This calculator estimates body fat percentage using the US Navy circumference method, developed by Hodgdon and Beckett at the Naval Health Research Center in 1984. It requires only a tape measure: height and the circumferences of the waist and neck (men) or waist, hip, and neck (women). Results are estimates with a standard error of approximately ±3–4% compared to hydrostatic weighing.
The formula
Men:
%BF = 495 / (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450
Women:
%BF = 495 / (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450
All measurements in centimeters. Convert inches to cm by multiplying by 2.54 (the formula handles this automatically when you select inches).
Practical examples
Example 1 — Male: Height 178 cm, waist 84 cm, neck 38 cm. waist − neck = 46 cm. %BF = 495 / (1.0324 − 0.19077 × log₁₀(46) + 0.15456 × log₁₀(178)) − 450 = 495 / (1.0324 − 0.19077 × 1.6628 + 0.15456 × 2.2504) − 450 = 495 / (1.0324 − 0.3171 + 0.3479) − 450 = 495 / 1.0632 − 450 ≈ 15.6%
Example 2 — Female: Height 165 cm, waist 71 cm, hip 96 cm, neck 33 cm. waist + hip − neck = 134 cm. %BF = 495 / (1.29579 − 0.35004 × log₁₀(134) + 0.22100 × log₁₀(165)) − 450 ≈ 24.2%
Example 3 — Imperial inputs: Height 70 in, waist 34 in, neck 15 in. The calculator converts to cm before applying the formula: 177.8 cm, 86.4 cm, 38.1 cm → waist − neck = 48.3 cm → ~18% body fat.
Common mistakes
Measuring at the wrong site. Men measure waist at the navel. Women measure at the narrowest point of the torso. Measuring at the wrong location introduces a consistent error in every result.
Compressing the tape. Pull the tape snug but do not indent the skin. Compressing reduces the circumference reading and underestimates the measurement — producing an unrealistically low body fat estimate.
Flexing the neck. Measure with the neck in a neutral, relaxed position, tape perpendicular to the spine. Flexing makes the neck appear larger, which lowers the estimated body fat.
Expecting precision. The standard error of ±3–4% means a reading of 18% could reflect anywhere from 14% to 22% actual body fat. Use this tool for trend tracking over time, not for a single-point diagnosis.
Comparison of body fat assessment methods
| Method | Equipment | Typical error (vs. reference) | Practical notes |
|---|---|---|---|
| US Navy circumference | Tape measure | ±3–4% | Self-measured, no cost, reproducible |
| Skinfold (3–7 site) | Calipers + trained measurer | ±3–4% | Technique-dependent; site selection matters |
| Bioelectrical impedance (BIA) | BIA scale or handheld device | ±3–5% | Sensitive to hydration level |
| DEXA scan | Clinical X-ray machine | ±1–2% | High accuracy; requires clinical setting |
| Hydrostatic weighing | Underwater weighing tank | Reference method | Gold standard; requires specialized facility |
General reference ranges
These are population-level reference ranges from the American Council on Exercise (ACE) — not diagnostic thresholds:
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletic | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Acceptable | 18–24% | 25–31% |
| Obese | ≥25% | ≥32% |
For a full reference covering ACSM body fat ranges by sex, measurement method accuracy comparison, and the Navy formula equations, see the Body Fat Percentage Reference Chart →.