Weight tells you how much of you there is, not what it is made of. This estimates the proportion that is fat from three or four tape measurements, using the equations the U.S. Department of Defense uses for its own body composition standard.
Key terms
- Circumference method — body fat predicted from girths rather than from calipers, water displacement or a scan. It needs a tape measure and nothing else.
- The girth — the number the equation actually works on. For men it is waist minus neck; for women it is waist plus hips minus neck.
- Neck correction — subtracting the neck adjusts abdominal girth for overall body size, which is why a tall broad person and a short slight one with the same waist do not get the same answer.
How it works
DoD circumference equations
men: %BF = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
women: %BF = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
All measurements in inches, logarithms base 10. A metric entry is converted with the exact international inch, because inches are the unit the equations were fitted in.
The two equations are not the same shape, and it matters: the female one takes three girths rather than two. Running a woman’s measurements through the male form gives a number that looks reasonable and is wrong.
Where the error actually comes from
Almost none of it is in the arithmetic. A circumference estimate sits several percentage points away from a DEXA scan, and most of that gap is the tape: held at a slight angle, pulled tight enough to compress, or placed an inch away from where the equation expects. Measure after breathing out, without pulling in, tape level all the way round and snug without denting the skin. Take each measurement twice.
Because the error is largely consistent for one person measuring themselves the same way, the number is far more useful as a series than as a single reading. Two points a few months apart tell you something real; one point on its own tells you roughly where you are.
Why there is no category on this page
Labels like “athletic”, “fitness” or “acceptable” for a given body fat percentage come from trade bodies rather than from a source that could be read and cited, so this page gives the number and stops. That is the same rule the waist circumference calculator follows: a health threshold with no citation behind it does not go on a page carrying a reviewer’s name.
How to use this calculator
- Pick your sex — the equations differ, and women need the hip measurement as well.
- Measure the neck just below the larynx, with the tape sloping slightly down at the front.
- Measure the waist at the navel for men, at the narrowest point for women.
- Women: measure the hips at the widest point of the buttocks.
- Repeat in a few months under the same conditions and compare, rather than reading anything into one number.
Frequently asked questions
How accurate is this compared with a DEXA scan?
It is an estimate with several percentage points of spread, and it was developed on military populations rather than on everyone. It will not match a scan. It is good at telling you which direction you are moving in, provided you measure the same way each time.
Why does the women’s equation include hips?
Because body fat distributes differently, and hip girth carries information for women that the waist alone does not. It is not an optional extra: the coefficients were fitted with it in, so leaving it out breaks the equation rather than simplifying it.
My result seems too high, or impossible. What happened?
Usually the neck or waist measurement. If the neck comes out wider than the waist, the logarithm has nothing to work on and this page declines rather than showing a number. Outside the range the equations were fitted over, a result can also fall below zero or above 100, and those are declined too.
Does this work for very muscular or very lean people?
Less well. The equations were fitted across a broad population, and at the extremes — a bodybuilder’s neck, a very lean waist — a girth-based prediction drifts. A scan is the honest answer if you are at either end.
Should I use inches or centimetres?
Either. The equations are in inches, so a metric entry is converted with the exact definition of an inch before anything else happens — the answer is identical, and no rounding is introduced along the way.
