Body fat calculator
Estimates body fat from tape measurements using the US Navy method. This separates fat from lean tissue, which is the main thing BMI cannot do — though it remains an estimate with meaningful error.
Built by Mubashir. Uses a published standard formula, named below. Not medical advice — see the limitations section.
Measurements
Estimated body fat
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Fat mass
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Lean mass
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Category
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Show the working
—An estimate with a typical error of three to four percentage points. The US Navy method infers body fat from circumferences, so it cannot see your actual composition and performs worst at the extremes and for people whose fat distribution is unusual. A DEXA scan is the accepted reference standard. Do not use this figure to set aggressive goals, and speak to a doctor or dietitian before making significant changes.
How to measure properly
The result is only as good as the tape work, and measurement error is the largest source of inaccuracy here. A centimetre off on the waist moves the estimate by roughly a full percentage point.
- Neck — just below the larynx, with the tape sloping slightly downward at the front. Do not flex.
- Waist — for men, at the navel; for women, at the narrowest point. Measure after a normal exhale, without pulling in.
- Hip — women only, at the widest point of the buttocks, with feet together.
Keep the tape snug but not compressing the skin, level all the way round, and measure at the same time of day each time. Morning before eating is most consistent.
The formula
The Navy method uses the logarithm of circumference differences, which is why the arithmetic looks unfamiliar. For men, body fat is derived from the difference between waist and neck, adjusted for height. For women, hip circumference is added, since fat distribution differs. Both were developed and validated by the US Naval Health Research Center as a practical field alternative to hydrostatic weighing.
The key insight is that the method is not measuring fat. It is measuring circumferences that correlate with fat in the population it was validated on, and inferring from there.
Healthy ranges
Women carry substantially more essential fat than men, for reproductive and hormonal reasons, so the categories differ.
For men, roughly: 2 to 5% is essential fat, 6 to 13% athletic, 14 to 17% fitness, 18 to 24% average, and 25% or above is generally classified as excess. For women, add about ten points to each band: 10 to 13% essential, 14 to 20% athletic, 21 to 24% fitness, 25 to 31% average, 32% and above excess.
Very low body fat is not a goal. Dropping toward essential levels disrupts hormone production, immune function and, in women, menstrual regularity. The lean end of these ranges is not healthier than the middle.
Where this method fails
It assumes a typical relationship between circumference and fat, so anyone atypical gets an unreliable number. Very muscular people with thick necks are systematically underestimated. People carrying weight primarily on the limbs rather than the trunk are underestimated too, since the tape never sees it. Accuracy degrades at both very low and very high body fat.
For tracking change over time it is more useful than it is for a single absolute reading. The measurement error is reasonably consistent, so a downward trend across several months is meaningful even if the absolute figure is off.
Frequently asked questions
What is a healthy body fat percentage?
Roughly 14 to 24% for men and 21 to 31% for women covers the fitness and average bands. Women require more essential fat than men for hormonal and reproductive function, so the ranges are not comparable across sexes.
How accurate is the US Navy method?
Typically within three to four percentage points for people of average build. It is less reliable for very muscular individuals, at extremes of body fat, and where fat distribution is unusual.
Is body fat percentage better than BMI?
More informative, since it separates fat from lean tissue. Also harder to measure accurately — a tape estimate carries real error while BMI at least computes a precise number from two easy measurements.
What is the most accurate way to measure body fat?
DEXA scanning is the practical reference standard, with hydrostatic weighing and air displacement close behind. All require clinic equipment. Home bioelectrical impedance scales are notably unreliable, varying with hydration.
Can body fat be too low?
Yes. Approaching essential fat levels disrupts hormone production, immune function and menstrual regularity, and is associated with poor bone density. Lean is not automatically healthier.
How fast can body fat change?
Slowly. Losing more than about half a percentage point a month while preserving muscle is difficult. Larger apparent swings are usually measurement error or shifts in hydration.