Body Fat Calculator

How the tape-measure method works

The most accurate ways to measure body fat — a DXA scan or underwater weighing — need a lab. In 1984, James Hodgdon and Marcie Beckett at the US Naval Health Research Center measured hundreds of Navy men and women both ways: with underwater weighing and with a tape measure. They then found the combination of tape measurements that best predicted the lab result. Equations from that work have been used in US military body composition programs for decades.

The waist (and, for women, the hips) is where much body fat is stored, so a bigger waist raises the estimate. The neck grows mainly with frame and muscle, so it is subtracted. Height scales the result to body size.

How to measure

• Use a flexible tape on bare skin. Keep it level and snug without pressing into the skin.
• Neck: just below the larynx (Adam’s apple), with the tape sloping slightly down toward the front.
• Waist, men: around the belly at the level of the navel. Waist, women: at the narrowest point, usually a little above the navel. Stand relaxed and measure after a normal breath out — don’t pull the stomach in.
• Hips (women): around the widest part of the buttocks, feet together.
• Measure each site two or three times and use the average.

The calculator asks for sex because separate equations were fitted for men and women, using different measurements. They were not developed for people whose body composition differs from both groups.

US Navy body fat formulas

With all measurements in inches:

In metric mode, the calculator divides each centimeter measurement by 2.54 and uses the same equations, so switching units never changes the answer. The original reports expressed the equations as body density from centimeters, turned into a percentage with Siri’s formula (495 ÷ density − 450). That route is not algebraically identical to the inch equations, but for typical measurements the two agree within about one percentage point.

From the percentage, the split of your weight follows directly:

If you enter your age, the calculator also shows the adult formula of Deurenberg and colleagues (1991), which estimates body fat from BMI alone:

S

1 for men, 0 for women

BMI

weight (kg) ÷ height (m)²

It comes from a study of 1,229 people whose body fat was measured by underwater weighing; this is the adult version. Its authors found it slightly overestimated body fat in people with obesity, and because it cannot see muscle, it is shown only as a cross-check.

Worked example

A 35-year-old man, 5 ft 10 in (70 in) tall and 180 lb, with a 15 in neck and a 34 in waist:

1. Waist − neck = 34 − 15 = 19 in
2. BF% = 86.010 × log₁₀(19) − 70.041 × log₁₀(70) + 36.76 = 86.010 × 1.27875 − 70.041 × 1.84510 + 36.76 = 109.99 − 129.23 + 36.76 = 17.5%
3. Fat mass = 180 lb × 17.5% = 31.5 lb; lean mass = 180 − 31.5 = 148.5 lb
4. ACE category for men: 14–17% is “fitness”, so 17.5% sits at the top of that band.
5. BMI-based comparison: BMI 25.8, so 1.20 × 25.8 + 0.23 × 35 − 10.8 − 5.4 = 22.8%. The gap shows how much a BMI-only estimate depends on assumptions about build.

For a woman 5 ft 5 in (65 in) tall with a 13 in neck, 30 in waist and 40 in hips: 163.205 × log₁₀(57) − 97.684 × log₁₀(65) − 78.387 = 286.57 − 177.09 − 78.39 = 31.1%, in the “average” band for women.

Body fat categories

The result is placed on the general body fat chart published by the American Council on Exercise (ACE). The chart is a fitness guide, not a medical classification, and it doesn’t change with age even though body fat usually rises as people get older.

Values that fall between two published bands (13.5% for a woman, for example) are placed in the lower band. Essential fat is the minimum the body needs for hormones, nerves and cell membranes; readings below it are labelled separately and are more likely to be a measurement error than a real value.

Limits of the estimate

• Individual error. In a 2022 study of 430 men and 179 women in the US Marine Corps, the tape equations read on average about 2.5 percentage points lower than DXA for men and 1.3 to 2.3 points higher for women, with individual differences of several points either way. Lean people tended to be overestimated and people with more body fat underestimated.
• Not for everyone. The equations were built on adults. They don’t apply to children and teenagers, during pregnancy, or when fluid retention or a medical condition changes body shape.
• Unusual builds. A thick, muscular neck lowers the estimate; a large waist from bloating or posture raises it.
• Technique. Small reading errors matter: for the man in the example, an extra half inch on the waist adds about one percentage point. Measure carefully and the same way each time.