WithusToolsWithusTools

Body fat (tape estimate)

U.S. Navy circumference method—not DXA or BIA

Body fat (tape estimate)

Not a lab value. This page uses the U.S. Navy circumference equations. Numbers from DXA, BIA, or underwater weighing often sit several points away—sometimes more—because they measure different things with different errors. Use this as a rough tape-based guess, not a diagnosis.

Neck: slimmest point, standing tall. Waist: around the abdomen at navel level (Navy protocol). Hip (women only): widest part of the hips. Height: barefoot standing height.

Sex

Equations (log base 10, inches inside the log)

Published Navy screening forms list coefficients for men and women. This page converts your centimetres or feet and inches into total height in inches, converts metric circumferences to inches, then evaluates the same expressions you would find in a PDF of the instructions.

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

Women: BF% = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387

log10 means base-10 logarithm, same as in the code.

Waist is abdomen at the navel; hip is the maximal buttock breadth; neck is minimal circumference; height is barefoot. If waist minus neck (men) or waist plus hip minus neck (women) is not clearly positive, the logarithm is undefined— the calculator stops with an error instead of inventing a number.

Informal chart bands (chip only)

The label colours borrow loose buckets similar to ACE reference material (for example athlete, fitness, average ranges). They are here so you have language to talk about a percentage band—they are not returned by the Navy formula and they are not clinical classes.

Why we lead with the limitations

  • BIA can swing with hydration; DEXA costs money and still has slice thickness assumptions; underwater weighing is rare. Tape is cheap but blunt—honesty about that trade-off matters more than pretending three circumferences equal a scan.
  • If you already have a reliable BIA or DEXA number you trust, treat this page as a second opinion for curiosity, not a tie-breaker.
  • For waist-to-hip patterning without percent fat, see the waist-to-hip ratio calculator. For weight versus height only, use BMI.

Quick guide

1. What do I measure first?

  1. Select male or female, then enter standing height, neck at the narrowest point, and waist at the navel (Navy-style horizontal tape).
  2. Women also enter hip girth at the widest part of the buttocks—the female Navy equation uses waist plus hip minus neck inside a logarithm.
  3. Pick metric or US units and wait a moment after typing; the estimate updates on a short delay like the other calculators here.

2. What math runs here?

The tool converts everything to inches, applies the published Navy logarithmic coefficients, then shows percent body fat to one decimal.

The coloured chip under the percentage maps loosely to ACE-style population bands; those bands are only a visual aid, not part of the Navy math.

3. Why might this disagree with a scan?

DXA splits fat, lean, and bone by X-ray attenuation. BIA predicts water compartments with electricity. A tape cannot see visceral fat directly, so disagreement of several percentage points is normal.

Age is not an input here because the classic Navy tape equations do not include an age term. Older adults and very muscular people are exactly where tape methods drift most.

4. Why use a browser-only tool?

  • No scale beyond a tape and no server upload.
  • Same worksheet layout as the rest of the calculator hub so you are not learning a new UI pattern.
  • Copy button pastes the headline percentage and the informal category label for quick notes.

5. Typical uses

  • Checking whether your hand math matches what a recruiter or trainer wrote on a form.
  • Seeing how sensitive the percentage is when you move the waist tape up or down an inch.
  • Teaching students why circumference models and scanner models rarely match line for line.