Relative Fat Mass CalculatorFree online fitness & health calculator — instant results, no sign-up
Estimate adult fat percentage using height, waist and sex.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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About the Relative Fat Mass Calculator
The Relative Fat Mass Calculator estimates adult body fat percentage from height, waist circumference, and the sex used by the formula. It uses the height-to-waist ratio rather than body weight to calculate relative fat mass (RFM). The result is an educational estimate, not a direct measurement of body fat.
- Sex used by formula
- Select Male or Female to set the formula’s unitless sex coefficient; the default is Male.
- Height (cm)
- Enter your standing height in centimeters, measured without shoes; the default is 175 cm.
- Waist circumference (cm)
- Enter your waist circumference in centimeters, measured with a level tape around the abdomen at the top of the hip bones; the default is 85 cm.
Formula and method
RFM (%) = 64 − 20 × (H / W) + 12 × S
- RFMRelative fat mass: estimated body fat percentage.
- HHeight in centimeters.
- WWaist circumference in centimeters; must be greater than zero.
- SSex coefficient: 0 for Male and 1 for Female.
Divide height by waist circumference, multiply that ratio by 20, and subtract the result from 64. Add 12 if Female is selected, or add zero if Male is selected. With the default inputs, RFM = 64 − 20 × (175 / 85) + 12 × 0 = 22.823529…%, displayed as 22.82%. Keep full precision during calculation and round the final result to two decimal places.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Sex used by formulaMale
- Height175 cm
- Waist circumference85 cm
Results
- Relative fat mass22.82 %
- NoteEducational estimate for adults; not a diagnosis or treatment recommendation.
How to use the Relative Fat Mass Calculator
- 1Select the sex used by the formula: Male or Female; the default is Male.
- 2Measure your standing height without shoes and enter it in centimeters; the default is 175.
- 3Measure your waist at the top of the hip bones after a normal exhalation, keeping the tape horizontal and snug without compressing the skin; enter centimeters, with 85 as the default.
- 4Calculate relative fat mass using positive height and waist measurements.
- 5Read the estimated percentage and the educational-use note together; the default inputs produce 22.82%.
Reading your result
The “Relative fat mass” row reports estimated body fat as a percentage of total body mass. For Male, height 175 cm, and waist circumference 85 cm, it displays 22.82%; this is an estimate, not a measured percentage or a health classification. The “Note” row states: “Educational estimate for adults; not a diagnosis or treatment recommendation.”
Limitations
RFM is a population-derived prediction and can differ from an individual’s measured body fat percentage. Accuracy depends on consistent waist measurement, and measuring at the navel or narrowest waist instead of the specified site can change the result. The equation uses only two sex categories and does not account directly for age, muscle mass, fat distribution, or hormone therapy. It is intended for adults, not children, and should not be relied on during pregnancy or when abdominal swelling distorts waist size. Unusual body proportions or invalid inputs can produce implausible percentages; a result outside the plausible range should not be treated as a meaningful body fat estimate.
Common questions
Why does the Relative Fat Mass Calculator not ask for weight?+
The RFM equation predicts body fat percentage from the height-to-waist ratio and a sex coefficient. Weight is not part of this equation, so it is not needed for the calculation.
Where should I measure my waist for RFM?+
Place the tape around your abdomen at the top of the hip bones, keep it horizontal, and read it after a normal exhalation. Keep the tape snug without pressing into the skin, and use the same measurement site each time.
How does the Male or Female selection affect RFM?+
Male sets S to 0, while Female sets S to 1. For identical height and waist measurements, selecting Female adds 12 percentage points to the estimate; this is a fixed feature of the equation, not a measurement of individual physiology.
Does the default result of 22.82% mean my body fat is exactly that percentage?+
No. For Male, 175 cm height, and 85 cm waist circumference, 22.82% is the rounded equation output. The two decimal places describe display precision, not clinical accuracy, and the result alone cannot determine health status.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.