Doctors are warning that cheap step counters or pedometers should not be used for public health measures.
A report by researchers at Ghent University in northern Belgium, says cheap pedometers are inaccurate.
They have based their findings on research into almost 1000 inexpensive pedometers which were tested by 35 volunteers with an age of between 20 and 60 years.
Each volunteer was given 30 cheap pedometers as well as a sophisticated automated step count log against which to compare the performance of the gadgets.
Over a period of six days each one wore five cheap pedometers and counted the steps recorded daily with each of the pedometers.
They then compared the figures against those of the automated log.
The researchers were prepared to accept a variation in performance of 10%, but it was found that only one in four of the pedometers fell within that range.
Three out of four of the pedometers either exceeded or fell below 10%, and more than one in three had a variation that was greater than 50%.
In almost two thirds of these, the pedometers overestimated the actual steps taken.
The authors say this is important because an error of 20% in 10,000 daily steps adds up to 2,000 steps, so either 8,000 or 12,000 steps will be recorded.
Pedometers have become very popular as a cheap and easy way of encouraging fitness and weight loss, and the researchers say while their use should be encouraged inexpensive pedometers provide incorrect information which makes them inappropriate for physical activity promotion targets.
They suggest some form of quality marker would be helpful for consumers and patients.
Fitness experts say they are not surprised by the findings as cheap pedometers can log motion triggered by other than that associated with walking and sometimes do not respond to the motion of every step taken.
But there is a consensus of opinion that pedometers encourage people to exercise, and achieve targets and goals.
The report is published in the the British Journal of Sports Medicine.