Including gardening, DIY, vacuuming or cleaning in a tally of weekly exercise may give people a false sense of how active they have been.
Until now, studies have suggested housework counts towards a weekly round-up of moderate activity.
This is despite NHS recommendations saying it does not raise the heart rate enough to count towards the goal of 150 minutes of moderate intensity activity every week.
In the study, published in the journal BMC Public Health, experts researching the idea found that those who counted housework were actually heavier than people choosing other forms of exercise.
Professor Marie Murphy, from the University of Ulster, who led the study, said: "Housework is physical activity and any physical activity should theoretically increase the amount of calories expended.
"But we found that housework was inversely related to leanness which suggests that either people are overestimating the amount of moderate intensity physical activity they do through housework, or are eating too much to compensate for the amount of activity undertaken."