How the calculation works
The main result uses the US Navy method, which turns your height and a few circumferences into a body fat percentage. As a second opinion the calculator shows an estimate from your BMI and age.
Men: 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
Women: 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
BMI-based: 1.20 × BMI + 0.23 × age − 10.8 (men only) − 5.4
All lengths in centimetres. Fat mass = body weight × body fat percentage; lean mass is the rest.Worked example
Woman, 35, 165 cm, 65 kg, neck 36 cm, waist 80 cm, hip 98 cm.
- Waist + hip − neck = 142 cm, so log10 of that is 2.152 and log10 of the height is 2.217.
- 1.29579 − 0.35004 × 2.152 + 0.221 × 2.217 = 1.0325, and 495 ÷ 1.0325 − 450 = 29.4%.
- Fat mass is about 19.1 kg and lean mass about 45.9 kg. The BMI-based estimate is 31.3%.
What this calculator does not include
- Where your fat sits, and muscle mass, which can make the result misleading.
- Age-related and ethnic differences in body composition.
- Clinical measurements such as DEXA, which are more accurate.
Results are estimates for information, not medical advice. Last updated: 12 November 2026.
Frequently asked questions
How accurate is a body fat calculator?
It gives a rough estimate. Formulas based on a tape measure can be several percentage points off compared with clinical methods such as a DEXA scan. They are best used to follow your own changes over time with the same way of measuring.
How do I measure my waist, neck and hip?
Use a soft tape and keep it level and snug but not tight. Measure the waist at the navel, the neck just below the larynx, and for women the hip at the widest point. Stand relaxed and breathe out normally.
Why do the two methods give different results?
The US Navy method uses your circumferences, and the other formula only uses your BMI and age. They look at different things, so they often differ by several points. Neither is exact.
What is a healthy body fat percentage?
It depends on sex, age and fitness. The ranges shown are those of the American Council on Exercise and are only a guide. A doctor can tell you what is healthy for you.
Is this suitable for everyone?
No. It is for healthy adults aged 18 and over. It does not fit children, pregnancy or medical conditions, and it can be misleading for very muscular people.