Body Fat Calculator (Navy Method)

How this is calculated

This is the Hodgdon-Beckett equation, developed at the Naval Health Research Center in 1984 as a practical field method requiring only a tape measure. THE FORMULAS, in INCHES: Men: 86.010 x log10(waist - neck) - 70.041 x log10(height) + 36.76 Women: 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387 ⚠️ THOSE CONSTANTS ARE FOR INCHES, AND A GREAT MANY SOURCES SAY OTHERWISE. There is a separate metric form of the same method with completely different constants - the 495-over-something version. Feeding centimetres into the inch constants does not fail; it quietly returns a plausible-looking wrong answer. A man measuring 90 cm waist, 38 cm neck and 178 cm tall is 20.1 percent by the correct metric formula and reads 26.7 percent if those centimetres are pushed through the inch equation. Six and a half percentage points, and nothing looks broken. So: measure in inches, or convert first. If you have seen this formula quoted with centimetres, that source is wrong. WHY IT USES LOGARITHMS: body density does not vary linearly with circumference, and the log terms fit the observed relationship far better than a straight line would. That is also why you cannot sanity-check it in your head. HOW TO MEASURE, because this is where the error actually comes from: NECK - just below the larynx, tape sloping slightly downward at the front, and do not flex. WAIST - men measure at the navel; women measure at the narrowest point, which is usually above the navel. These are different sites and swapping them changes the answer materially. HIP (women) - the widest point, feet together. Measure at the end of a normal exhale, with the tape snug but not compressing the skin. Take each measurement three times and use the average, because a half-inch difference on the waist moves the result by roughly 0.9 to 1.1 percentage points on the men's formula and 0.6 to 0.7 on the women's. ⚠️ IT IS AN ESTIMATE AND IT HAS A KNOWN WEAKNESS. Circumference methods infer body composition from shape, so two people with identical measurements and very different builds get the same answer. It tends to be least accurate at the extremes - very lean and very heavy - and it cannot distinguish muscle from fat at the waist. Compared against a laboratory method it typically lands within a few percentage points, which is useful and is not precision. WHAT IT IS ACTUALLY GOOD FOR: tracking a direction over time, on the same person, measured the same way. The absolute number is the least reliable part; the trend across months is the reliable one. ⚠️ AND IT IS NOT A HEALTH ASSESSMENT. A body fat percentage is one crude number about one aspect of a body, and it says nothing about blood pressure, blood sugar, cholesterol, fitness, strength or how you actually feel. It cannot tell you whether you are healthy and it cannot tell you what to change. If a figure here worries you, or if you are considering a serious change to how you eat or train, that is a conversation to have with a clinician who can measure the things that actually matter - not a decision to make from a tape measure and a formula.

Common questions

Inches or centimetres?

INCHES, for these constants, and this catches a remarkable number of calculators. There is a separate metric form of the same method with entirely different constants. The failure mode is nasty because nothing breaks: feed centimetres into the inch equation and you get a plausible-looking wrong answer. A man at 90 cm waist, 38 cm neck and 178 cm tall is 20.1 percent by the correct metric formula, and reads 26.7 percent if those centimetres go through the inch one. If you have seen this formula published with centimetres and these constants, that source is wrong.

Where exactly do I measure?

Neck just below the larynx, with the tape sloping slightly downward at the front, and do not flex. Waist at the NAVEL for men and at the NARROWEST POINT for women, which is usually above the navel - those are genuinely different sites and swapping them changes the answer materially. Hip at the widest point with feet together. Measure at the end of a normal exhale, tape snug but not compressing the skin, and take each one three times and average.

How accurate is it?

Useful, and not precise. Compared against laboratory methods it typically lands within a few percentage points, which is genuinely good for something requiring only a tape measure. Its known weakness is that it infers composition from shape, so two people with identical measurements and very different builds get the same answer, and it cannot distinguish muscle from fat at the waist. It tends to be least accurate at the extremes - very lean and very heavy.

Why does it use logarithms?

Because body density does not vary linearly with circumference, and logarithmic terms fit the observed relationship far better than a straight line. That is not a complication for its own sake - it is what makes a two-measurement estimate as good as it is. The side effect is that you cannot sanity-check the result in your head, which is precisely why the tool shows the single combined circumference it reduces your measurements to.

What is the half-inch sensitivity output for?

To show you how much your measuring technique matters relative to the result. A half-inch difference on the waist moves the answer by about 0.9 to 1.1 percentage points on the men's formula and about 0.6 to 0.7 on the women's - which is the same order as the change you might be trying to detect over a month. That is the argument for measuring three times and averaging, always at the same time of day, and for treating a single reading as noisier than it looks.

What is this actually good for?

Tracking a direction over time, on the same person, measured the same way. The absolute number is the least reliable part of the output and the trend across months is the most reliable. If it says 22 percent it might really be 19 or 25 - but if it says 22 in March and 19 in June, measured identically both times, something real has changed. Use it as an instrument for change rather than a verdict on a number.

Should I make decisions based on this number?

Not on its own. A body fat percentage is one crude estimate of one aspect of a body, and it tells you nothing about blood pressure, blood sugar, cholesterol, cardiovascular fitness, strength or how you feel day to day - all of which matter more to your health than this figure does. It is genuinely useful for tracking a direction over months when you measure the same way each time, and genuinely poor as a verdict on anything. If the number concerns you, or you are planning a significant change to how you eat or train, take it to a clinician who can measure what a tape measure cannot.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Body Fat Calculator (Navy Method) at https://www.bfcbrilliance.com/tools/body-fat-calculator.

What I entered:
- Formula: ___
- Height (in): ___
- Waist (in): ___
- Neck (in): ___
- Hip (women only) (in): ___
- Body weight (lb): ___

What it calculated:
- Body fat: ___
- Fat mass: ___
- Lean mass: ___
- The circumference the formula uses: ___
- Effect of a half-inch waist error: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Keep this general and do not give medical advice — where it matters, tell me what to raise with a clinician.

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