Body mass index and body composition
Your data
Leave the tape loose enough not to press the skin, and breathe out normally before reading the waist. A couple of centimetres of difference there move the estimate by about one point.
Results
Your body mass index
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| Body fat estimated from the tape | — |
| Which reference range that falls in | — |
| Everything that is not fat: muscle, bone, water | — |
| Fat mass | — |
| What you would weigh at each edge of the normal range | — |
The same body shape at other heights
| Height, in centimetres | Weight of the same shape | What the index would read |
|---|
Those rows are the same person scaled up and down: identical shape, identical proportion of fat. The index still climbs, because it divides by the height squared while a body grows in three dimensions. Being tall pushes the number up on its own, and being short pulls it down.
The index was built to describe populations, not to diagnose a person, and the tape formula is an estimate with a margin of a few points. Neither replaces a measurement made by someone qualified.
Why does the body mass index get athletes wrong?
Because it only knows weight and height, and muscle weighs more than fat for the same volume. Two men of ninety five kilograms and one metre eighty have the very same index of twenty nine point three.
Run a tape over them and they turn out to be opposite bodies: a waist of eighty five and a neck of forty give fourteen point five per cent of fat and eighty one kilograms of lean mass, while a waist of one hundred and ten and a neck of thirty eight give thirty two per cent and sixty four kilograms. The index cannot tell those two apart, and never could.
Does being tall change the index on its own?
It does, and by an amount you can work out exactly. Take a body and scale it up keeping the same shape: the weight grows with the cube of the height, while the index divides by the height squared, so the index climbs in a straight line with height.
A one metre ninety copy of someone one metre seventy, same shape and same proportion of fat, reads one point one one eight times the index, which is exactly one hundred and ninety divided by one hundred and seventy. Twelve per cent more index for no change in the body at all.
How does the tape estimate body fat without a scale?
It uses circumferences instead of weight. The method here is the one the United States Navy uses, published by Hodgdon and Beckett, and it works from the neck, the waist, the height, and the hips for women.
It is an estimate with a margin of a few points, not a measurement, and it assumes fat sits in the usual places. What makes it useful next to the index is precisely that it never looks at the scale, so muscle does not push it up.
What should I do with these numbers?
Read them as two different descriptions of the same body, and notice when they disagree. This page names the range each one falls in and says nothing beyond that on purpose.
The index was built to describe populations, not to diagnose a person. Anything that follows from these numbers is a conversation with someone qualified, not with a calculator.
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