How it works
This tool estimates body fat percentage from three or four circumference readings you take with a plain tape measure — no calipers, scales, or lab visits required. The underlying equations come from U.S. Navy anthropometric research, which correlated tape measurements against hydrostatic weighing across thousands of subjects. The result is a civilian-friendly screening tool that's popular among lifters, runners, and self-tracking hobbyists who want a repeatable number without expensive equipment.
The appeal is simplicity: if you can measure your neck, waist (and hips, for women), and height, you get a body fat estimate in seconds. It's not a clinical diagnostic — but for tracking whether your cut is working or your off-season bulk is getting away from you, it's one of the most accessible options going.
| Measurement | Men | Women |
|---|---|---|
| Neck | ✓ | ✓ |
| Waist (at navel) | ✓ | ✓ |
| Hip (widest point) | — | ✓ |
| Height | ✓ | ✓ |
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 circumference inputs use the same unit (inches for US, centimeters for metric) — just stay consistent across every field.
Worked example
A 30-year-old woman tracking her progress through a 12-week cut wants a quick read on her composition shift. She measures herself first thing in the morning, before eating, using a flexible tape on bare skin.
Her inputs in US units:
Waist: 30 in
Neck: 13 in
Hip: 40 in
Height: 65 in
First, the combined circumference term inside the logarithm:
Waist + Hip − Neck: 30 + 40 − 13 = 57 in
Now the two logarithms the equation needs:
log₁₀(57) ≈ 1.7559
log₁₀(65) ≈ 1.8129
Plug into the female equation:
Denominator: 1.29579 − 0.35004 × 1.7559 + 0.22100 × 1.8129
0.35004 × 1.7559 ≈ 0.6146
0.22100 × 1.8129 ≈ 0.4007
Denominator: 1.29579 − 0.6146 + 0.4007 ≈ 1.0819
495 / 1.0819 ≈ 457.56
Body fat: 457.56 − 450 ≈ 7.56%
That result — roughly 7.6% — sits below the typical healthy floor for women (around 10–13% essential fat), which would warrant a closer look at measurement technique or a second method like DEXA.
Let's re-run with corrected measurements: waist 32, neck 13, hip 42, height 65.
Waist + Hip − Neck: 32 + 42 − 13 = 61 in
log₁₀(61) ≈ 1.7853
log₁₀(65) ≈ 1.8129
0.35004 × 1.7853 ≈ 0.6249
0.22100 × 1.8129 ≈ 0.4007
Denominator: 1.29579 − 0.6249 + 0.4007 ≈ 1.0716
495 / 1.0716 ≈ 461.92
Body fat: 461.92 − 450 ≈ 11.9%
That lands in a much more plausible range for an actively training woman.
Common mistakes
Inconsistent tape placement is the single biggest source of error. Measure your waist at the navel every time — not at the narrowest point, which shifts as you lean out. The same goes for neck: just below the Adam's apple, tape horizontal, not tilted.
Pulling the tape too tight compresses tissue and artificially shrinks the reading. The tape should make contact without indenting skin. If you see flesh bulging above or below the tape, it's too tight.
Mixing units silently breaks the math. If you enter height in inches but waist in centimeters, the logarithm produces nonsense. Pick US or metric and use it for every field.
Measuring at random times introduces noise from meals, hydration, and posture. Morning readings after the restroom, before food, give the most stable trend line week to week.
This calculator gives an estimate, not professional advice — for clinical body composition assessment, consult a qualified practitioner or use DEXA/hydrostatic weighing.