Body Fat Calculator
Estimate body fat percentage from height, waist, neck and (for women) hip measurements with the US Navy tape method, rated on the ACE chart.
US Navy Method: This calculator uses the US Navy circumference formula. Measure consistently, to the nearest quarter inch or half centimeter. The result is an estimate and can differ from a DEXA scan or other clinical measurement by several percentage points.
Updated Formula covered by automated tests
This tool estimates the share of your body weight that is fat using nothing more than a soft tape measure. It applies the US Navy circumference equations: men enter height, waist and neck; women add a hip measurement. The result is shown as a percentage and placed in the matching band of the American Council on Exercise (ACE) chart, which uses different cut-offs for men and women.
The tape method appeals to people who want more than a bathroom scale can tell them but have no access to a DEXA scanner or hydrostatic tank. Tracking the same three or four circumferences every few weeks shows whether a training or diet change is shifting fat rather than just water or muscle, even when the scale barely moves.
The formula
- waist
- Abdominal circumference in inches; men measure level with the navel, women at the narrowest point of the torso
- neck
- Neck circumference in inches, taken just below the larynx with the tape sloping slightly down toward the front
- hip
- Women only: circumference in inches around the widest part of the buttocks
- height
- Standing height in inches without shoes
- log10
- Base-10 logarithm, which compresses large circumference differences so that each extra inch of waist adds a little less than the one before
The equations come from work by James Hodgdon and Marcie Beckett at the Naval Health Research Center in San Diego in the 1980s. They measured body density by underwater weighing in a large group of Navy personnel and then searched for the combination of tape measurements that best predicted it. Waist minus neck captures trunk fat while correcting for overall frame size, and height scales the result for taller people.
The US Department of Defense adopted these inch-based equations for its body composition program, which is why they are often called the military or Navy formula. The metric version predicts body density first and converts it with the Siri equation (495 ÷ density − 450); for typical measurements it agrees with the inch form to within a few tenths of a percent. The calculator works in inches internally and converts centimeter entries by dividing by 2.54.
Several branches have since revised their own tape tests, for example toward single-site waist equations, so the version here is the classic multi-site formula rather than any service’s current regulation.
Worked example
Daniel is 5 ft 10 in (70 in) tall. His waist at the navel measures 36 in and his neck measures 15 in.
- 1 Subtract neck from waist: 36 − 15 = 21 in, and log10(21) = 1.3222.
- 2 Multiply by the waist coefficient: 86.010 × 1.3222 = 113.72.
- 3 Take the height term: log10(70) = 1.8451, and 70.041 × 1.8451 = 129.23.
- 4 Combine: 113.72 − 129.23 + 36.76 = 21.25, which rounds to 21.3%.
- 5 Compare with the ACE chart for men: 18% to 24% is the average band.
Result: Daniel’s estimated body fat is 21.3%, in the ACE average range for men. Losing about an inch from his waist while his neck stays the same would bring the estimate down by roughly two points.
Before you rely on the result
- The equations were fitted to young, mostly fit military personnel, so error grows for older adults, children, pregnant women and people with unusual body shapes.
- A quarter-inch error in waist or neck placement moves the estimate by roughly half a percentage point, so small month-to-month changes can be measurement noise rather than real fat loss.
- Body fat percentage is not a diagnosis. Anyone concerned about weight-related health risks should discuss them with a clinician who can order proper tests.
Frequently asked questions
Why does my tape estimate differ from my smart scale?
Bioelectrical scales send a small current through the legs and infer fat from water content, so a large drink or a hard workout can move the reading by several points. The Navy formula ignores hydration but depends heavily on where the tape sits. Neither is a reference method; pick one, measure under the same conditions each time, and watch the trend.
Where exactly should I put the tape on my waist?
For the male equation, measure horizontally at the level of the navel after a normal breath out, without pulling in the stomach. For the female equation, measure at the narrowest point between the ribs and the hips. Taking each measurement three times and averaging them reduces the error from a slightly tilted tape.
What body fat percentage counts as healthy?
The ACE chart describes 14% to 24% as the fitness and average bands for men and 21% to 31% for women, with essential fat at roughly 2% to 5% and 10% to 13% respectively. These bands are descriptive rather than medical targets. A doctor can interpret your number alongside blood pressure, blood sugar and waist size.
Is the Navy method accurate for very lean or very heavy people?
It is least reliable at the extremes. Very lean, muscular people with thick necks can come out lower than their true value, and people with a lot of abdominal fat may get an estimate that is off by several points in either direction. A DEXA scan gives a more trustworthy figure if the exact number matters.