Body fat percentage calculator

BMI knows your mass and nothing else. This method uses circumferences, so it has some information about shape, and shape is where the useful signal lives. Two people at an identical BMI can differ by twenty points of body fat.

It is still an estimate built from a regression, not a measurement of your actual tissue. Expect it to be within about three or four percentage points of a proper scan, which is close enough to be useful and not close enough to argue over a single point.

Almost all the error in this method comes from the tape, not the maths. Measure on bare skin, keep the tape level and snug without compressing, and breathe out normally rather than sucking in.

Take each measurement twice and use the average. If two attempts differ by more than a centimetre, take a third. Measuring at the same time of day, ideally in the morning, removes another chunk of noise.

These bands are conventional rather than official, and they shift with age. Essential fat is higher in women for physiological reasons, so the categories differ by sex rather than by any judgement about build.

Track the trend across months and ignore small movements. A change of a point between two Tuesdays is measurement noise, not progress.

The most useful thing this figure does is separate weight loss from fat loss. Someone whose weight holds steady while body fat drops is gaining muscle, and the scale alone would have told them nothing was happening.