Navy Body Fat Calculator

men: density = 1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height) · women: density = 1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height) · body fat % = 495 ÷ density − 450

neck

Neck circumference, in centimetres, measured just below the larynx (cm)

waist

Waist circumference, in centimetres. Men measure at the navel, women at the narrowest point (cm)

hip

Hip circumference, in centimetres — women only; it is not part of the men's equation (cm)

height

Standing height, in centimetres, with no shoes (cm)

Use it when you have a tape measure and want a body fat percentage that does not depend on a tester's skill with calipers, or when you want a number you can take again in a month and compare. It is the right choice for tracking over a long period, because the same three measurements taken the same way are repeatable. Reach for the caliper page when you have calipers and a tester, and for a DEXA scan when you need the number itself rather than a direction — the two tape methods disagree with each other by several percentage points, and neither is the reference standard.

1. Man, 180 cm, 80 kg, neck 38 cm, waist 90 cm

   1. The men's equation uses waist minus neck: 90 − 38 = 52 cm
   2. Density: 1.0324 − 0.19077 × log10(52) + 0.15456 × log10(180) = 1.0536
   3. Body fat: 495 ÷ 1.0536 − 450 = 19.8 per cent
   4. Fat mass: 80 × 0.198 = 15.8 kg
   5. Lean mass: 80 − 15.8 = 64.2 kg

   The hip measurement in the inputs above changes nothing at all for a man — the men's equation never reads it. It is filled in here only because the field is on the page for everyone.

2. Woman, 165 cm, 60 kg, neck 32 cm, waist 72 cm, hip 98 cm

   1. The women's equation uses waist plus hip minus neck: 72 + 98 − 32 = 138 cm
   2. Density: 1.29579 − 0.35004 × log10(138) + 0.22100 × log10(165) = 1.0368
   3. Body fat: 495 ÷ 1.0368 − 450 = 27.4 per cent
   4. Fat mass: 60 × 0.274 = 16.5 kg
   5. Lean mass: 60 − 16.5 = 43.5 kg

   Drop the hips out of that sum and the women's equation stops working altogether: the log term collapses and the percentage comes back as a large negative number. For women the hips field is not optional, and for men it is not read at all.

Every one of these numbers is a tape reading, and a tape reading depends on where you put it. The waist is the hard one: the men's equation is fitted to an abdominal measurement at the navel, the women's to the narrowest natural waist, and those two sites are several centimetres apart on the same body. Measure the wrong one and the percentage shifts by points, not tenths. The equations themselves carry a standard error of roughly three to four percentage points against underwater weighing, which is wider than most of the differences anyone is trying to detect, and they were fitted to military samples that were mostly white and mostly young — several later reviews find the method overestimates body fat in women and in people the original sample did not represent. This is a field estimate, not a diagnosis, and it does not apply to children, adolescents, or pregnancy, where the equations were never validated.

How accurate is the US Navy body fat method?

Against underwater weighing the equations carry a standard error of roughly three to four percentage points, which is the same order as calipers and considerably worse than a DEXA scan. That sounds poor, and it is — but the number is not the point. What the circumference method does better than almost anything else is repeat: three measurements taken the same way each month give a trend that is more reliable than any single reading, and the trend is what tells you whether a plan is working.

Why does the neck measurement go into a body fat formula?

Because the equations were fitted, not derived. Hodgdon and Beckett measured twelve circumferences and eight skinfolds on hundreds of sailors, compared them against body density from underwater weighing, and kept whichever combination predicted best. The neck survived because it is a place where fat does not accumulate much, so it works as a stand-in for frame size: the same waist on a thicker neck means less of that waist is fat.

Does the hip measurement matter for men?

No. The men's equation subtracts the neck from the waist and never reads the hips, so the field can be left as it is. The women's equation adds the hips in, and leaving them out of it moves the answer by several percentage points, so for women it is not optional. This page keeps one waist field for everybody, which is also why the measuring site differs by sex: men at the navel, women at the narrowest point of the waist.

Where exactly should I measure?

The neck just below the larynx, with the tape sloping slightly downward at the front. Men measure the waist at the navel, with the tape level and the abdomen relaxed — not sucked in. Women measure the narrowest point of the waist, which is usually above the navel, and the hips at the widest point of the buttocks. Keep the tape snug against the skin but not compressing it, measure at the end of a normal breath out rather than at the bottom of a deep exhale, and take each one twice.

Why is my result different from my smart scale?

Because they are measuring different things badly in different directions. A bioelectrical scale sends a current through your legs and infers from the resistance, which makes its reading move with hydration, recent exercise and the time of day; a tape measure does not care about any of those. Neither is a reference value. Pick one method, use only that one from then on, and compare each new reading against your own earlier ones rather than against the other device.

• Prediction of body fat content in military personnel from body circumferences and height — Hodgdon JA, Beckett MB — Medicine & Science in Sports & Exercise, 1984
• Nutrition and Athletic Performance — joint position statement — Academy of Nutrition and Dietetics, Dietitians of Canada and American College of Sports Medicine, 2016