They are the must-have gadgets for the health-conscious striving to walk 10,000 steps a day.
But new research suggests activity monitors such as Fitbits may be over-estimating the number of steps users take by up to 25 per cent.
In tests, some gadgets registered "movements" when volunteers were sitting at a desk typing or standing still washing dishes or stacking books, reports the Daily Mail.
The gadgets contain a tiny device called an accelerometer to detect motion, with the number of steps often based on counting arm movements forwards and backwards, which can be deceptive.
Findings from the National University of Ireland in Galway suggest some users who think they have racked up the recommended 10,000 steps a day - roughly five miles - may actually have achieved only 7,500.
Researchers filmed 37 men and women and compared readings on five different devices with the activity actually done.
The results, published in the Public Library of Science journal PLOS One, showed the £20 Jawbone UP device registered a false positive - counting a step when there isn't one - up to 25 per cent of the time, and the £70 Fitbit One about ten per cent of the time.
They noted: "All monitors registered a significant number of false positives."
Sales of fitness trackers have soared in recent years. In 2015, British consumers bought an estimated three million monitors and smart watches - more than double the previous year.
Professor Ian Swaine, head of sports science at Greenwich University, said that even though technology is improving, fitness trackers can only ever give an estimate. But he added that they still encourage exercise.
Fitbit said its trackers "are not intended to be scientific or medical devices" but monitor "overall health and fitness trends" to help users get in shape.