What this calculator does
Relative fat mass estimates body fat percentage from just two measurements: height and waist. It was published in 2018 as a simpler alternative to BMI that correlates better with DEXA-measured body fat, particularly in women.
Waist does all the work. At a fixed 175 cm height, a waist of 75 cm estimates 17.3 per cent body fat for a man and a waist of 100 cm estimates 29.0 per cent, with weight never entering the calculation.
The formula
Height divided by waist, both in the same units, is multiplied by 20 and subtracted from a constant: 64 for men and 76 for women. The constants were fitted against DEXA measurements.
| Term | Meaning |
|---|---|
| RFM | Relative fat mass, an estimate of body fat as a percentage. |
| Woolcott-Bernstein | The 2018 equation used here, fitted against DEXA body composition data. |
| Height-to-waist | The single ratio the estimate is built on. |
The inputs explained
| Field | What to enter |
|---|---|
| Height (cm) | Height in centimetres. |
| Waist circumference (cm) | Waist circumference in centimetres, measured at the level of the navel. |
| Sex | Sex, which selects the constant used. The two equations differ only in that constant. |
When to use it
Estimating body fat without equipment
A tape measure is all this needs, unlike calipers or impedance scales.
Tracking change over time
Waist is a more responsive measure than weight for changes in body composition.
Comparing against BMI
RFM often separates people that BMI treats as identical.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does waist drive the estimate?
The same height at three waist measurements.
Questions
Why does weight not appear?
Because the equation was fitted to predict DEXA body fat from height and waist alone, and adding weight did not improve it enough to justify the extra input. Waist already captures most of what varies between people at a given height.
How accurate is it?
The original study reported closer agreement with DEXA than BMI, particularly for women. It remains an estimate from a tape measure, and it cannot distinguish a muscular build from a lean one at the same waist.
Where exactly should the waist be measured?
At the level of the navel, standing relaxed, at the end of a normal breath out. Measuring at a different level, or pulling the tape tight, changes the result noticeably.
Why do men and women use different constants?
Because body fat distribution differs on average, so the same height-to-waist ratio corresponds to a higher fat percentage in women. The 12 point gap between the constants reflects that average difference.
For waist adjusted for overall size, see the ABSI calculator. For a caliper or impedance-based estimate, see the body fat calculator.