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Body Fat Calculator

Enter your height, neck and waist (plus hips for women) in inches or centimeters to estimate body fat percentage with the US Navy method, plus fat mass and lean mass.

The US Navy tape method

The US Navy circumference method was developed by Hodgdon and Beckett in 1984 so that body composition could be checked with nothing but a tape measure. It works because fat is not distributed at random: for a given height, the circumference of the neck and waist predicts total body fat reasonably well in men, and adding the hip measurement does the same for women.

With every measurement in inches, the formulas are: men, body fat % = 86.010 x log10(waist - neck) - 70.041 x log10(height) + 36.76; women, body fat % = 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387. If you enter centimeters the calculator converts them first, which is why the constants above only apply to inches. Measure the neck just below the larynx, sloping slightly downward at the front; the waist at the navel for men and at the narrowest point for women; and the hips at the widest part of the buttocks. Keep the tape level, snug but not compressing the skin, and measure at the end of a normal exhale.

A worked example

Take a 30-year-old man who is 69 inches tall, weighs 175 lb, and measures 15 in at the neck and 34 in at the waist. Waist minus neck is 19 in, log10(19) = 1.2788 and log10(69) = 1.8388. So body fat = 86.010 x 1.2788 - 70.041 x 1.8388 + 36.76 = 109.99 - 128.79 + 36.76 = 18.0%, which sits right at the border between the fitness and average bands for men. Fat mass is 175 x 0.18 = 31.5 lb and lean body mass is 143.5 lb.

The same man's BMI is 703 x 175 / 692 = 25.8. The Deurenberg BMI equation gives 1.20 x 25.8 + 0.23 x 30 - 10.8 - 5.4 = 21.7%, about four points higher. That gap is typical: BMI-based estimates cannot see where the weight sits, so they overestimate fat in muscular people and underestimate it in people with little muscle. When the two disagree sharply, the tape result is usually the better guide.

Reading the number

The American Council on Exercise publishes the categories most US gyms use. For men: 2-5% essential fat, 6-13% athletes, 14-17% fitness, 18-24% average and 25% or more obese. For women, whose bodies require more essential fat: 10-13% essential, 14-20% athletes, 21-24% fitness, 25-31% average and 32% or more obese. Sustained levels near the essential-fat floor are unhealthy and, in women, often stop menstruation.

No tape-based estimate is exact. Validation studies put the Navy method within about 3-4 percentage points of hydrostatic weighing for most adults, with larger errors for very lean athletes, people with a lot of loose skin after major weight loss, and during pregnancy. Body scales that use bioelectrical impedance are similarly rough and swing with hydration. What the tape does well is track change: if your waist drops an inch while your weight holds steady, you are losing fat and keeping muscle, whatever the absolute percentage says.

Frequently asked questions

How accurate is the US Navy body fat formula?

It comes within about 3-4 percentage points of hydrostatic (underwater) weighing for most adults, which is good enough to track progress but not precise enough to argue over a single point. DEXA scans are the practical gold standard if you need an exact number.

Where exactly do I measure my waist?

Men measure at the navel, women at the narrowest part of the torso, usually just above the navel. Stand relaxed, do not suck in, keep the tape horizontal, and read it at the end of a normal breath out. Measuring twice and averaging cuts most of the error.

What is a healthy body fat percentage?

For men, roughly 14-24%; for women, roughly 21-31%. Below 6% for men or 14% for women is athlete territory and hard to hold safely, while 25% (men) or 32% (women) and above is classified as obese by the American Council on Exercise.

What is lean body mass?

Everything that is not fat: muscle, bone, organs, connective tissue and body water. It equals your weight minus your fat mass. At 175 lb and 18% body fat, fat mass is 31.5 lb and lean mass is 143.5 lb. Protein targets are often set per pound of lean mass.

Why does the BMI estimate differ from the tape result?

BMI only knows your height and weight, so it cannot tell muscle from fat. The Deurenberg equation converts BMI to a fat percentage using age and sex, which works for an average person but overestimates fat in lifters and underestimates it in sedentary people with a normal weight.

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