Free Body Fat Calculator — The US Navy Tape Method, Done Right

Your bathroom scale can’t tell 80 kg of muscle from 80 kg of fat. That’s the whole problem with judging health by weight alone — and it’s exactly what body-fat percentage fixes. The new Body Fat Calculator estimates that number two ways, with no smart scale and no subscription.

Why body fat beats BMI

BMI is just weight ÷ height². A powerlifter and a sedentary person can share the same BMI while having opposite metabolic health. Body-fat % closes that gap by telling you what fraction of your weight is fat versus muscle, bone, and water. Even a ±3% estimate catches things a scale misses entirely:

• Body recomposition — lose 5 kg of fat and gain 3 kg of muscle, and the scale barely moves while body-fat % drops noticeably.
• “Skinny fat” with age — BMI looks fine while fat mass quietly rises at a stable weight.
• Cuts vs bulks — whether a calorie deficit is preserving muscle (good) or burning through it (bad).

For a side-by-side on where BMI falls short, read Body Fat % vs BMI.

Two methods, one screen

• US Navy tape method (default) — uses a soft tape around neck, waist (and hip for women) plus height. Derived from Naval Health Research Center data and validated against hydrostatic weighing, it’s accurate to ±3–4% versus DEXA when measured correctly.
• BMI-derived (Deurenberg 1991) — needs only height, weight, age, and sex. A handy fallback when you don’t have a tape, though it shares BMI’s blind spot for muscular people.

Both give you a body-fat percentage, an ACE category (Essential, Athletes, Fitness, Average, Obese) with sex-specific ranges, and — if you enter weight — a fat-mass / lean-mass split. Measure at the same time of day, snug but never pulled tight, for the most reproducible results.

A $5 tape beats a smart scale

Impedance smart scales sell convenience but their body-fat readings swing with hydration and time of day, often by 10 percentage points. The US Navy method, with a tape that costs a few dollars, is more reliable. The calculator on the tool page is free and stores nothing — the maths runs in your browser and no measurement is sent anywhere. A future Pro tier could add saved measurements, a body-fat trend chart, and a multi-client roster for trainers, but the core tool stays free.

This is an informational estimate, not medical advice. Tape and BMI-based methods approximate composition within a few points; they are not a clinical scan. Talk to a qualified professional before making decisions about your health, diet, or training.

Frequently Asked Questions

How accurate is the US Navy method? Within ±3–4% of DEXA when measurements are taken correctly — far better than impedance smart scales, which can be off by 10 percentage points.

Should I use the US Navy or BMI-derived method? US Navy if you have a soft tape measure; it is more accurate. BMI-derived is a useful quick check when you don’t, but it can’t tell muscle from fat.

Is this medical advice? No. It is an informational estimate, not a clinical measurement like DEXA. Consult a qualified professional before making health decisions.

Try it now — Open the Body Fat Calculator — no signup required.