Body Fat Calculator
Estimate your body-fat percentage two ways — the trusted US Navy tape method, or a no-tape CUN-BAE estimate from your BMI and age. See your fat mass, lean mass, category, and fat-free mass index (FFMI) for real body-composition context.
Your measurements
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Body composition
How your weight splits between fat and everything else — muscle, bone, organs and water.
Two ways to estimate body fat
Body-fat percentage tells you what BMI can't: how much of your weight is actually fat versus fat-free mass — muscle, bone, organs and water. It's the single most useful number for tracking real fitness progress, because it shows whether a diet is stripping fat or costing you muscle. Clinical methods like DEXA and underwater weighing are most accurate but expensive; this calculator gives you two accessible, zero-cost estimates instead.
The US Navy circumference method
Developed at the Naval Health Research Center in 1984, the US Navy method needs only a flexible tape. Because fat is distributed differently by sex, the formulas differ: men measure neck and waist; women add the hips. Measured carefully on bare skin, it lands within about ±3.5% of laboratory methods for most adults.
Men: %BF = 495 ÷ [1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)] − 450
Women: %BF = 495 ÷ [1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)] − 450
All measurements are in centimetres (this tool converts imperial input for you). To measure well: keep the tape snug but not tight, take the neck just below the larynx, the waist horizontally at the navel for men or at the narrowest point for women, and the hips at the widest point. Measure twice and average.
The no-tape CUN-BAE estimate
No tape? Switch to the CUN-BAE method, which estimates body fat from your BMI, age and sex — reflecting that body fat rises with age and differs between the sexes at the same BMI. Validated on thousands of adults, it outperforms older BMI-only formulas, though it's naturally less precise than a direct circumference measurement. It's a solid quick estimate when you can't measure.
Fat mass, fat-free mass and FFMI
From your body-fat percentage and weight, the tool derives your fat mass (weight × body-fat %) and fat-free mass (the rest). Dividing fat-free mass by your height in metres squared gives the Fat-Free Mass Index (FFMI) — think of it as BMI for lean mass. It's especially useful for athletic people, whose muscle can push BMI into "overweight" while their body fat is low. FFMI typically ranges from around 18 for an average person to the low-to-mid 20s for well-trained lifters, with about 25 often cited as the natural ceiling for men.
Body-fat categories
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25%+ | 32%+ |
Limitations
Both methods are estimates. The US Navy formula assumes typical fat distribution and can be thrown off by an unusually muscular or slim neck, or by measuring inconsistently; CUN-BAE inherits BMI's blind spots. For either one, the real value is in tracking change — measure the same way, at the same time of day, every few weeks, and watch the trend rather than fixating on a single reading. For medical-grade accuracy, a DEXA scan is the gold standard.
Frequently asked questions
How accurate is the US Navy body fat test?
Measured carefully with a non-stretch tape on bare skin, it's within about ±3.5% of lab methods for most adults — less precise than DEXA or hydrostatic weighing, but far more accessible. Consistent technique matters most when tracking change.
Can I calculate body fat without a tape or calipers?
Yes — switch to the no-tape method, which uses the CUN-BAE equation to estimate body fat from your BMI, age and sex. Less precise than the tape method, but handy with no equipment.
What is fat-free mass index (FFMI)?
Fat-free mass (kg) ÷ height (m)² — like BMI, but for lean mass. It contextualises muscularity: ~18 is average, low-20s is well-trained, and ~25 is often cited as the natural ceiling for men.
What is a healthy body-fat percentage?
Men: fitness ~14–17%, average 18–24%, obese 25%+ (essential 2–5%). Women: fitness ~21–24%, average 25–31%, obese 32%+ (essential 10–13%). Women naturally carry more essential fat.
How do I measure for the US Navy method?
On bare skin, tape snug not tight. Neck just below the larynx; waist at the navel (men) or narrowest point (women); women also measure hips at the widest point. Measure twice and average.