Body Fat Percentage Calculator

Body fat percentage answers a question BMI cannot: how much of your weight is actually fat. Lab methods like DEXA answer it precisely but cost money and appointments; this calculator uses the U.S. Navy circumference method, which gets a useful estimate from nothing but a flexible tape measure — height, neck, and waist, plus hips for the female formula.

The method is a pair of published equations built on circumference ratios: measurements are converted to inches internally and run through logarithmic formulas the Navy developed for fitness assessment. The result is shown to one decimal place with a broad interpretation band — essential, athletic, fitness, average, or higher range, with different thresholds for the male and female formulas — plus a colour gauge showing where the estimate sits. An age field is included for context only; the Navy formula does not use it, and the tool says so rather than pretending otherwise.

Accuracy lives and dies with the tape work, which is why a measurement guide is built in: neck just below the larynx, waist at navel level, hips at the widest point, tape snug but never tight. Input validation catches implausible combinations (the male formula, for instance, requires waist greater than neck), and results outside the plausible display range trigger a re-measure warning. It remains an estimate for general information — as the prominent disclaimer states, not a diagnosis, and not comparable one-to-one with DEXA, calipers, or smart-scale readings, each of which measures differently. Your measurements stay in your browser throughout.

Measurements are converted to inches, then run through the published Navy equations: for the male formula, 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76; for the female formula, 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387. The result is rounded to one decimal place.

The logic behind the ratios: waist (and hip) circumference grows with fat storage while neck circumference tracks lean frame size, so their relationship — scaled against height — correlates with body-fat percentage across the populations the Navy studied. The interpretation bands then classify the estimate on sex-specific thresholds (for the male formula: under 6% essential, 6–14 athletic, 14–18 fitness, 18–25 average, above 25 higher; the female bands sit higher because essential fat is higher). Validation runs before calculation — including the mathematical preconditions like waist exceeding neck — and results outside 2–65% raise a re-measure warning rather than being presented as fact.

The Navy method is a population-fitted estimate, not a measurement of your actual fat mass: accuracy depends heavily on tape technique, and results legitimately differ from DEXA, hydrostatic weighing, calipers, and smart scales — methods disagree with each other by several percentage points. The interpretation bands are broad educational labels, not clinical categories, and age is displayed as context only since the formula ignores it. As the built-in disclaimer states, this is general information, not medical advice or diagnosis — anyone with health concerns, and especially teenagers or pregnant people, should rely on professional assessment instead.

Client-side computation: metric inputs convert at 2.54 cm/in, then the standard Navy equations run via Math.log10 (male: 86.010·log10(waist−neck) − 70.041·log10(height) + 36.76; female: 163.205·log10(waist+hip−neck) − 97.684·log10(height) − 78.387). Results round to one decimal. Validation enforces measurement ranges and the formulas' mathematical preconditions before computing; values outside 2–65% attach a plausibility warning. Calculation is button-triggered rather than live, and the age field is excluded from the math by design.

How the Navy method compares

Numbers from different methods are not interchangeable — pick one and compare its readings only with each other.

People who want a body-composition signal without lab access or equipment beyond a tape measure; lifters who find BMI misleading for their build; and anyone tracking a recomposition over months who needs a repeatable, free check-in. It suits trend-watchers more than precision-seekers — for a clinical-grade number, DEXA and professional assessment are the route.