Body Fat Percentage Calculator
Estimate body fat with the US Navy tape method from height, neck, waist, and hip.
What it does
The US Navy method estimates body fat from a few tape measures instead of a scan. It compares waist (plus hips for women) against neck and height, on the idea that the gap between trunk girth and neck tracks fat stored around the middle.
The tool above runs that formula in inches under the hood and converts your centimeters automatically when you pick metric. Enter each measure twice and average the rounds; half an inch of tape slack moves the answer by a full point or more.
Expect an estimate, not a lab value. Against DEXA scans, the Navy method usually lands within 3 to 4 points for typical builds and drifts more for very lean or very heavy people. It still works well for tracking change, since the same tape error repeats each time.
How to use
- Pick units and sex. Women need the hip measure, men do not.
- Measure height standing tall. Waist goes around the navel with the tape level.
- Measure the neck just below the Adam's apple, with shoulders relaxed and eyes forward.
- Measure twice, average each site, then enter the averages. Read the percentage and the band.
Formula
- waist - neck
- Trunk girth minus neck, the core fat signal for men.
- waist + hip - neck
- Extended signal for women, adding lower body fat.
- log10
- Base ten logarithm. It compresses girth into body fat scale.
- height
- Stature in inches. Taller frames carry girth differently.
All three inputs must use inches inside the formula, so metric values are divided by 2.54 first. The logarithms compress the scale: doubling a girth gap adds far less than double the fat. Waist minus neck must stay positive, or the tape values are off.
Worked example
Example: man, 70 in tall, 34 in waist, 15 in neck
Waist minus neck is 19 inches. Height is 70 inches.
- First term: log10(19) = 1.2788, times 86.010 = 110.0.
- Second term: log10(70) = 1.8451, times 70.041 = 129.2.
- Combine: 110.0 - 129.2 + 36.76 = 17.5 after rounding.
- Result: about 17.5% body fat, in the fit band for men.
Reference table
| Band | Men | Women |
|---|---|---|
| Essential fat | 2 to 5% | 10 to 13% |
| Athletic | 6 to 13% | 14 to 20% |
| Fit | 14 to 17% | 21 to 24% |
| Average | 18 to 24% | 25 to 31% |
| High | 25% and above | 32% and above |
Limits
- Field estimate, not a lab value. Tracks change better than absolutes.
- Drifts for very lean and very heavy builds.
- Posture, meals, and time of day move tape by whole points.
- Cannot place fat. Belly versus hips need a waist ratio too.
Common mistakes
- Pulling the waist tape tight like a belt, which understates fat by two points or more.
- Measuring the waist at the narrowest point instead of around the navel.
- Letting the neck tape ride up over the jaw, which shrinks the waist minus neck gap.
- Measuring once, after a meal, and treating that single number as final.
- Comparing a morning measure to an evening measure and reading water as progress.
Try it yourself
Quick checks. Answers sit tucked under each question.
Man, 72 in tall, 36 in waist, 16 in neck?
log10(20) x 86.010 = 111.9, log10(72) x 70.041 = 130.1. Then 111.9 - 130.1 + 36.76 = 18.6%, average band.
Woman, 65 in, waist 30, hip 40, neck 13?
log10(57) x 163.205 = 286.6, log10(65) x 97.684 = 177.1. Then 286.6 - 177.1 - 78.387 = 31.1%, top of the average band.
Questions
How accurate is the Navy method?
Within about 3 to 4 points of DEXA for typical adults in most comparisons. It is a field estimate, so use it for tracking change rather than as a medical grade value.
Why do women add the hip?
Hip girth carries part of the signal for lower body fat that the waist alone misses. Leaving it out understates the result for women.
Can I use centimeters?
Yes. The tool divides each centimeter value by 2.54 to reach inches before applying the formula, which keeps the constants valid.
What if waist minus neck is zero or negative?
That means a tape error: the waist must exceed the neck in this method. Re-measure both sites with the tape flat and level.
How often should I retest?
Every two to four weeks, in the morning, averaged over two rounds. Weekly testing mostly shows water and posture noise.