Body Fat Calculator

Seventeen percent, from a tape measure

A man of 175 cm with a 38 cm neck and an 85 cm waist has an estimated body fat of about 16.9%.

The method is the US Navy circumference formula, developed to assess personnel without laboratory equipment. It uses logarithms of circumference measurements:

Men: body fat % = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450

Women additionally require the hip measurement, because fat distribution differs and waist alone is a poorer predictor.

Why it uses waist minus neck

The logic is that the waist reflects abdominal fat while the neck reflects frame size with relatively little fat. The difference between them therefore approximates fat carried around the midsection more reliably than waist alone, which would penalise larger-framed people.

This also explains the method's sensitivity: because the formula works on the *difference*, a one-centimetre error in either measurement moves the result by roughly a full percentage point. Measurement technique matters more here than in most body-composition estimates.

Measuring properly

Waist — at the navel for men, at the narrowest point for women. Standing relaxed, at the end of a normal exhale, tape snug but not compressing the skin.

Neck — just below the larynx, with the tape sloping slightly downward at the front.

Hips (women) — at the widest point of the buttocks.

Measure at the same time of day, ideally morning before eating, and take the average of two or three readings. Measuring after a meal, after exercise, or while holding the stomach in all produce materially different answers.

What the result means

Commonly cited categories for men:

• Essential fat — 2–5%
• Athletic — 6–13%
• Fitness — 14–17%
• Average — 18–24%
• Obese — 25% and above

For women, each band sits roughly 10 percentage points higher, because essential fat requirements are greater — women's essential fat is around 10–13%, reflecting reproductive and hormonal function. A woman at 12% body fat is not the equivalent of a man at 12%; she is below essential levels, with genuine risk of hormonal disruption and amenorrhoea.

Accuracy

The Navy method is typically accurate to within about 3–4 percentage points against laboratory methods for most people — reasonable for a tape measure and free.

The reference methods are DEXA scanning, considered the practical gold standard, and hydrostatic weighing. Bioelectrical impedance scales are convenient and highly sensitive to hydration status, food and time of day, which is why the same person can read several percentage points differently across a single day.

The method is least accurate at the extremes — very lean and very obese individuals — and for people whose fat distribution is unusual for their sex.

The useful way to use it

Track the trend, not the absolute. Whatever the method's systematic error, it is consistent, so a fall from 21% to 18% measured the same way is real information even if neither figure is exactly right.

Body fat percentage is a considerably better measure than BMI for anyone with meaningful muscle mass, since it distinguishes what BMI cannot.

This is an estimate for general information and is not medical advice.

Common mistakes

Measuring inconsistently. Time of day, technique and tape tension all shift the result.

Pulling the tape tight. It compresses tissue and understates the measurement.

Comparing across methods. Navy, BIA and DEXA give different numbers; pick one and stay with it.

Applying men's categories to women. The bands differ by about ten points.

Chasing very low body fat. Below essential levels carries real health consequences, particularly for women.