Body Fat Calculator (US Navy Method)
Estimate body fat using the US Navy method from neck, waist, and hip measurements in metric or imperial.
Returns fat %, lean mass, and visual gauge.
The US Navy body fat method estimates body fat percentage using simple tape-measure circumferences. It eliminates the need for underwater weighing or DEXA scans (a clinical-grade body composition scan). It’s accurate to within about ±3-4% of laboratory methods, which is good enough for tracking progress.
The Formulas (Inches)
The original formulas use inches, so this calculator converts metric inputs to inches internally before applying them.
Men:
% Body Fat = 86.010 × log₁₀(abdomen − neck) − 70.041 × log₁₀(height) + 36.76
Women:
% Body Fat = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
How to Measure
The accuracy depends entirely on getting the measurements right.
- Neck — measured just below the larynx (Adam’s apple), perpendicular to the neck’s long axis. Don’t pull tight.
- Abdomen (men) — at the navel level, horizontal, after a normal exhale. Don’t suck in.
- Waist (women) — at the narrowest point of the torso, horizontal.
- Hip (women) — at the widest point of the buttocks, horizontal.
- Height — without shoes.
A flexible cloth or fiberglass tape gives the cleanest reading. Avoid stretchy fabric tapes. Repeat each measurement twice and average if they differ.
Body Fat Reference Ranges
| Category | Men | Women |
|---|---|---|
| Essential fat | 2 to 5% | 10 to 13% |
| Athletes | 6 to 13% | 14 to 20% |
| Fitness | 14 to 17% | 21 to 24% |
| Average | 18 to 24% | 25 to 31% |
| Obese | 25%+ | 32%+ |
Women carry more essential fat for biological reasons (reproductive function), which is why the women’s “athletic” range starts higher than the men’s.
What the Calculator Returns
- Body fat percentage — your estimate
- Fat mass — your weight × body fat % (the kilograms or pounds that are fat tissue)
- Lean mass — your weight minus fat mass (muscle, bone, organs, water)
- Category — where you fall on the reference ranges
Worked Example
Male, 70 inches (5'10"), neck 15 inches, abdomen 36 inches:
% Body Fat = 86.010 × log₁₀(36 − 15) − 70.041 × log₁₀(70) + 36.76 = 86.010 × log₁₀(21) − 70.041 × 1.8451 + 36.76 = 86.010 × 1.3222 − 129.19 + 36.76 = 113.72 − 129.19 + 36.76 = 21.3% body fat — Average range
Same numbers in metric (178 cm, 38 cm neck, 91 cm abdomen): the calculator converts to inches and gets the same answer.
Limitations
The Navy method assumes “normal” body proportions. Outliers it doesn’t handle well:
- Bodybuilders with thick necks may score artificially low
- People with significant abdominal muscle hypertrophy (large rectus abdominis) may score higher than reality
- Pregnant women — the formula isn’t valid; use pre-pregnancy values
- Very short or very tall people — accuracy drops at extremes
For tracking progress, what matters most is consistency. Use the same tape, same measurement spots, same time of day. The week-over-week change is more reliable than the absolute number.