It's not a dieter's myth after all. You really do weigh more if you put your scales on the carpet. And now we know why.

David MacKay, a physicist at the University of Cambridge, heard about the strange effect after a chance conversation with a fellow academic.

So MacKay and his student Jon Pendergast brought in some standard analogue bathroom scales and tried them out on different surfaces. Sure enough, they found they weighed in at around 10 per cent more on thick carpet than on the hard floor.

Why We Weigh More When The Scales Are On A Carpet

To find out why, Pendergast took several sets of scales apart and measured the movement of the internal mechanisms when loaded on different surfaces. Inside each set of scales, four levers or "fulcrums", each pointing inwards from one of the corners, transmit the weight of the person to a spring-loaded metal plate at the back of the scales. The movement of the plate is then transferred via a metal rod to turn the dial on the scales.

Pendergast found that on a hard surface, the base of the scales bows. This makes the fulcrums at each corner of the scales tilt in slightly, shortening the distance between each fulcrum and the point at which the load pushes onto the lever.

"Huge Difference"

Why we weigh more when the scales are on a carpet (Click to enlarge)

Put the scales on a deep carpet, however, and the scales sink into it, so the carpet supports the base, which prevents it from bending. This increases the distance between each fulcrum and the point at which its lever is loaded, so for the same force the lever moves further. Even a small increase in this distance can add several kilograms to the weight registered on the display.

The manufacturers calibrate the scales on a hard surface, not a carpet. And this makes sense, says MacKay, as more people have easy access to a solid floor than to a deep shag-pile carpet.

"I've always thought this was an urban myth," says a spokeswoman for Weight Watchers. "But it sounds like it makes a huge difference."

Pendergast found that digital scales were far less prone to the effect because of a slight difference in their internal mechanism. He finished his investigation by using a hacksaw to cut tiny notches in each lever for the fulcrums to lock into. This cut the weight change effect on carpet from over 10 per cent to just 2.5 per cent.

New Scientist June 30, 2002