How to use it
- Pick US units (inches and pounds) or metric, and your sex.
- Measure your neck and waist; women also measure the hips. Enter height and weight.
- Read your estimated body fat, its ACE category, and fat and lean mass.
Measure on bare skin, in the morning, with the tape snug but not pressing in. Taking each measurement twice and using the average helps.
The formula
The calculator uses the circumference equations of Hodgdon and Beckett (Naval Health Research Center, 1984). They give body density from logarithms of measurements in centimetres:
- men: density = 1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)
- women: density = 1.29579 − 0.35004 × log₁₀(waist + hips − neck) + 0.22100 × log₁₀(height)
Density becomes body fat with Siri’s equation (1961): body fat % = 495 ÷ density − 450. Inches are converted to centimetres first (1 in = 2.54 cm).
Categories follow the American Council on Exercise chart:
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13 % | 2–5 % |
| Athletes | 14–20 % | 6–13 % |
| Fitness | 21–24 % | 14–17 % |
| Average | 25–31 % | 18–24 % |
| Obese | 32 % + | 25 % + |
Worked example
A man 175 cm tall with a 38 cm neck and an 85 cm waist: waist − neck = 47 cm. Density = 1.0324 − 0.19077 × 1.6721 + 0.15456 × 2.2430 = 1.0601, and 495 ÷ 1.0601 − 450 = 16.9 % – the “fitness” range. At 80 kg that is 13.6 kg of fat and 66.4 kg of lean mass.
Limits
- The method was developed on young, mostly fit adults; it is less reliable for older people, very lean or very muscular people, and during pregnancy.
- Body fat percentage is one indicator among many. If you are worried about your weight or health, a doctor can put it in context with blood pressure, blood tests and other measures.