Body Fat Calculator (US Navy Method) Body composition
Measure your neck, waist, and (for women) hip with a tape, add your height, and this calculator returns your estimated body fat percentage, fat mass, and fat-free mass — the number that feeds the Katch–McArdle BMR equation.
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Body weight alone does not tell you much about composition. Two people at 80 kg can be structurally different — one carrying 25% body fat, the other 12% with substantially more muscle — and every downstream calorie number (BMR, TDEE, protein targets) depends on that difference. Body fat percentage is the single most useful composition number you can get at home with no equipment beyond a tape measure.
This page implements the US Navy circumference method, developed by Hodgdon and Beckett at the Naval Health Research Center in 1984 (Reports 84-11 for men and 84-29 for women). It estimates body fat from neck, waist, and height for men, and adds hip for women. The method was built against underwater weighing in young, fit Navy personnel, and it remains the standard tape-measure technique because it needs only a tape and two minutes. Its honest weakness: like every circumference equation, it carries an error of a few percentage points, because it infers internal fat from external shapes. Treat the result as a good estimate band, not a clinical measurement — and treat the tape placement discipline (described below) as the largest error source you control.
The output you should pay most attention to is fat-free mass (FFM). FFM is what drives resting energy expenditure, and it is the required input for the Katch–McArdle BMR equation on our BMR calculator — the equation that beats height-and-weight formulas for anyone who knows their body fat reasonably well. If you also want the weight-height context, the BMI calculator runs the complementary check.
The equations were published in inches and we keep the constants exactly as published. Your centimeter entries are divided by 2.54 before the formula runs; nothing else is rescaled. For men: BF% = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76. For women: BF% = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387.
Worked example (men): height 180 cm, waist 90 cm, neck 38 cm. Converted to inches: 70.87, 35.43, 14.96. BF% = 86.010 × log10(20.47) − 70.041 × log10(70.87) + 36.76 ≈ 19.9%. If that weight were 80 kg, fat mass is about 15.9 kg and fat-free mass about 64.1 kg.
From FFM to calories: the Katch–McArdle equation (commonly known as Katch–McArdle; the same form appears in Cunningham 1991, PMID 1957828) estimates BMR as 370 + 21.6 × FFM. With the FFM above that is about 1,752 kcal/day — noticeably different from what height-and-weight formulas give the same person, which is exactly the point of measuring composition. The BMR calculator runs both side by side.
Why the tape protocol matters more than the arithmetic: the equation is only as good as the difference (waist − neck) or (waist + hip − neck). A 2 cm systematic error at the waist moves the result by roughly 2–4 percentage points — more than the equation's own error. Measure in the morning, on bare skin, tape horizontal, and take each measurement twice.
Frequently asked questions
How accurate is the Navy body fat method?
It is a good estimate, not a measurement. In the original 1984 development reports, the standard error of estimate against underwater weighing was a few percentage points of body fat, and independent validations since have found similar or slightly larger error for lean and very lean individuals. Read your result as a band several points wide. For tracking progress, the trend across consistent measurements is far more informative than any single reading.
Why does the women's equation need a hip measurement?
Fat distribution differs by sex: men carry a larger share of fat centrally (which the waist captures), while women carry more at the hips and thighs. The Hodgdon–Beckett female equation adds the hip term so that gluteofemoral fat is not mistaken for lean mass. Using the male equation on female measurements — or vice versa — produces systematically wrong numbers, which is why the sex selection switches the entire equation set rather than tweaking one constant.
My result says waist minus neck is zero or negative. What happened?
The measurement protocol was not followed, almost always the waist: the tape must be horizontal at the navel, snug against the skin, at the end of a normal exhale — not loose over clothing, not angled up or down, not after a maximal exhale. Re-measure all three sites. If the numbers still disagree, the method is simply not fitting your body shape well; use the BMR page's Mifflin or Harris–Benedict mode instead, which need only height, weight, and age.
Is a body fat of X% healthy?
Essential fat is roughly 2–5% for men and 10–13% for women — below that is not sustainable. Population guideline ranges (for example Gallagher et al., Am J Clin Nutr 2000) put healthy ranges by age and sex well above those floors, with the healthy band rising with age. Athletes sit lower within those ranges; contest-level leanness is a temporary performance state, not a maintenance target. If you are concerned about a low reading in the context of disordered eating, that is a clinician conversation, not a calculator question.
Why does the page also ask for my weight?
Because percentage alone is abstract — 15% at 60 kg and 15% at 100 kg are very different bodies. Weight converts the percentage into fat mass and fat-free mass in kilograms, and FFM is the number the Katch–McArdle BMR equation runs on. Your inputs never leave the browser; the calculator runs entirely on your device.
References & sources
Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men from body circumferences and height. Naval Health Research Center Report 84-11, San Diego, 1984.
Supports: The male circumference equation used here (waist, neck, height, log10 form), developed against underwater weighing.
Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy women from body circumferences and height. Naval Health Research Center Report 84-29, San Diego, 1984.
Supports: The female circumference equation used here (waist, hip, neck, height).
Cunningham JJ. Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation. Am J Clin Nutr. 1991;54:963–969.
Supports: The 370 + 21.6 × FFM resting-energy equation (commonly known as Katch–McArdle) that consumes this page's FFM output.
Gallagher D, et al. Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. Am J Clin Nutr. 2000;72:694–701.
Supports: Age- and sex-specific healthy body fat percentage bands referenced in the FAQ.