Body Fat Calculator

Body fat percentage estimates how much of total body weight is fat rather than lean tissue such as muscle, bone, organs, and water. It can be more informative than BMI when tracking body composition, because two people with the same weight and height may have very different muscle and fat distribution. This calculator supports circumference-based and BMI-based estimates, so it can support home fitness tracking when lab methods are not available. Tape placement, posture, hydration, recent meals, and measurement consistency can all affect the number. Treat one result as an estimate, not a clinical diagnosis. For medical, athletic, or high-precision needs, compare trends over time or use professional tests such as DEXA.

Body fat percentage estimation relies on indirect anthropometric formulas because direct measurement (cadaver dissection by Clarys, Martin, and Drinkwater 1984) is the only gold standard, and even DEXA (Dual-Energy X-ray Absorptiometry) drifts by 1-3% body fat compared to a four-compartment model. The two methods on this page cover the field-estimate and the regression-estimate approach. The US Navy method, derived by Hodgdon and Beckett at the Naval Health Research Center in 1984 and codified in OPNAVINST 6110.1, fits linear regression to log10-transformed circumferences: men BFP = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76; women BFP = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387. The method assumes subcutaneous fat distribution is the main driver and is most accurate for normal-weight adults; for obese or extremely lean individuals, the residual error reaches ±5% body fat, which is why the US military pairs it with a taping test for borderline cases. The Deurenberg formula (Deurenberg, Weststrate, and Seidell 1991, British Journal of Nutrition 65(2): 105-114) is the standard BMI-based estimator: BFP = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4 where sex = 1 for males and 0 for females. It was derived from a Dutch cohort of 1,229 subjects and validated on a separate population, with a residual standard error of about 4% body fat. The age coefficient reflects the well-documented shift in body composition with age (sarcopenia and visceral fat accumulation in older adults), and the sex coefficient reflects the essential-fat difference. The Deurenberg method overestimates body fat in muscular athletes (because BMI cannot distinguish muscle from fat) and underestimates in older sarcopenic adults, so a separate body composition model is the right tool for those populations. The page surfaces both formulas because they are complementary: Navy is a field measurement and reflects subcutaneous distribution, while Deurenberg is a regression on BMI and age. The American Council on Exercise (ACE) classification bands in use here - essential 2-5% men / 10-13% women, athletes 6-13% / 14-20%, fitness 14-17% / 21-24%, average 18-24% / 25-31%, obese ≥ 25% / ≥ 32% - are the consumer-grade version of the WHO/NHANES ranges, not clinical thresholds. For an athlete, 10% is well above essential; for a sedentary adult, 28% is the same number but the risk profile is very different. A useful subtlety: impedance-based smart scales use the BIA (Bioelectrical Impedance Analysis) method with their own proprietary equation, often Segal 1988 or Sun 2003, and can disagree with each other by 3-5% body fat because the hydration assumption differs. Migration advice: for a real body composition assessment, prefer DEXA (gold standard for bone density and lean/fat mass), BodPod (air displacement plethysmography, ~2% accuracy), or hydrostatic weighing (~1.5% accuracy); anthropometric formulas are the field tool, not the lab tool.

• US Navy method (Hodgdon and Beckett, Naval Health Research Center, 1984; OPNAVINST 6110.1): men BFP = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76; women BFP = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387.
• Deurenberg formula (Deurenberg, Weststrate, and Seidell, British Journal of Nutrition 65(2): 105-114, 1991): BFP = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4; sex = 1 male, 0 female; residual SE ≈ 4% body fat.
• ACE classification bands: men essential 2-5%, athletes 6-13%, fitness 14-17%, average 18-24%, obese ≥ 25%; women essential 10-13%, athletes 14-20%, fitness 21-24%, average 25-31%, obese ≥ 32%; these are consumer-grade ranges, not clinical cutoffs.
• Gold standards: DEXA scan (1-3% residual error vs four-compartment), BodPod (air displacement plethysmography, ~2%), hydrostatic weighing (~1.5%); cadaver dissection (Clarys 1984) is the only true reference, with 24-30% body fat range across 12 cadavers.
• Tape measurement protocol: neck at the narrowest point below the larynx, waist at the narrowest point above the navel for men, at the level of the iliac crest for women (NIH 1998 protocol), hip at the widest point; tape horizontal, snug but not compressing, end-expiration; a 1 cm error in waist moves the result by ~1.5% body fat.
• Pitfall: BIA smart scales use proprietary equations (Segal 1988, Sun 2003) and assume a fixed hydration fraction (73% lean body mass); they disagree with each other by 3-5% body fat, and the assumption breaks for dehydrated athletes, menstruating women, and post-exercise measurements.
• Skinfold (Jackson-Pollock 1978, 3-site or 7-site) is the third common field method: sum of 3 or 7 skinfolds entered into age- and sex-specific regression, residual SE ~3.5%; requires a 7-10 mm constant-tension caliper like Harpenden or Lange.