch reported 26.2 miles travelled on marathon day, you'd actually find yourself around the 19 mile mark near Canary Wharf.

Huawei told us that results may vary depending on test conditions and runner variances, that it is committed to providing an accurate and better running experience for users, and will continue to work to optimise existing and new generation products.

The importance of accuracy in wearables

Distance accuracy while running is just one of a range of metrics we test to determine the very best and most accurate wearables. Distance accuracy while walking can actually vary dramatically from this, and we also test step count accuracy, and calories.

Step counter accuracy (while walking)

One of the most basic uses for smartwatches and fitness trackers is counting steps - it's a helpful way to get an overview of your activity and set some simple exercise targets. Hitting that magic 10,000 isn't easy though, and it's even tougher if your wearable isn't giving you accurate data.

Calorie counter accuracy (while walking)

If losing weight is your main goal, you're going to be interested in how many calories all that hard work is actually burning off. Be careful if you're relying on a wearable though - we've seen some pretty shocking inaccuracy here as well:

For more on accuracy across all these metrics, read our guide to the most and least accurate fitness trackers.

How we test activity trackers and fitness watches

We go the extra mile when it comes to testing fitness watches and activity trackers. Our expert tests sort the diamonds from the duds, with a strong focus on accuracy and other important features that matter to people the most:

• How accurately does the fitness watch or activity tracker capture steps? - we use a calibrated treadmill to compare the trackers' abilities to log steps taken and distance travelled.
• How well does it track sports and exercise? - All fitness watches with built-in GPS are tested for accuracy on a 1km route. We've also found that accuracy when swimming can vary wildly.
• Is it comfortable and will I want to wear it? - Putting up with a wearable that digs into your skin is no fun at all. We score each product on how well it sits on different-sized wrists.
• How easy is it to use? - Being easy to set up and use is a must - a wearable that's awkward to get to grips with will just end up sitting in a drawer.
• Battery life - how long does it last? Will you be charging once a week, or once a day? Our tests reveal whether battery life claims stand up to real life use.

To see which wearables have soared through our test lab with top marks, see our guide on top activity trackers and fitness watches for 2019.