Summary
Body fat percentage is the proportion of your total body weight that is fat tissue. Unlike BMI, it distinguishes between fat and lean mass, making it a better indicator of health and fitness. The calculator estimates body fat using the US Navy circumference method, which requires only a tape measure and provides results within 3-4% of clinical methods like DEXA.
How it works
US Navy method
The US Navy developed a formula based on body circumference measurements:
Men: Body fat % = 86.010 x log10(waist - neck) - 70.041 x log10(height) + 36.76
Women: Body fat % = 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387
Measurements are in centimetres. Waist is measured at the navel, neck at the narrowest point, and hips at the widest point.
Body fat categories
| Category | Men | Women |
|---|---|---|
| Essential fat | 2-5% | 10-13% |
| Athletes | 6-13% | 14-20% |
| Fitness | 14-17% | 21-24% |
| Average | 18-24% | 25-31% |
| Obese | 25%+ | 32%+ |
Women carry higher essential fat due to reproductive function.
Other estimation methods
- Skinfold calipers (Jackson-Pollock 3 or 7 site): measures fat fold thickness at specific body sites. Accuracy depends on the tester’s skill.
- Bioelectrical Impedance (BIA): sends a small current through the body. Affected by hydration, food intake, and time of day.
- DEXA scan: clinical gold standard. Uses X-rays to measure bone, lean, and fat mass separately. Accuracy within 1-2%.
Worked example
Male: height 180 cm, waist 88 cm, neck 38 cm
- log10(88 - 38) = log10(50) = 1.6990
- log10(180) = 2.2553
- Body fat = 86.010 x 1.6990 - 70.041 x 2.2553 + 36.76
- = 146.14 - 157.93 + 36.76
- = 24.97% (Average category)
Inputs explained
- Gender — different formulas for men and women
- Height — in cm or feet/inches
- Waist circumference — measured at the navel
- Neck circumference — measured at the narrowest point
- Hip circumference — women only, measured at the widest point
Outputs explained
- Body fat percentage — estimated proportion of body weight that is fat
- Fat mass — body fat in kg
- Lean mass — everything else (muscle, bone, organs, water)
- Category — essential, athlete, fitness, average, or obese
- Comparison to healthy range — how far from the healthy range you are
Assumptions & limitations
- The Navy method has an accuracy of ±3-4% compared to DEXA. It is a screening tool, not a diagnostic measurement.
- Measurement consistency is key. Measure at the same time of day, same hydration level, and same anatomical landmarks.
- Formulas may be less accurate for very lean (bodybuilders) or very obese individuals.
- Does not distinguish between visceral fat (around organs, high health risk) and subcutaneous fat (under skin, lower risk).
- Muscle mass is not assessed — two people at the same body fat % can have very different muscle levels.
Sources
Related calculators
BMI
Calculate your BMI with NHS categories and ethnic-adjusted thresholds for South Asian and Chinese populations.
Ideal Weight
Calculate your ideal body weight using Devine, Robinson, Miller, and Hamwi formulas. See the range across all four methods and compare to BMI healthy weight.
Waist-to-Hip Ratio
Calculate your waist-to-hip ratio (WHR) and see your cardiovascular risk category based on WHO guidelines. A stronger predictor of heart disease risk than BMI.
Lean Body Mass
Calculate your lean body mass using Boer, James, and Hume formulas. See your body composition breakdown - lean mass vs fat mass - with optional body fat % input for higher accuracy.
FFMI
Calculate your Fat-Free Mass Index to see how muscular you are relative to your height. FFMI above 25 (men) or 21 (women) exceeds natural limits identified by Kouri et al. (1995).