Body Fat Calculator: Estimate Body Fat % With a Tape Measure

How this calculator works

This is the US Navy circumference method, the tape test the US Navy has long used for body composition screening (Peterson, 2015). It assumes that the bigger your waist is relative to your neck (and hips, for women), the more fat you carry. The calculator runs the metric version of the equations to get body density, then converts density to body fat with the Siri equation (495 ÷ density, minus 450):

• Men: density = 1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)
• Women: density = 1.29579 − 0.35004 × log10(waist + hip − neck) + 0.221 × log10(height)

All measurements are in centimetres. Fat mass is your weight × body fat %, and lean mass is everything else (muscle, bone, organs, water). If the result comes out at 2% or lower, or above 70%, you get “n/a” instead, which almost always means a typo or a neck bigger than the waist.

How to read your number

The label under your result uses these bands:

Category	Men	Women
Essential fat	under 6%	under 14%
Athletic	6-13.9%	14-20.9%
Fit	14-17.9%	21-24.9%
Average	18-24.9%	25-31.9%
Above average	25% and up	32% and up

These are a fitness-industry chart, not a diagnosis. The tape is also a blunt tool. In 31 college men, the Navy equation produced an unacceptable total error of 5.2 percentage points against a three-compartment lab model (Moon et al., 2008). In 1,407 army recruits, tape equations read about 6 points lower than DXA on average (Foulis et al., 2023). In women aged 40-60, the Navy equation’s average (30.3%) was close to underwater weighing (29.5%), yet individual readings agreed poorly (Bathalon et al., 1995). Groups average out. Individuals don’t.

So use the result as a ballpark. The most useful number on the page is often your raw waist measurement, tracked monthly.

Using it to plan a cut or bulk

The lean mass figure is what matters for programming. Feed it into the FFMI calculator to see how much muscle you carry for your height, or into the calorie calculator to set intake. When a cut is going well, the waist drops while lifts hold steady. The tape can miss part of that change: in the recruit study, the tape equations underestimated the fat loss DXA detected over 8 weeks in men and did not detect it at all in women. Compare measurements across months, not days.

Who this is not for

Very muscular lifters with thick necks can read leaner than they are, and people who carry fat mainly on the hips and thighs (common in women) may get misleading numbers. The equations were developed on US military personnel; Asian Indians tend to carry more fat at the same BMI than Europeans (Rush et al., 2009), and the tape method has not been well validated in Indian adults. Pair it with the waist-to-height ratio calculator. It is not suitable for under-18s, during pregnancy, or with fluid retention from heart, kidney or liver disease. If you want a precise number for a medical reason, ask your doctor about a DXA scan.