Body Fat Calculator

This estimates body fat percentage from a handful of tape-measure circumferences, using the method the U.S. Navy uses for body composition assessment. It needs a height, weight, neck, and waist measurement, plus a hip measurement for women. Measurement technique affects accuracy meaningfully, so read the instructions for each measurement before you start.

Body Fat Percentage: ...
Category: ...
Fat Mass: ...
Lean Mass: ...

Assumptions

This uses the U.S. Navy circumference method (Hodgdon and Beckett, 1984), which estimates body fat from waist, neck, and height for men, adding hip for women, not from a direct measurement of body fat itself.
The circumference method's published coefficients are for measurements in inches. Height, neck, waist, and hip are converted to inches internally regardless of which unit system you select, since the same coefficients are also circulated online mislabeled as centimetres on some sites, which produces a materially different, incorrect result.
Accuracy depends heavily on consistent measurement technique: where you place the tape, how snug it is, and whether you measure at the same point each time. Circumference-method estimates typically run about 3 to 4 percentage points off a DEXA scan, in either direction, and that gap can widen with inconsistent technique.
Category labels (Essential fat, Athletes, Fitness, Acceptable, Obesity) use the American Council on Exercise's published ranges, which differ between men and women. Other authorities draw these lines differently: some academic references, for example, use narrower 'athletic' and 'good' bands that shift at different percentages. A category label describes where a figure sits on one published chart, not a diagnosis.
Body fat below roughly 2% for men or 10% for women is generally considered below the minimum needed for basic physiological function (essential fat), which itself carries health risk rather than indicating extra fitness.
Weight is used only to convert the percentage into fat mass and lean mass. It does not change the body fat percentage itself, which comes entirely from the circumference and height measurements.

Sources

How the circumference method estimates body fat

The method uses the natural log relationship between body fat and the girth of the waist relative to the neck (for men), or waist plus hip relative to the neck (for women), scaled against height. It was developed by fitting these measurements against body density measured by underwater weighing in several hundred Navy personnel, then converting that density to a percentage. It does not measure fat directly the way a DEXA scan or skinfold caliper does; it estimates it from the shape and scale of a few circumferences.

Why measurement technique matters so much

Because the formula multiplies circumference differences by large coefficients, a small measurement error gets magnified. Measuring the waist an inch too high or too low, pulling the tape too tight, or measuring after eating a large meal can shift the result by several percentage points, often more than the method's own margin of error against a DEXA scan. For a trend over time, measure at the same time of day, in the same state (ideally first thing in the morning), and at the exact same landmarks each time, rather than treating any single reading as precise on its own.

Worked example

A man measuring 34 in (86 cm) at the waist, 15 in (38 cm) at the neck, 70 in (178 cm) tall, weighing 180 lb (82 kg).

Body fat % = 86.010 x log10(34 - 15) - 70.041 x log10(70) + 36.76 = 86.010 x log10(19) - 70.041 x log10(70) + 36.76 = 109.99 - 129.25 + 36.76 = 17.5%. That falls in the Fitness range on the ACE chart for men. At 180 lb, 17.5% body fat works out to about 31.5 lb of fat mass and 148.5 lb of lean mass.

Frequently asked questions

How does the Navy body fat calculator actually work?

It plugs your waist, neck, and height into a formula for men, or waist, hip, neck, and height for women, developed from a 1984 U.S. Navy study that compared circumference measurements against body density from underwater weighing. It's an estimate derived from body shape, not a direct measurement of fat tissue.

Why does this show a category alongside the percentage?

The category (Essential fat, Athletes, Fitness, Acceptable, or Obesity) shows where your percentage falls on a published reference chart from the American Council on Exercise, so the raw number has some context. Different organizations draw these category boundaries in slightly different places, so treat the label as one reference point, not a fixed verdict.

How accurate is this compared to a DEXA scan or caliper test?

The circumference method typically runs about 3 to 4 percentage points off a DEXA scan, in either direction, when measurements are taken carefully. That margin can widen noticeably with inconsistent technique, so it's better used to track your own trend over weeks or months with consistent measuring than to compare a single reading against someone else's.

Is this the same formula the Army uses?

It's the U.S. Navy version of the tape-measure method specifically. Other military services have used circumference-based estimates too, sometimes with adjusted measurement sites or additional sites like the wrist, so figures from a different service's official body composition test may not match this tool's result exactly.