Body fat calculator

The two equations

The US Department of Defense's 2002 body fat instruction used a tape and a height, with no scale and no skinfold caliper. The equations come from Hodgdon and Beckett's 1984 work for the Naval Health Research Center and are printed in that instruction (DoD Instruction 1308.3). They take measurements in inches and use base-10 logarithms:

Men:
% body fat = 86.010 x log10(abdomen - 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

The Department of Defense rounds the answer to a whole number, because decimals would suggest more precision than the method has. This page rounds to the nearest whole number too, and shows a range for the same reason.

How to measure

• Use a non-stretch tape (fiberglass is best), touching the skin but not squeezing. Keep it level with the floor.
• Neck: just below the larynx, tape level, looking straight ahead with shoulders down.
• Men: abdomen at the belly button. Women: natural waist at its narrowest point, and hips at the widest point over the buttocks.
• Measure at the end of a normal breath out, arms at your sides. Take each site three times in turn and average them.

The Department of Defense notes that differences between estimates "have frequently been traced to improper measurement techniques": handling the tape badly, picking the wrong site, or a miscalibrated height gauge.

The honest error range

• In the data the equations were built from, the standard error of estimate was 3.52 percentage points for men and 3.72 for women (Institute of Medicine, 1998). About two thirds of people land within one standard error, so 3.5 points is the usual miss, not the worst case.
• Against a DXA scan, in 609 US Marines, the circumference method read about 2.5 points too low for men and 1.3 to 2.3 points too high for women on average, with a spread (standard deviation) of 3.7 to 4.8 points (Potter and colleagues, 2022). The authors concluded it suits field screening, not research-grade measurement. Lean people tended to read high and high-fat people low.
• These were fit adult service members. The estimate is less certain for teenagers, older adults and very different builds.

The range this tool shows is the estimate plus and minus one standard error. It is a fair picture of the usual miss and a poor one of the worst.

A worked example

A man 178 cm tall (70.1 in) with a 38 cm neck (15.0 in) and an 85 cm waist (33.5 in). Waist minus neck is 18.5 in.

• 86.010 x log10(18.5) = 86.010 x 1.267 = 109.0
• 70.041 x log10(70.1) = 70.041 x 1.846 = 129.3
• 109.0 - 129.3 + 36.76 = 16.5%, shown as 16% with a likely range of about 13% to 20%.

What to do with it

Treat it as a rough marker, not a score. If you track it, measure the same way at the same time of day, and watch the trend and your waist number itself, which is a real measurement rather than a calculation. If you use it with the BMR calculator, remember its error carries through.

Sources

1. U.S. Department of Defense. DoD Instruction 1308.3: DoD Physical Fitness and Body Fat Programs Procedures, November 5, 2002, Enclosure 3 (archived copy of the original document). The equations come from Hodgdon JA and Beckett MB, Naval Health Research Center reports 84-11 (men) and 84-29 (women), 1984. archived PDF
2. Institute of Medicine. Assessing Readiness in Military Women: The Relationship of Body Composition, Nutrition, and Health. National Academies Press, 1998. Chapter 2, Table 2-1 (the Navy equations with their standard errors). chapter 2
3. Potter AW, Tharion WJ, Holden LD, Pazmino A, Looney DP, Friedl KE. Circumference-based predictions of body fat revisited: preliminary results from a US Marine Corps body composition survey. Front Physiol 2022;13:868627. doi:10.3389/fphys.2022.868627

Written November 5, 2026, with the sources read that day. An estimate, not medical advice.