Body fat calculator

Body fat percentage is the share of your total weight that is fat tissue. Everything else (muscle, bone, organs, water) counts as lean mass. This calculator gives you three estimates side by side, because each method fails in a different way and seeing them together tells you more than any single number.

The first tab uses the US Navy circumference method, which is the main one. The second uses the formula the US Army adopted in 2023. The third estimates body fat from your BMI, age and sex.

What it measures

None of the three tabs measures fat directly. Each takes a few body dimensions and plugs them into an equation that researchers fitted against a reference method, usually underwater weighing. The result is a prediction for people who resemble the original sample. That sample matters. The Navy equations were built on 602 male and 214 female Navy personnel, aged 18 to 56 for men and 18 to 44 for women.

How to measure with a tape

Use a flexible, non-stretch tape (fiberglass is what the military regulation specifies). Lay it against bare skin without squeezing. Keep it level all the way around, and measure at the end of a normal breath out. Take each measurement more than once and average them; the Army regulation text asks for three readings, and the Navy procedure document for two.

Neck: just below the larynx (Adam's apple), with the tape sloping slightly down toward the front. Look straight ahead, drop your shoulders, and keep the tape off the larynx itself.

Men's abdomen: across the navel, arms relaxed at your sides.

Women's waist: at the narrowest point of the abdomen, usually about halfway between the navel and the bottom of the breastbone. If you cannot see where that is, take a few readings and use the smallest.

Hips (women): around the widest part of the buttocks, viewed from the side, with the tape level front to back.

Height: barefoot against a wall, in inches for the US formulas.

One practical warning. The equations take the difference between two circumferences, so a half inch of error in a neck reading moves the answer more than you would expect. Measure twice on different days if the result matters to you.

The Navy formula in plain words

For men, take the abdomen minus the neck and look up the base-10 logarithm of that gap. Multiply it by 86.010. Then take the log of your height, multiply by 70.041, and subtract. Add 36.76. All lengths are in inches.

For women, add waist and hip, subtract neck, take the log, multiply by 163.205, subtract 97.684 times the log of height, then subtract 78.387.

Metric inputs are converted to inches first. These are the equations printed for Department of Defense use in a 1999 Naval Health Research Center report. They were re-fitted from the original 1984 equations, which predicted body density in centimetres.

Worked example

A man is 70 inches tall with a 15 inch neck and a 34 inch abdomen.

1. Abdomen minus neck: 34 - 15 = 19.
2. 86.010 x log10(19) = 86.010 x 1.2788 = 109.99.
3. 70.041 x log10(70) = 70.041 x 1.8451 = 129.23.
4. 109.99 - 129.23 + 36.76 = 17.5.

His Navy estimate is about 17.5 percent. A woman who is 65 inches tall with a 30 inch waist, 38 inch hips and a 13 inch neck gets 28.6 percent.

The Army tab

The Army formula needs weight and abdomen only. For men it is -26.97 minus 0.12 times weight in pounds plus 1.99 times abdomen in inches. For women it is -9.15 minus 0.015 times weight plus 1.27 times abdomen. A 210 lb man with a 35 inch abdomen comes out at 17.48 percent. A 165 lb woman with a 30 inch abdomen comes out at 26.475 percent. Those two figures match the official Army worked examples.

There is an important status note. The Army introduced this one-site test in Army Directive 2023-11, then dropped body-fat estimation altogether in July 2026. Army Directive 2026-13 made waist-to-height ratio the only standard, and a ratio below 0.55 is the pass mark. A 70 inch soldier passes with a waist under 38.5 inches. We keep the old formula because many people still search for it and it works as a second estimate, but it no longer decides anything for the Army. See Navy vs Army body fat methods for the full history.

The BMI tab

This tab uses an equation from Deurenberg and colleagues (1991): 1.20 times BMI, plus 0.23 times age, minus 10.8 if you are male, minus 5.4. It was fitted on 1,229 people in the Netherlands and had a standard error of 4.1 percentage points. The authors noted it slightly overestimated fat in obese subjects.

For a 30 year old man at 70 inches and 180 lb, BMI is 25.8 and the estimate is 21.7 percent. Compare that with the 17.5 percent Navy example above. A four point gap between methods is normal and sits inside the stated error of both.

How accurate is it

In the Navy's own comparison against a four-compartment reference, the male equation had a standard error of about 3.2 percentage points and the female equation about 3.1. In the same newer sample the equations read higher than the reference on average, by 0.8 points for men and 2.0 for women. Against underwater weighing in the original studies, the errors were 3.5 (men) and 3.7 (women).

So a Navy result of 18 percent is better read as roughly 14 to 22. DEXA scans and underwater weighing sit closer to the truth, although they have their own error sources, such as hydration and the machine used. They also cost money and need an appointment. The tape method wins on cost and repeatability: if you measure the same way each time, changes over weeks mean more than any single reading.

Who it does not suit

Tape-based estimates struggle with people who are very muscular, since a thick neck or a trained waist can distort the gap. They also struggle with people whose fat sits somewhere other than the abdomen, with anyone pregnant, with children and teenagers, and with people over about 60, who were outside the original samples. A distended abdomen from fluid or illness will inflate the number. The BMI tab is the weakest for athletes and older adults, because it cannot tell muscle from fat.

If you have a medical condition, an eating disorder history, or are asked to hit a number by a coach or employer, use a qualified professional's assessment instead of this page.