There are two ways that a fitness watch can measure the distance a person has ran, walked or jogged.
Some step trackers don’t have a built-in GPS – so will calculate distance travelled on the number of steps that have been taken.
This is based on number of steps multiplied by average stride length – which can affect accuracy.
Other watches use GPS models or connect to your smartphone to use GPS.
GPS models are more accurate – but Which? results showed some that overestimated distance travelled by up to 20 per cent.
How was accuracy measured?
The test participants walked on a treadmill at 4.8km (3 miles) and hour for 10 minutes and run at 9.0-10km (5.6 – 6.2 miles) per hour for 10 minutes for Which?’s tests.
They were also put through a series of daily tasks, such as loading and unloading the dishwasher and carrying shopping.
To test calorie’s burned, testers wore a face mask that was linked to a gas-analysis system.
This measured oxygen intake and carbon dioxide production for each breath, meaning calories were measured as accurately as possible.
Tracker results were then compared with gas-analysis data to find the most accurate devices.