Free tool · Body
Body fat calculator (US Navy method)
Short answer
The US Navy method estimates body fat from your height and a few tape measurements: neck and waist for men, plus hips for women. A man who is 5 ft 10 in with a 16-inch neck and a 34-inch waist comes out at about 15.5% body fat. It is a quick field estimate, so use it to track change with the same tape rather than as an exact figure.
Last reviewed 27 October 20263 published sourcesNot medical advice
Reps keeps track of this for you on iPhone, iPad and Apple Watch.
Get Reps freeBody fat calculator (US Navy method)
Use a soft tape, snug but not compressing the skin. Measure twice and use the average.
Enter your height, weight and tape measurements to estimate your body fat.
Runs entirely in your browser. Nothing you type is saved or sent anywhere.
The equations
With every measurement in inches, the US Navy equation for men is: body fat % = 86.010 × log10(abdomen − neck) − 70.041 × log10(height) + 36.76 (Hodgdon & Beckett, 1984 (men)).
For women it is: body fat % = 163.205 × log10(waist + hips − neck) − 97.684 × log10(height) − 78.387 (Hodgdon & Beckett, 1984 (women)). The calculator converts centimetres to inches first.
Worked examples: a man of 70 in with a 16 in neck and 34 in waist gives 15.5%. A woman of 65 in with a 13 in neck, 30 in waist and 38 in hips gives 28.6%.
| Waist at navel | Estimated body fat |
|---|---|
| 30 in (76 cm) | 6.1% |
| 32 in (81 cm) | 11.1% |
| 34 in (86 cm) | 15.5% |
| 36 in (91 cm) | 19.4% |
| 38 in (97 cm) | 23.0% |
| 40 in (102 cm) | 26.2% |
| 42 in (107 cm) | 29.2% |
Using the number
There is no single official healthy body fat range. For a health check, your waist measurement already does a lot of the work: NICE advises keeping your waist under half your height (NICE NG246, 2025). Check it with the waist-to-height ratio calculator.
Your body fat result also feeds the lean body mass calculator and the FFMI calculator.
Pair the tape with what you eat
Reps logs meals by photo, voice or text and tracks the workouts your Apple Watch records, so you can see what is behind a change in your measurements.
Free to download · iPhone, iPad and Apple Watch
Common questions
How accurate is the Navy body fat method?
It is a regression equation fitted to US Navy personnel whose body fat was measured by underwater weighing, so it gives a reasonable estimate for most adults but can be several points off for an individual. Measuring technique matters most: the same tape, the same spots and the same time of day keep your trend meaningful.
Where do I measure my neck, waist and hips?
Neck just below the Adam's apple, with the tape sloping slightly down to the front. Men measure the waist around the navel; women measure at the narrowest point of the waist and the hips at the widest point of the buttocks. Stand relaxed and measure after a normal breath out.
Why does the women's formula use hips?
Women tend to store more fat on the hips and thighs, so Hodgdon and Beckett's equation for women adds a hip measurement to the waist and subtracts the neck. The men's equation uses abdomen minus neck.
Sources
Figures on this page were checked against these publications on 27 October 2026.
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men from body circumferences and height. Naval Health Research Center, San Diego, CA; 1984. Defense Technical Information Center report ADA143890. doi.org/10.21236/ADA143890
- Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy women from body circumferences and height. Naval Health Research Center, San Diego, CA; 1984. Defense Technical Information Center report ADA146456. doi.org/10.21236/ADA146456
- National Institute for Health and Care Excellence. Overweight and obesity management (NG246). Section 1.9: Identifying and assessing overweight, obesity and central adiposity. 2025. www.nice.org.uk/guidance/ng246/chapter/Identifying-and-assessing-overweight-obesity-and-central-adiposity
More body tools
- BMI calculatorBody mass index with standard and lower thresholds, and when not to trust it.
- Waist-to-height ratioCheck central fat with the 'keep your waist under half your height' rule.
- Lean body mass calculatorLean mass from height and weight with three equations, or from your body fat.
- FFMI calculatorFat-free mass index and the height-adjusted value, with the 25 limit in context.