Comprehensive Calculator Guide
The relative fat mass (RFM) calculator estimates your body fat percentage from just your height and waist — no scales or gadgets. A man 178 cm tall with a 90 cm waist comes out at 24.4% body fat; a woman 165 cm tall with an 80 cm waist at 34.8%. Add your weight to see how many kilos are fat and how many lean mass.
Where the formula comes from
Researchers at Cedars-Sinai published RFM in 2018, building it on thousands of US adults whose body fat was measured by DEXA scan. It estimated body fat better than BMI did, especially at identifying obesity.
Men: 64 − 20 × (height ÷ waist). Women: 76 − 20 × (height ÷ waist). Results are shown against the American Council on Exercise bands: for men 18–24% is average and 25%+ high; for women 25–31% average and 32%+ high.
How to Use
- Choose your sex and enter your height and waist in the same unit.
- Add your weight if you want fat and lean mass in kilos (optional).
- Press “Calculate body fat” to see your percentage, its band and what it means.
Formula Used
Men: RFM = 64 − 20 × (height ÷ waist) | Women: RFM = 76 − 20 × (height ÷ waist)
Practical Examples
- Example 1: a man
- 178 cm, 90 cm waist, 82 kg: 24.4% body fat — about 20 kg of fat and 62 kg lean mass.
- Example 2: a woman
- 165 cm with an 80 cm waist: 34.8%, in the high band for women.
Important Tips
- Measure your waist at the navel, tape level and snug, at the end of a normal breath out.
- Measure the same way each time so you can compare month to month.
Common Mistakes to Avoid
- Mixing units, such as height in metres and waist in centimetres.
- Using it for children or during pregnancy; it was built on adults.
Frequently Asked Questions
- How can I estimate body fat without a device?
- With RFM: 64 − 20 × (height ÷ waist) for men, 76 − 20 × (height ÷ waist) for women. A tape measure is all you need.
- What is a normal body fat percentage?
- Around 18–24% is average for adult men and 25–31% for women.
- Is RFM more accurate than BMI?
- For body fat, yes: BMI can’t tell muscle from fat, while RFM uses the waist, which reflects belly fat.