Body fat calculator
What BMI cannot see, from three measurements and a tape — with the error bars stated rather than implied.
How this is calculated
The US Navy circumference equation (Hodgdon & Beckett, 1984), still used for fitness screening because it needs nothing but a tape:
men: 495 / (1.0324 − 0.19077·log₁₀(waist − neck) + 0.15456·log₁₀(height)) − 450 women: 495 / (1.29579 − 0.35004·log₁₀(waist + hip − neck) + 0.221·log₁₀(height)) − 450
If your waist is no larger than your neck the logarithm has nothing to work with, and this page says it cannot tell rather than printing a number. That case is rare in adults and usually means a mistyped measurement.
The cheaper number underneath
Waist-to-height ratio needs two of the same measurements and predicts metabolic risk at least as well: keeping your waist under half your height is the whole rule. It is shown above because for many people it is the more useful of the two.
These are the same numbers and the same guards the Talk2Fit app applies when it builds your plan — the calculator is the app's own arithmetic, not a separate approximation.
Questions
How accurate is the tape-measure method?
Within about 3–4 percentage points of hydrostatic weighing for typical builds, and worse at the extremes — very lean, very heavy, or unusually proportioned people are where it drifts. It measures circumferences, so it cannot tell a thick waist from a thick torso.
Where exactly do I measure?
Neck just below the larynx, sloping slightly down at the front. Waist at the navel for men, at the narrowest point for women. Hips at the widest point. Tape snug but not compressing, at the end of a normal exhale, skin bare. Measure twice.
Why do women need the hip measurement?
Because the equation was fitted separately by sex, and the female version uses waist plus hip minus neck. Fat distribution differs enough that one formula for both would be worse for everyone.
What number should I aim for?
Health-wise, most guidance puts men roughly 10–20 % and women 18–28 %, and both sexes have an essential floor below which things stop working — around 3 % and 12 %. Chasing a number below your genetic comfort costs more than it returns.