Health

Body Fat Calculator

Estimate body fat percentage from neck, waist, height, and (for women) hip circumferences using the US Navy formula — a field method that often beats BMI for muscular people.

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Result

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The formula

The US Navy method uses logarithms of circumference differences and height to estimate body fat percentage. Separate equations apply for men and women. Results are estimates with typical errors of a few percentage points versus laboratory methods.

Men: BF% = 86.010×log₁₀(waist−neck) − 70.041×log₁₀(height) + 36.76 (cm)

Worked example

  1. Male example: waist 85 cm, neck 38 cm, height 178 cm
  2. Compute log₁₀(waist − neck) and log₁₀(height), then apply the male coefficients
  3. Result ≈ 18% body fat

Result: ≈ 18% body fat (estimate)

How the Navy estimate works

Circumference equations were built to assess readiness without calipers or scanners. Measurement technique drives accuracy.

Inputs and units

Measure height and circumferences consistently (usually cm here; imperial inputs convert). Waist is typically at the navel level for this method; neck below the larynx. Women also measure hips at the widest point. Tape should be snug, not compressing tissue.

Why logarithms?

The published Navy equations are empirical fits relating girths to densitometry-based body fat. They are not a direct geometric volume of fat tissue.

Accuracy limits

Expect roughly ±3–4 percentage points versus DEXA for many adults, with larger errors at very low or very high body fat, and when measurements are inconsistent. Edema, pregnancy, and some medical conditions invalidate casual use.

BMI vs body fat

BMI uses only height and weight. Two people with the same BMI can differ substantially in fat percentage — which is why circumference methods exist.

Interesting facts

Built for field use

The U.S. Navy circumference formula estimates body fat from neck, waist, and (for women) hip measurements without calipers or scanners.

BMI is not body fat

Identical BMIs can hide very different fat-to-lean ratios. Fat mass and lean mass tell a clearer story than weight alone.

Essential fat

Some body fat is required for hormones and organ protection. Extremely low levels can harm health even if the number looks “athletic.”

Visceral vs subcutaneous

Fat around organs (visceral) carries different metabolic risk than fat under the skin — one reason waist appears in many screening tools.

Lab methods still win

DEXA, ADP, and hydrostatic weighing are more precise; circumference and BIA remain practical estimates.

Frequently asked questions

Often within about 3–4 percentage points of DEXA for many people, but less reliable at extreme leanness or obesity and when tape measurements vary.

For this method, measure at navel level, standing relaxed, tape horizontal and snug. Do not suck in your stomach.

The female Navy equation includes hip circumference to better capture sex-typical fat distribution patterns in the empirical fit.

No. It is a screening estimate. Discuss body composition goals with a qualified clinician or dietitian when health decisions depend on it.

References

  1. Navy Physical Readiness Program — body composition — U.S. Navy (MyNavy HR / PRP materials) Official program context for circumference-based standards.
  2. Assessing your weight and health risk — U.S. National Heart, Lung, and Blood Institute (NIH) Waist and BMI as complementary risk screens.
  3. Adult BMI categories — U.S. Centers for Disease Control and Prevention (CDC) Contrast with weight-only screening via BMI.