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Relative Fat Mass (RFM) Calculator

Estimate body fat from just height and waist

How it is calculated

The Relative Fat Mass index estimates body fat from two measurements: RFM = 64 − (20 × height / waist) + 12 × S, where height and waist circumference are in the same unit (centimetres is the usual choice) and S is 0 for males and 1 for females, so females get 12 added. The result is a percentage with one decimal.

Height and waist must be greater than zero. Sex is taken from the form: only the exact value female adds the 12; any other value, or none, is calculated with the male constant. Only the ratio of height to waist matters, so the unit cancels as long as both use the same one. The waist should be measured as the formula intends, around the navel. It is an estimate for adults, not a clinical measurement, and it does not replace body composition scans or medical advice.

Worked example

A man 175 cm tall with an 85 cm waist. Divide height by waist: 175 / 85 = 2.059. Multiply by 20: 41.18. Subtract from 64: 64 − 41.18 = 22.82, shown as 22.8 %. For a woman of 165 cm with an 80 cm waist the steps are 165 / 80 = 2.0625, times 20 = 41.25, and 64 − 41.25 + 12 = 34.75, shown as 34.8 %.

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Common mistakes

  • Entering height in centimetres and waist in inches. The ratio then breaks completely; use the same unit for both.
  • Measuring the waist at the narrowest point, over clothing, or at the hips. A different measuring point changes the result noticeably.
  • Forgetting to choose the sex. The female result is 12 points higher for the same measurements, and the tool uses the male formula unless female is selected.
  • Comparing it with a BMI or a weight. RFM uses only height and waist, so changing body weight without changing the waist changes nothing.
  • Treating one decimal as accuracy. It is a rough estimate, and readings from scales or scans can differ by several points.

About this calculator

Relative Fat Mass estimates body fat percentage from just two measurements: for men it's 64 − (20 × height ÷ waist), and for women 76 − (20 × height ÷ waist), both using consistent units (e.g., inches or cm) for height and waist circumference.

Published in 2018 as an alternative to BMI, RFM was designed specifically because BMI can't distinguish muscle from fat — a muscular athlete and a sedentary person of the same height and weight can get the same BMI but very different body fat. RFM's waist-based approach correlates more closely with DXA-scan body fat measurements, making it popular with trainers and researchers who want a quick, equipment-free estimate.

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