Body Fat Calculator — US Navy method + ACE categories

How it works

The Navy circumference method fits a formula to the girths that track fat: for men, the waist relative to the neck and height; for women, the waist and hips relative to the neck and height. It's the tape-measure test the US Navy uses in its fitness assessment because it needs no equipment beyond a tape, and it lands within about 3–4% of a DEXA scan for most people. This page also splits your weight into fat mass and lean mass, and shows where your result sits among ACE's categories — which differ by sex, since women carry more essential fat, so the same percentage means different things for a man and a woman.

Worked example

For a man 178 cm tall with a 38 cm neck and 90 cm waist, the formula is 495 ÷ (1.0324 − 0.19077 × log₁₀(90 − 38) + 0.15456 × log₁₀(178)) − 450 ≈ 20.2%. On an 82 kg body that's 82 × 0.202 ≈ 16.5 kg of fat and 65.5 kg of lean mass. At 20%, he's in ACE's "average" band for men (18–24%). A woman uses the waist-plus-hip version; measured the same way she'd typically read higher, because her essential fat is higher.

Frequently asked questions

How do I take the measurements?

Use a flexible tape, snug but not compressing, and measure relaxed after breathing out. Neck: just below the larynx, sloping slightly down to the front. Waist: at the navel for men, at the narrowest point for women. Hips (women): at the widest point. Average two or three readings — small tape errors move the result by a percent or two.

How accurate is the Navy method?

For most people it's within about 3–4% of a DEXA scan, which is good for a tape measure. It's least accurate at the extremes — very lean or very muscular people, and those with unusual fat distribution — because it infers fat from girths rather than measuring it. Treat it as a solid estimate to track over time, not a precise reading.

What's a healthy body fat percentage?

It depends on sex. ACE's categories put men's "fitness" range at 14–17% and "average" at 18–24%; for women those are 21–24% and 25–31%. Below the essential minimum (about 2–5% for men, 10–13% for women) is unsafe, and above the "obese" threshold (25% men, 32% women) raises health risk. These are general bands, not personal targets.

Why are the categories different for men and women?

Women's bodies require more essential fat — for hormonal and reproductive function — so a healthy percentage is naturally higher than for men. ACE's categories build this in, which is why the same 20% is "athletic" for a woman but "average" for a man. Comparing your percentage to the band for your sex is the only meaningful comparison.

Body fat percentage vs BMI — which is better?

They answer different questions. BMI uses only height and weight and can't tell muscle from fat, so a muscular person can have a high BMI and low body fat. Body fat percentage targets fat directly, which is often more useful for fitness — but it depends on measurement technique. Using both, plus a waist measure, gives a fuller picture than any one.

Can I lower body fat without losing weight?

Yes — gaining muscle while losing fat keeps weight steady but lowers the percentage, which is why the scale can stay flat while your body composition improves. That's a reason to track body fat and lean mass, not just weight. This calculator shows both, so you can see the split shift even when the total doesn't.

How accurate is this, and what does it exclude?

The formula is exact for the measurements entered, but it's an estimate of body fat, not a measurement, with a few percent of uncertainty and less reliability at the lean and obese extremes. It excludes fat distribution beyond the measured girths, hydration, and the precision a DEXA or hydrostatic test would give. It's a screening estimate, not a medical assessment.