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

Its own validation against underwater weighing reported a standard error of about 3.5 points for men and 3.7 for women, with a correlation of 0.90. That makes it good for tracking change over time when you measure the same way each time, and only rough for a single absolute number — which is why this page shows the band, not just the point.

Why does it read low for muscular people?

Because it reads only circumferences, and a waist thick with muscle measures the same as one thick with fat. For muscular builds — roughly a BMI above 27 — the estimate runs low by about 2.5 to 4.5 points. For very lean women it tends to run slightly high.

Where exactly do I measure?

Neck: just below the larynx, tape sloping slightly down at the front. Waist: at the navel for men, at the narrowest point for women. Hip (women): at the widest point. Keep the tape snug but not compressing the skin, and do not hold your breath or pull your stomach in — the whole result turns on these few centimetres.

Is a body-fat scale more accurate than this?

Not reliably. Bioelectrical-impedance scales carry their own error and drift with hydration, time of day and recent meals. No field method is exact. The honest approach with any of them is to pick one, use it under the same conditions each time, and read the trend rather than the absolute figure.

Health

Body Fat Calculator

The tape gives a percent; the method gives a band around it. This page shows both, because the band is wide enough to matter.

The link carries your figures, so it reopens on exactly these numbers.

What a slip of the tape does to the answer

The same person, with the waist mis-measured by a centimetre or two — the commonest error. A 2 cm slip moves the estimate about 1.5 points, close to half the whole error band. This is measurement noise, on top of the method’s own.

Waist measured as	Body fat	Lower end of the band	Upper end of the band
4 cm under	14.5	11	18.1
2 cm under	16.1	12.6	19.6
as measured (88 cm)17.6%, band 14.1 to 21.1.	17.6	14.1	21.1
2 cm over	19.1	15.6	22.6
4 cm over	20.5	17	24

The formula

men: 495 ÷ (1.0324 − 0.19077·log₁₀(waist − neck) + 0.15456·log₁₀(height)) − 450

women: 495 ÷ (1.29579 − 0.35004·log₁₀(waist + hip − neck) + 0.22100·log₁₀(height)) − 450

circumferences and height in cm

error band = ± 3.52 points (men) / 3.72 (women), from the method’s own validation

Worked example

A man 180 cm tall, 88 cm waist, 39 cm neck, 80 kg

waist − neck = 88 − 39 = 49 cm
495 ÷ (1.0324 − 0.19077·log 49 + 0.15456·log 180) − 450 = 17.6%
the validated band: 14.1% to 21.1% — a 7-point spread
fat mass 14.1 kg, lean mass 65.9 kg

Where this goes wrong

Acting on the single percent as if it were measured

The tape returns 17.6%, and it is easy to treat that as your body fat. It is not — it is the middle of a band. The method’s own validation against underwater weighing, on 1,022 people, put its standard error at 3.52 points for men and 3.72 for women, so 17.6% honestly means somewhere from 14.1% to 21.1%. Whether that reads as lean or merely average depends on which end you land, and the tape cannot say. Worse, it reads low for muscular builds — by 2.5 to 4.5 points once BMI passes 27 — because a waist thick with muscle is counted the same as one thick with fat. The number is genuinely useful for one thing: measured the same way each month, the change is more reliable than the absolute. Chase the trend, not the decimal.

Questions

How accurate is the U.S. Navy body fat method?: Its own validation against underwater weighing reported a standard error of about 3.5 points for men and 3.7 for women, with a correlation of 0.90. That makes it good for tracking change over time when you measure the same way each time, and only rough for a single absolute number — which is why this page shows the band, not just the point.
Why does it read low for muscular people?: Because it reads only circumferences, and a waist thick with muscle measures the same as one thick with fat. For muscular builds — roughly a BMI above 27 — the estimate runs low by about 2.5 to 4.5 points. For very lean women it tends to run slightly high.
Where exactly do I measure?: Neck: just below the larynx, tape sloping slightly down at the front. Waist: at the navel for men, at the narrowest point for women. Hip (women): at the widest point. Keep the tape snug but not compressing the skin, and do not hold your breath or pull your stomach in — the whole result turns on these few centimetres.
Is a body-fat scale more accurate than this?: Not reliably. Bioelectrical-impedance scales carry their own error and drift with hydration, time of day and recent meals. No field method is exact. The honest approach with any of them is to pick one, use it under the same conditions each time, and read the trend rather than the absolute figure.