Body fat calculator
Estimate body fat with the US Navy circumference method (Hodgdon & Beckett, 1984, used by OPNAVINST 6110.1) from height, neck and waist — plus hip for women — cross-checked against the Deurenberg (1991) BMI equation, with fat mass, lean mass and ACE categories.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
Body measurements
Circumference methods carry roughly ±3% error; where you place the tape at the waist matters most.
Result
Fill in height, neck and waist, then press “Calculate body fat”.
What this tool does
- Estimate body fat with nothing but a tape measure, so you can tell whether a diet is dropping fat or muscle instead of just watching the scale.
- Track a cut or a bulk every few weeks using body fat and lean mass trends, which are far less noisy than day-to-day weight.
- Cross-check the circumference result against the Deurenberg BMI equation: a big gap usually means the tape was placed differently.
- Get lean mass before using a Katch-McArdle BMR or setting protein targets, both of which are driven by fat-free mass.
Example
Input
Male, height 180 cm, neck 40 cm, waist 90 cm, weight 80 kg, age 30
Output
Body fat 18.4%, ACE category “Average”, fat mass 14.7 kg, lean mass 65.3 kg, BMI 24.7, BMI cross-check 20.3%
This is the most widely quoted US Navy example (180 cm / neck 40 / waist 90); public calculators land in the same 18% neighbourhood.
Frequently asked questions
Why does this disagree with my smart scale?
Smart scales use bioelectrical impedance, which shifts with hydration, food and body temperature — several points of drift in a single day. The tape measure depends only on your measurements, so it repeats better, but it still carries roughly ±3% systematic error. Both are trend tools, not absolute truth.
Where exactly do I measure the waist?
Men measure at the navel, women at the narrowest point, tape flat against the skin but not compressing it, reading at the end of a normal exhale. The waist dominates the formula: 2 cm of difference moves the result by about one percentage point, so keep the spot identical every time.
Where do these formulas come from?
The circumference method is from Hodgdon & Beckett (1984) at the Naval Health Research Center and is the one used by OPNAVINST 6110.1, the US Navy fitness standard. The BMI cross-check is the Deurenberg et al. (1991) regression published in the British Journal of Nutrition.
Is lower body fat always better?
No. Essential fat is roughly 2–5% for men and 10–13% for women; going below that harms hormone production and temperature regulation. The “essential fat” band is a survival floor, not a target.
Is my data uploaded anywhere?
No. Everything runs in the browser, the page makes no network requests, and a refresh clears your inputs.
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