A report from Which? has claimed that some fitness trackers and apps are so inaccurate that they could measure as many as eight miles too short, or too far, over the London Marathon course. So how can you measure how far you have really run?

Don’t extrapolate

The Which? team used data from a short treadmill run (and some daily chores such as washing up), then extrapolated from this to 26.2 miles. This really isn’t a particularly helpful test – GPS doesn’t work indoors on a treadmill and most people don’t stop running a marathon to wash the dishes. Besides, there’s a really good way to know exactly where you are on the London marathon course: the giant signs and banners that mark every mile. These have been measured using the most accurate method, a special counter on a bike along the shortest possible route. Of course, runners rarely take this “racing line” – they weave through other runners and water stations, adding distance – and that’s before factoring in any app inaccuracies.

Why are they so inaccurate?

Trackers that are based solely on your step-count are, at best, a rough guide. Wrist-based ones extrapolate from your arm movements, which can also be inaccurate. As a rough rule of thumb to encourage daily activity, such metrics are fine, but as an accurate distance measure they can be pretty useless. In fairness, though, given how novel wearable tech is, they’ve come a long way in a short time in terms of accuracy.

Watches v apps

GPS watches are usually more accurate than apps, and don’t kill your phone’s battery. GPS – global positioning satellites – work by trilateration: your receiver picks up signals from a number of satellites to work out your exact position. However, they too are fallible, and can be foxed by atmospheric conditions or tall buildings and tunnels. One study found the error was about 6% – fine for a rough guide but not enough to distinguish between a nine- and a 10-minute mile. When running a race, people should always treat their phone or watch as a simple stopwatch, and trust the distance markers for accuracy. Of course, human error can creep in, but this is still more likely to produce an accurate distance than your phone app.