Body Fat Calculator — U.S. Navy Method (% Body Fat)

Body fat percentage is often a more useful measure than weight alone, because it separates fat from muscle, bone, and water. This body fat percentage calculator uses the U.S. Navy circumference method — a tape measure and a few numbers, no special equipment required.

How it works

The Navy method estimates body fat from the ratio of your circumferences (neck and waist, plus hips for women) to your height. Larger waist-to-neck measurements relative to height indicate more fat:

Men: use neck, waist, and height.
Women: use neck, waist, hip, and height.

From your body fat percentage and weight, the calculator also splits your weight into fat mass and lean mass (everything that isn’t fat).

The Navy formula

The method comes from research at the Naval Health Research Center, and it is just circumferences and height run through a logarithm:

Men: %fat = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450
Women: %fat = 495 ÷ (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.221 × log₁₀(height)) − 450

All measurements are in centimetres — if you measure in inches, the calculator converts automatically before applying the formula. You never have to touch this yourself, but it shows why the waist-to-neck gap (and, for women, the hips) drives everything: the larger that gap is relative to your height, the higher the estimate.

Body fat categories

The result is classified using American Council on Exercise (ACE) ranges, which differ by sex because women naturally carry more essential fat:

Category	Men	Women
Essential fat	2–5%	10–13%
Athletes	6–13%	14–20%
Fitness	14–17%	21–24%
Average	18–24%	25–31%
Obese	25%+	32%+

Your ideal body fat, and the gap to it

Alongside the percentage, the calculator shows an age-based ideal body fat from the Jackson & Pollock generalized-equations research — because a healthy target drifts upward with age (a fit 20-year-old man sits near 8–9%, a fit 50-year-old nearer 18–19%). If you’re above that ideal, it also estimates the kilograms of fat you’d need to lose to reach it, adjusted for the fact that losing fat lowers your total weight along the way.

No tape measure? The BMI method

If you can’t take your circumferences, the calculator also gives a rougher estimate from the BMI method (Deurenberg equation), which needs only your height, weight, age and sex. It’s less accurate than the tape method — it can’t see where you carry weight — but it’s a reasonable fallback, and seeing both numbers side by side is a useful sanity check.

Measuring accurately

The number is only as good as your measurements. Use a flexible tape, keep it level and snug (not tight), and measure at the same time of day. Because the method has a margin of a few percent, it is best used to track change over time rather than to chase a single exact figure.

For the official military version of this test, the army body fat calculator uses the newer single-site (abdomen) tape standard. To see how fat is distributed rather than just how much, try the waist-to-hip ratio, and for the muscle side of the ledger, lean body mass.

Frequently asked questions

How do I calculate body fat percentage?

Without a scan, the most practical no-equipment method is the U.S. Navy formula: measure your neck and waist (plus hips for women) and your height, then feed those circumferences through the Navy equation. This calculator does that for you and cross-checks it against the BMI-based Deurenberg method.

How can I calculate my body fat at home?

All you need is a flexible tape measure. Measure your neck just below the larynx, your waist at the navel (men) or narrowest point (women), your hips at the widest point (women), and your height — then enter them above in centimetres or inches. The Navy formula behind the calculator does the rest.

How accurate is the U.S. Navy body fat method?

In the original Naval Health Research Center validation, the equations came within a standard error of roughly 3–4% compared with hydrostatic (underwater) weighing, the lab standard of the time. Accuracy depends on measuring in the right places with a snug (not tight) tape. It's a useful way to track changes over time, even if the absolute number isn't perfect.

What is a healthy body fat percentage?

It differs by sex. For men, roughly 14–24% is considered fitness-to-average and 25%+ is obese. For women, who carry more essential fat, roughly 21–31% is fitness-to-average and 32%+ is obese. Athletes are often below these ranges.

Where do I measure my waist and neck?

Measure your neck just below the larynx, your waist at the navel for men and at the narrowest point for women, and (women) your hips at the widest point. Keep the tape level and snug against the skin without compressing it.