MultiConvers

Body Fat Calculator

Estimate percent body fat from a tape, not a scale: the U.S. Navy / Hodgdon & Beckett circumference equations. Height, neck and waist (plus hip for the female equation) go in as centimetres or inches — never feet for the tape sites.

Updated

How to use

  1. Pick metric or US units and the sex whose equation you want. Hip appears only for the female equation.
  2. Measure height, neck, and waist at the navel / abdomen. Add hip when that field is shown. Use the same unit for every tape length.
  3. Read estimated body fat to one decimal. A 70 in, 16 in neck, 34 in waist male example is about 15.4%.
  4. If the result is rejected, waist (or waist + hip) is not larger than neck — the log argument would not be positive. This is not a DEXA scan.

How it works

A scale knows mass. A tape knows circumferences. The Navy method turns those lengths into a percent-fat estimate with base-10 logs. All lengths are converted to inches first, then the male equation is 86.010 log₁₀(waist − neck) − 70.041 log₁₀(height) + 36.76. The female equation adds hip: 163.205 log₁₀(waist + hip − neck) − 97.684 log₁₀(height) − 78.387. Those coefficients are Hodgdon & Beckett’s published fit, not a clinic’s proprietary model.

Site of measurement matters more than the third decimal. The male waist is the abdomen at the navel, not the narrowest point above the hip bones. Neck is below the larynx. Hip, when required, is the largest protrusion of the buttocks. A tape pulled at an angle or over a thick shirt changes the log argument. The equation cannot fix a bad path.

The logs require a positive argument. Waist must exceed neck for men; waist plus hip must exceed neck for women. If they do not, the page rejects the inputs instead of printing a nonsense percent. That is arithmetic, not a judgement that the measurements “look wrong.”

Error versus DEXA, Bod Pod or hydrostatic weighing is well known and often several percentage points. The tape exists because it is cheap and repeatable enough for a field screen. It is the same family of equations the Army tape page uses; that page only changes the regulatory label (AR 600-9 / DoD) and still does not issue a pass or fail.

BMI is height and weight only. Lean body mass on this site is Boer, another height-and-weight estimate. Neither replaces a tape or imaging. Not medical advice.

Formula

Male: 86.010 log₁₀(waist − neck) − 70.041 log₁₀(height) + 36.76
Female: 163.205 log₁₀(waist + hip − neck) − 97.684 log₁₀(height) − 78.387
(all lengths in inches)

Worked examples

  • Male tape example

    70 in height, 16 in neck, 34 in waist → about 15.4%

Useful notes

  • Same family of equations as the Army tape test; that page labels the AR 600-9 context.
  • Waist (or waist + hip) must exceed neck or the result is rejected.
  • Not medical advice and not a DEXA scan.

FAQ

Where do I measure waist?

The Navy male equation uses the abdomen at the navel. Follow a consistent tape path; the equation cannot correct a bad measurement.

Is this as accurate as DEXA?

No. It is a circumference estimate with known error, used because a tape is cheap.

Why was my result rejected?

The log arguments must be positive. Waist has to be larger than neck (and hip is required for the female equation).

Do I enter neck in feet and inches?

No. Neck, waist and hip are centimetres or inches only — not feet. Height is the field that can use feet and inches.

Is this the same as BMI?

No. BMI uses only height and weight. This page uses circumferences and logs. They can disagree.

Guides

Related tools

See all Fitness tools.

Sources