A pavement is just as good, but you need to put in 100 steps per minute
You should be taking 100 steps each minute for half an hour a day if you want to achieve "moderate" exercise by walking, a study shows.
A US team reached the figure after measuring the body's oxygen demand in some 100 people walking on a treadmill.
They wrote in the American Journal of Preventive Medicine that a pedometer alone was not enough to gauge exercise as it gave no data on intensity.
US and UK guidelines urge half an hour of moderate exercise five days a week.
But there is often confusion as to what constitutes moderate exercise, and the amount of gardening, housework or walking needed to confer health benefits.
Researchers at the San Diego State University based their conclusions on exercise tests given to 97 healthy adults who had an average age of 32.
In general, men needed to walk at a pace of 92 to 102 steps per minute to achieve a moderately intense workout for their hearts. The range for women was between 91 and 115 steps per minute.
"Because health benefits can be achieved with bouts of exercise lasting at least 10 minutes, a useful starting point is to try to accumulate 1000 steps in 10 minutes, before building up to 3000 steps in 30 minutes," said Simon Marshall, lead researcher.
A pedometer was not useless, but should be used in conjunction with a wristwatch to work out how many steps were being taken.
Gary O'Donovan, lecturer in sport and exercise medicine at the University of Exeter said: "Regular physical activity is important for health and well-being and brisk walking is a great way to start.
"Most pedometers don't provide a measure of intensity, but Dr Marshall's team has identified a simple and effective method to ensure that every step counts."
Ken Fox, professor of exercise and health science at Bristol University, warned however that while 100 steps per minute was a good target for healthy walkers, it would be "dangerous to make this an across the board recommendation.
"We should note that the study was conducted on healthy young adults. People who are overweight or obese - which is the majority of middle to older adults - are working harder in order to carry their weight for any walking speed. They will need to down grade their speeds accordingly."