This calculator estimates your body fat percentage from a few tape measurements using the legacy U.S. Navy circumference equation, then reports a fitness category and derived fat and lean mass. Body fat percentage attempts to separate fat mass from lean mass, unlike weight or BMI alone. It is still an estimate rather than a clinical measurement.
Fitness reference categories
| 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%+ |
These ACE-associated fitness categories provide broad context. They are not clinical diagnoses, universal targets, or current Navy pass/fail standards.
How to take circumference measurements
Use a non-stretch tape on bare skin when practical. Stand relaxed, keep the tape level, and apply consistent tension without compressing the skin. The older Navy procedure called for repeated measurements, directional half-inch rounding, and specific anatomical sites. This calculator accepts decimals and evaluates the equation directly, so it does not reproduce that official table procedure.
- Neck: measure below the larynx with the tape sloping slightly downward at the front.
- Men's abdomen: measure horizontally at the navel with the abdomen relaxed.
- Women's waist: measure at the natural waist, usually the narrowest point.
- Women's hip: measure around the largest horizontal circumference of the buttocks.
- Height: stand upright without shoes and use the same unit system as the circumferences.
Record measurements under similar conditions if you are comparing results over time. A single change smaller than the likely tape-placement variation may not represent a real change in body composition.
What this estimate can and cannot tell you
The equation predicts body-fat percentage from the relationship between height and selected circumferences. It does not directly observe fat tissue, account for every body shape, or identify where fat is stored. The calculated fat mass and lean mass are derived from the same estimated percentage, so they do not become independent measurements.
Compare the result with the BMI Calculator for a height-and-weight screen and the Lean Body Mass Calculator for several other predictive formulas. Agreement between estimates does not turn them into a diagnosis; disagreement is a reminder that each method has different inputs and assumptions.
Examples
Legacy equation: man, 5 ft 10 in, 165 lb, neck 18 in, waist 34 in
log10(34 − 18) = 1.204 and log10(70) = 1.845, so %fat = 86.010 × 1.204 − 70.041 × 1.845 + 36.76 ≈ 11.1% — within the "Athletes" range. Fat mass is about 18.3 lb and lean mass about 146.7 lb.
Woman, 165 cm, 60 kg, neck 32 cm, waist 70 cm, hip 95 cm
Using the metric women's formula, %fat ≈ 24.9% — the "Fitness" range — with roughly 14.9 kg of fat mass.
Formula
The legacy U.S. Navy equation uses log-transformed circumference measurements. The imperial (inches) formulas are:
Men: %fat = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
Women: %fat = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
Fat mass is %fat × weight, and lean body mass is weight − fat mass. The
metric (cm) variants use a separately published coefficient form, so switching
between equivalent rounded measurements can change the result slightly.
Sources
- MyNavy HR: current December 2025 Body Composition Assessment guide
- MyNavy HR: January 2024 guide documenting the older neck/waist/hip procedure
- Deurenberg, Weststrate, and Seidell: BMI age and sex prediction formula
- American Council on Exercise: context for commonly quoted fitness categories
- CDC: BMI Frequently Asked Questions