π Body Fat Calculator
A tape measure is enough. The US Navy circumference formula estimates body fat from height, neck and waist β within about 3β4 percentage points of a DEXA scan.
π Body fat percentage chart (ACE)
| Category | Men | Women |
|---|---|---|
| Essential fat | 2β5% | 10β13% |
| Athletes | 6β13% | 14β20% |
| Fitness | 14β17% | 21β24% |
| Average | 18β24% | 25β31% |
| Obese | 25%+ | 32%+ |
Every band sits higher for women, and that is physiology rather than a softer standard: essential fat for hormonal and reproductive function is roughly 10β13% in women versus 2β5% in men.
How to use this calculator
Pick your units, your sex, then enter four numbers taken with a soft tape measure. Stand relaxed, breathe out normally, and keep the tape snug against the skin without pressing in. Measure each site twice and average β that single habit removes most of the error.
- Neck: just below the larynx, tape sloping slightly downward at the front. Don't flex.
- Waist (men): at navel level. Waist (women): at the narrowest point of the torso, usually just above the navel.
- Hip (women only): the largest circumference around the glutes, feet together, tape level front to back.
How the math works
Two log-linear regressions, one per sex, fitted by the Naval Health Research Center in 1984 against hydrostatic weighing:
Men: %BF = 495 / (1.0324 β 0.19077Β·log10(waist β neck) + 0.15456Β·log10(height)) β 450
Women: %BF = 495 / (1.29579 β 0.35004Β·log10(waist + hip β neck) + 0.22100Β·log10(height)) β 450
All circumferences in centimetres; this page converts your input when you choose imperial. The result feeds into Siri's equation (%BF = 495/density β 450), which is why the constants look unusual. Fat mass is simply weight Γ %BF, lean mass is the rest.
Worked example
A 32-year-old man, 178 cm tall, 86 cm waist, 39 cm neck: waist β neck = 47, log10(47) = 1.6721, log10(178) = 2.2504, denominator = 1.0324 β 0.19077(1.6721) + 0.15456(2.2504) = 1.0612, so %BF = 495 / 1.0612 β 450 β 16.5% β the fitness band. At 82 kg that is about 13.5 kg of fat and 68.5 kg of lean mass.
Assumptions and limitations
Circumference is a proxy, not a measurement of fat. Two people with identical measurements can have different body composition, and the formula says nothing about where the fat sits β visceral fat carries more metabolic risk than the same amount on the hips. Accuracy is best in the middle of the range (roughly 12β30% body fat) and degrades at both extremes. Not intended for pregnancy, competitive bodybuilders, people with significant oedema, or anyone under 18. This is not medical advice.
Frequently Asked Questions
How accurate is the Navy tape measure method?
Typically within 3β4 percentage points of a DEXA scan for adults in the middle of the body fat range, with a standard error of estimate around 3.5% for men and 4.0% for women. That is roughly as good as skinfold calipers and better than most consumer bioimpedance scales. Measurement technique matters more than the formula: one centimetre of error at the waist moves the answer by about a full percentage point, so measure twice and average.
Why is this different from my smart scale?
Smart scales use bioelectrical impedance, which sends a small current through your body and infers fat from resistance. That reading depends heavily on hydration, recent exercise, food and even skin temperature, so it can swing several points within a day. The Navy method depends on your circumference and your tape technique instead. Neither is "right" β pick one method and track the trend with it.
What is a healthy body fat percentage?
For men, roughly 14β17% (fitness) to 18β24% (average); for women, 21β24% (fitness) to 25β31% (average). Athletes sit lower, and essential fat floors are about 2β5% for men and 10β13% for women. Dropping toward the essential floor is not a fitness achievement: it disrupts hormones, and in women it can stop menstrual function.
Is body fat percentage better than BMI?
It carries more information, because BMI cannot tell muscle from fat β a lifter at 12% body fat can register as "overweight". But body fat percentage is an estimate with a few points of error, while BMI is a hard calculation from two numbers. Read them together: high BMI with normal body fat usually means muscle; normal BMI with high body fat is the pattern sometimes called normal-weight obesity.
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