People should not rely on cheap pedometers to monitor their exercise levels, say researchers.
Pedometers measure steps
Tests on almost 1,000 pedometers - which measure step count - by Ghent University found most were inaccurate.
Three out of four gadgets were more than 10% out in their measurements - and more than one in three were more than 50% out.
Experts said the British Journal of Sports Medicine study did not mean pedometers served no purpose.
Thirty-five volunteers, aged between 20 and 60, each tested 30 cheap pedometers.
The readings given by the gadgets were compared to those produced by a sophisticated automated step count log.
In many cases the pedometers over-estimated the actual number of steps taken.
The researchers say that pedometers have a role to play as an easy way of measuring exercise levels.
But they call for a quality kite mark system so people can have confidence in the accuracy of the gadgets they use.
John Brewer, director of the Lucozade Sports Science Academy, said he was not surprised by the findings.
He said cheap pedometers could be triggered by motion other than that associated with walking, or might not respond to the motion of every stride.
However, he added: "The whole idea behind pedometers is to get people excited about taking exercise, and achieving targets and goals, and that is a good thing."
Mr Brewer said the best way to use a pedometer was to set oneself a succession of targets of increasing difficulty.
He said that walking one mile (1.6km) on average burned up 100 calories. However, to lose 1kg in weight required the use of 8,000 calories.
"There is no point in using a pedometer simply to count your daily activities, it must be an encouragement to do more."
Ellen Mason, a cardiac nurse at the British Heart Foundation, agreed that pedometers were a popular and inexpensive way of encouraging people to get fit.
She said: "Many of us are motivated to exercise if we have a target to work towards, so such step counters can be a useful aid.
"While accuracy is important for public health targets, it is less of an issue when getting the UK to think about exercise, and this is where pedometers can really help."