Body Fat Percentage Calculator
Estimate your body fat percentage from a tape measure using the US Navy circumference method — the same protocol the US military uses for fitness compliance. Enter your waist, neck, height (and hip if female) and the calculator returns body fat percentage, accurate to within ±3–4 percentage points compared to a DEXA scan.
About this calculator
The US Navy formula derives body fat percentage from circumference measurements that act as proxies for subcutaneous fat distribution. For men: BF% = 495 / (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450. For women: BF% = 495 / (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450. All measurements are in centimetres. The equations were developed by Hodgdon and Beckett at the Naval Health Research Center in the 1980s using a sample of 4,000+ Navy personnel, validated against hydrostatic (underwater) weighing — at the time the gold-standard reference method. The waist − neck subtraction isolates the trunk-fat-related component of the waist measurement by removing the influence of frame size (proxied by neck circumference). The female formula adds hip circumference because women store a meaningful fraction of fat in the gluteal-femoral region, which a waist-only measure would miss. The log₁₀ transformations reflect the geometric non-linearity between circumference and volume. Measurement technique matters: measure the waist at the navel level (men) or at the narrowest point above the navel (women) on a normal exhale, the neck just below the larynx, and the hip at the widest gluteal protrusion. Keep the tape snug but not compressing the skin, level with the floor. Edge cases: the formula can return implausibly low or even negative values for very lean, athletic builds where waist ≈ neck — these are mathematical artefacts of the log term collapsing, not real body composition. It tends to overestimate body fat in very muscular people whose waist circumference is high due to abdominal muscle development, and underestimate in skinny-fat individuals whose visceral fat is high but waist still looks modest. Standard error vs DEXA is ±3–4 percentage points, fine for tracking trends over weeks but not adequate for a single-point clinical diagnosis.
How to use
Example 1 — Adult male. You are 178 cm tall with an 85 cm waist and 38 cm neck. log₁₀(85 − 38) = log₁₀(47) ≈ 1.6721. log₁₀(178) ≈ 2.2504. Denominator = 1.0324 − (0.19077 × 1.6721) + (0.15456 × 2.2504) = 1.0324 − 0.3190 + 0.3478 = 1.0612. BF% = 495 ÷ 1.0612 − 450 ≈ 16.5%. ✓ That places you in the "fitness" range (14–17%) for adult men per ACE benchmarks. Example 2 — Adult female. You are 165 cm tall, waist 75 cm, hip 100 cm, neck 32 cm. log₁₀(75 + 100 − 32) = log₁₀(143) ≈ 2.1553. log₁₀(165) ≈ 2.2175. Denominator = 1.29579 − (0.35004 × 2.1553) + (0.22100 × 2.2175) = 1.29579 − 0.7544 + 0.4900 = 1.0314. BF% = 495 ÷ 1.0314 − 450 ≈ 30.0%. ✓ That sits in the "acceptable" range (25–31%) for adult women — for context, the "fitness" range for women is 21–24% and "athlete" is 14–20%.
Frequently asked questions
How accurate is the Navy method compared to DEXA?
Validation studies place the standard error of estimate for the US Navy method at roughly ±3–4 percentage points compared to DEXA or hydrostatic weighing, with R² values around 0.85 in the original Hodgdon and Beckett samples. This is more than accurate enough to track changes in body composition over weeks or months — if your number drops from 22% to 18% across three months of consistent measurement, that is a real trend, not noise. It is not accurate enough to claim a precise single-point body fat percentage; a Navy-method "15%" could realistically be 12% or 18% in a DEXA scan. Consistency matters more than absolute accuracy for tracking: same tape, same time of day (first thing in the morning is best), same measurement technique, same measurer every time. For competitive bodybuilding, modelling, or clinical applications, use DEXA, hydrostatic, or air-displacement (BodPod) testing instead.
What body fat percentage is actually healthy?
The American Council on Exercise publishes the most widely cited reference bands. For men: essential fat 2–5%, athletes 6–13%, fitness 14–17%, acceptable 18–24%, obese ≥25%. For women: essential fat 10–13%, athletes 14–20%, fitness 21–24%, acceptable 25–31%, obese ≥32%. Women’s ranges are systematically higher because of essential reproductive and hormonal fat that men do not need. Going below the essential floor (especially for women) suppresses sex hormones, disrupts menstrual cycles, and increases stress-fracture risk — a cluster of symptoms known as RED-S (relative energy deficiency in sport). At the other end, obesity-range body fat is associated with increased cardiometabolic risk independent of BMI. For most adults aiming at general health, the fitness or acceptable bands are sensible targets; the athlete and essential ranges are appropriate only for competitive athletes in season, not for year-round living.
Why does the female formula include hip but the male formula does not?
Sex differences in fat distribution are large enough that a single formula would not fit both. Men tend to deposit fat centrally in the abdomen ("android" distribution), so waist circumference alone captures most of the variance in their total body fat. Women tend to deposit fat in the gluteal-femoral region ("gynoid" distribution), so a measure that ignored hip circumference would systematically underestimate female body fat. Hodgdon and Beckett tested both single-site and multi-site formulas during the original derivation and found the female model needed the hip term to reach acceptable accuracy. Post-menopause, female fat distribution shifts toward the male pattern, which is one reason the formula loses some accuracy for older women — the equation was derived primarily from younger Navy personnel.
What are the most common measurement mistakes that throw off the result?
Measuring the waist at the belt line rather than at the navel (or, for women, at the narrowest point) — even a 2 cm difference at the waist can shift the result by a full percentage point. Measuring after eating or in the evening rather than first thing in the morning, when the abdomen is at its thinnest. Pulling the tape too tight (compressing the skin) or too loose (sagging away from the body). Forgetting to keep the tape level all the way around the body. Taking neck circumference at the wrong height (it should be just below the larynx, perpendicular to the long axis of the neck). And for repeat measurements, having different people take the readings — even small differences in technique between two measurers can produce a larger week-to-week change than any real body composition shift. Always have the same person measure under the same conditions.
When should I not use this calculator?
Skip it during pregnancy and the postpartum period — body composition is in flux and the Navy formula was not derived for this population. Skip it if you are under 18 or over about 65, both ends where validation data is sparse. The formula systematically overestimates body fat in very muscular athletes (bodybuilders, powerlifters, rugby players) whose waist circumference reflects developed abdominal muscle as much as fat, and underestimates in older sedentary adults with visceral fat concentrated around the organs but not bulging at the waist — for either group, DEXA gives a far truer picture. Do not use it as a diagnostic tool for eating disorders or as a daily anxiety check; the ±3–4 point error band makes day-to-day comparisons meaningless. And do not chase the "athlete" or "essential fat" bands as a general health goal — for women in particular, very low body fat suppresses reproductive function and is incompatible with year-round health.