Popular brands of fitness trackers can overestimate the number of calories burned while walking by more than 50%, tests have found.
Researchers at Aberystwyth University found all products – ranging from £20 to £80 in price – were inaccurate during walking and running tests.
Dr Rhys Thatcher said devices had an “inherent tendency” to over-measure.
Fitbit said its product was based on “extensive research”, but others said theirs were “not medical devices”.
The tests, carried out for the BBC X-Ray programme, measured the amount of oxygen a volunteer used during ten minute walking and running sessions on a treadmill, before comparing it to the various fitness trackers.