Body Fat Calculator (US Navy Method)
Estimate body fat percentage from neck, waist (and hip) circumferences with the US Navy formula — plus your fitness category.
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How Is Body Fat Percentage Calculated?
The US Navy method estimates body fat from circumference measurements rather than weight alone. For men, it uses the difference between waist and neck circumference together with height: %BF = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450, all measurements in centimetres. For women, the formula adds a hip term — waist + hip − neck replaces waist − neck — with a different set of logarithmic coefficients. These equations were validated against underwater (hydrostatic) weighing, the gold-standard method available when the Navy developed them, and typically land within about ±3–4 percentage points of that reference.
Worked example: a man 180 cm tall with a 90 cm waist and 40 cm neck has waist − neck = 50 cm. Plugging those numbers into the formula gives roughly 20% body fat — solidly in the "average" category. Drop the waist to 80 cm with the same neck and height, and the estimate falls to roughly 13%, moving into the "fitness" category — which illustrates how sensitive the result is to the waist measurement specifically.
What You Should Know
Measurement technique matters more than the formula itself. Waist should be measured at the navel for men and at the narrowest point of the torso for women, standing relaxed — not sucking the stomach in or pulling the tape tight. Neck is measured just below the larynx (Adam's apple), with the tape sloping slightly downward at the front, the way a shirt collar sits. Hip, for women, is measured at the widest point around the buttocks. A tape that is twisted, positioned too high or low, or pulled at inconsistent tension can shift the estimate by several percentage points — more than the day-to-day change you are actually trying to track.
Fitness categories commonly used alongside this method (ACE) are: for men, essential fat 2–5%, athletes 6–13%, fitness 14–17%, average 18–24%, obese 25% and above; for women every threshold runs roughly ten points higher, since female physiology requires more essential fat for hormonal and reproductive function. Because the method is a mathematical estimate rather than a direct tissue measurement, treat single readings loosely and rely on the trend across repeated, consistently-measured readings — ideally at the same time of day, before eating. This tool is informational and does not replace clinical body-composition assessment such as DEXA scanning or hydrostatic weighing.
Frequently Asked Questions
How accurate is it versus a DEXA scan?
Typically within about 3–4 percentage points of hydrostatic weighing — good enough for tracking your own trend monthly with consistent technique, though not precise enough to compare directly against someone else's DEXA result.
Why does my reading jump day to day?
Tape position and tension, bloating, recent meals, and hydration level can each shift your waist reading by a centimetre or more, which is enough to move the calculated percentage. Measure at the same time of day — ideally each morning before eating — and average two readings.
What body fat percentage should I aim for?
For general health, roughly 10–20% for men and 18–28% for women covers most goals. Visible abdominal muscle definition usually requires closer to 10–12% for men and 16–19% for women — going below the essential-fat minimum (about 2–5% men, 10–13% women) is dangerous, not a fitness achievement.
Is this method reliable for very muscular or very lean people?
Less so at the extremes. Because the formula relies only on waist, neck (and hip) circumference rather than a direct fat measurement, very lean or very heavily muscled bodies can produce estimates that drift further from DEXA or hydrostatic results than the typical ±3–4% margin. In these cases, skinfold calipers with a trained assessor or a DEXA scan give more trustworthy numbers.
What should I do if the result looks impossibly high or low?
Double-check your measurements first — the formula requires waist minus neck (plus hip minus neck for women) to be a positive number measured in the same unit, and a single misread centimetre near the collar or navel can swing the result noticeably. If the numbers still look off after remeasuring carefully, treat the tool as approximate and consider a body-composition scan for an accurate baseline.
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Body Fat Calculator (US Navy Method)
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