Health Body fat calculator
Estimate body fat percentage from a tape measure using the US Navy method, with fat and lean mass broken out and a BMI-based cross-check.
Measure just below the larynx.
At the navel.
Used only by the BMI cross-check.
Body fat
16.9%
Fitness range for your sex, by US Navy circumference method.
- Fat mass
- 11.9 kg
- Lean mass
- 58.1 kg
- BMI
- 22.9
- BMI-based estimate
- 18.1%
Measure relaxed, at the end of a normal exhale, with the tape snug but not compressing the skin. A centimetre of error at the waist moves the result by roughly a percentage point, so measure three times and take the median.
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Reembun. (2026, July 28). Body fat calculator. https://reembun.com/body-fat-calculator
How to use it
Set the units and sex
The measurements the form asks for change with sex at birth, because the US Navy formula uses different sites.
Measure carefully
Neck just below the larynx, waist at the navel, hips at the widest point. A centimetre out here moves the result more than anything else you do.
Add height, weight and age
Height is part of the formula. Weight and age drive the BMI cross check shown next to the result.
Read it as a trend
This is an estimate from tape measurements, so watch the direction it moves over weeks rather than trusting a single reading.
The US Navy method
Published by Hodgdon and Beckett in 1984 for the US Naval Health Research Center, this estimates body density from log-transformed circumference measurements, then converts density to fat percentage using the Siri equation.
Men: %fat = 495 / (1.0324 − 0.19077·log₁₀(waist − neck) + 0.15456·log₁₀(height)) − 450
Women: %fat = 495 / (1.29579 − 0.35004·log₁₀(waist + hip − neck) + 0.221·log₁₀(height)) − 450All measurements in centimetres. The logarithms are why the relationship is not linear: a 5 cm waist increase matters more at a small waist than a large one.
Measuring properly
The arithmetic is exact; the measurement is not. Technique is where the error comes from.
Neck. Just below the larynx, tape sloping slightly downward at the front. Do not flex.
Waist. Men measure at the navel. Women measure at the narrowest point, which is usually above the navel. This difference is part of the formula, not a suggestion.
Hips. Widest point, feet together.
Every measurement. Relaxed, at the end of a normal exhale, tape snug against skin but not compressing it. Measure three times and take the median, since a single reading is more variable than most people expect.
Reference categories
American Council on Exercise bands, which are descriptive rather than diagnostic:
| Category | Men | Women |
|---|---|---|
| Essential fat | 2-5% | 10-13% |
| Athletic | 6-13% | 14-20% |
| Fitness | 14-17% | 21-24% |
| Average | 18-24% | 25-31% |
| Above average | 25%+ | 32%+ |
Women carry more essential fat than men for physiological reasons, which is why the bands differ by roughly 8 to 10 points throughout.
Fat mass and lean mass
The percentage is less useful on its own than the split it implies. A 80 kg person at 20% body fat carries 16 kg of fat and 64 kg of lean mass: muscle, bone, organs and water.
Tracking both matters during weight change. Losing 5 kg is a good outcome if 4 kg of it was fat and a poor one if 3 kg was lean mass, and the scale cannot tell you which happened. Re-measuring circumferences every four to six weeks will.
How the methods compare
| Method | Typical error | Cost |
|---|---|---|
| DEXA scan | ±1-2% | Clinic visit |
| Hydrostatic weighing | ±2% | Specialist facility |
| Navy tape method | ±3-4% | A tape measure |
| Bioelectrical impedance scale | ±4-8% | Consumer scale |
| BMI-derived estimate | ±6-10% | Nothing |
Consumer impedance scales are more variable than their displays suggest, because they infer body composition from a current path through the lower body and are sensitive to hydration, recent food and even foot moisture. Measure at the same time of day under the same conditions if you use one.
Common questions
How accurate is the tape method?
Within roughly 3 to 4 percentage points against hydrostatic weighing for most people, which is respectable for a tape measure. It is far more accurate than any BMI-derived estimate, and less accurate than a DEXA scan. Its weakness is technique: a centimetre of error at the waist moves the answer by about a point.
Where exactly do I measure?
Neck just below the larynx, with the tape sloping slightly downward at the front. Waist at the navel for men and at the narrowest point for women. Hips at the widest point. Measure relaxed, at the end of a normal exhale, with the tape snug but not compressing the skin.
Why does the women's formula need a hip measurement?
Because women store proportionally more fat on the hips and thighs, a waist measurement alone predicts total body fat poorly. The Navy equation adds the hip circumference to capture that, which is why the two formulas are not interchangeable.
What is a healthy body fat percentage?
Roughly 14 to 24% for men and 21 to 31% for women counts as fitness-to-average by the American Council on Exercise categories. Below about 6% for men and 14% for women is essential fat only, which is not sustainable. These are descriptive bands, not targets.
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