RFM Calculator — Relative Fat Mass

Just height and waist. RFM was fitted to whole-body DXA scans and beats BMI at predicting actual fat, especially for women.

How the RFM method works

Body fat % = 64 − 20 × (height ÷ waist) for men, or 76 − 20 × (height ÷ waist) for women, with height and waist in the same unit.

Worked example

A man 175 cm tall with a 90 cm waist: 64 − 20 × (175 ÷ 90) ≈ 25%. Trimming the waist to 82 cm brings it to about 21.3%.

Body-fat ranges (adults)

Category	Men	Women
Essential fat	2–5%	10–13%
Athletes	6–13%	14–20%
Fitness	14–17%	21–24%
Average	18–24%	25–31%
Above average	25%+	32%+

Other ways to estimate body fat

U.S. Navy method — tape measure only, the quickest.
Skinfold, 3-site — Three caliper pinches. The Jackson-Pollock generalised equation.
Skinfold, 7-site — Seven caliper pinches — the most precise field method.
From BMI — Roughest estimate — from BMI, age and sex, nothing to measure but weight and height.

Whichever you pick, stick with it: the direction over weeks matters more than the exact figure. To lower body fat, run a modest deficit with the calorie calculator, keep protein high via the macro calculator, and lift.

Source: Woolcott & Bergman, Scientific Reports 2018. Siri body-density conversion: Siri 1961.

Related calculators

Related guides

Frequently asked questions

What is RFM?

Relative Fat Mass. Woolcott and Bergman published it in 2018 after testing many simple anthropometric ratios against whole-body DXA scans in about 12,000 US adults. The height-to-waist ratio came out best, and RFM is the linear equation they fitted to it.

Is RFM better than BMI?

For estimating actual body-fat percentage, yes — noticeably for women and modestly for men — because it uses waist size, which tracks fat, rather than total weight, which includes muscle. It does not replace BMI for population screening, where BMI's simplicity and long track record still matter.

Where do I measure my waist?

At the level of the navel, standing relaxed, tape horizontal and snug against the skin without compressing it, reading taken at the end of a normal breath out. Same spot every time.

How accurate is it?

In the original study RFM was within about 3.5 percentage points of the DXA value for most people, and misclassified obesity less often than BMI. It is a screening estimate, not a lab measurement.

Why is the women's number 12 points higher for the same ratio?

The equation has a fixed offset — 64 for men, 76 for women — reflecting that women carry more essential and subcutaneous fat at the same height-to-waist ratio. It is built into the formula, not something you adjust.

Last reviewed: September 2026. Figures and formulas are checked against their published sources; see the site's data notes.