What This Body Fat Calculator Does
Body fat percentage answers what the bathroom scale can't: what is your weight made of? Two people at identical height and weight can look and perform completely differently depending on the fat-to-muscle split — which is why composition beats weight as a progress metric.
This tool uses the U.S. Navy circumference method: developed for fleet-wide fitness assessment, it estimates body fat from a few tape measurements (height, neck, waist, plus hips for women). Validation studies put it within ~3–4% of lab methods for most physiques — remarkable for a $2 tape measure.
The Formulas and How to Measure
Women: BF% = 495 ÷ (1.29579 − 0.35004·log₁₀(waist + hip − neck) + 0.22100·log₁₀(height)) − 450
Measurement technique decides accuracy — follow these exactly:
- Neck: just below the larynx, tape sloping slightly downward to the front. Don't flare the tape over muscle.
- Waist: men at the navel; women at the narrowest point. Relaxed, end of a normal exhale — no sucking in.
- Hips (women): at the widest point of the buttocks.
- Snug tape, no skin compression; same time of day each check; average two readings per site.
Body Fat Categories
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletic | 6–13% | 14–20% |
| Fit | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25%+ | 32%+ |
Women's ranges run higher by design — essential fat supports hormonal and reproductive health, and dropping near essential levels carries real medical risk for either sex. "Visible abs" typically appear around 10–14% (men) / 18–22% (women), varying with genetics and muscle.
Lean Mass Math: The Numbers That Matter for Training
Once you have BF%, two derived numbers guide programming:
Example — man, 88 kg at 22%
Fat mass 19.4 kg, lean mass 68.6 kg. If he cuts to 15% while keeping lean mass: goal weight = 68.6 ÷ 0.85 ≈ 80.7 kg — a 7.3 kg target that's honest about what should be lost. Tracking lean mass across a diet (re-measure monthly) is how you verify the deficit is burning fat, not muscle — pair with protein targets from the TDEE Calculator.
How the Navy Method Compares
| Method | Typical error | Cost/access |
|---|---|---|
| DEXA scan | ±1–2% | Clinic, $50–150 |
| Hydrostatic/Bod Pod | ±2–3% | Lab facilities |
| Navy tape (this tool) | ±3–4% | Free, home |
| Bioimpedance scales | ±3–8%, hydration-sensitive | Consumer devices |
| Calipers | ±3–5% (skill-dependent) | Cheap, learning curve |
The practical strategy: use the same method consistently and track the trend — method-to-method absolute differences don't matter when you're comparing you to you.
The Bottom Line
A tape measure and thirty seconds give you a composition estimate good enough to steer any cut or bulk. Measure carefully, log the number monthly under identical conditions, and judge progress by the trend in lean vs fat mass — not by the scale alone, and never by a single reading. For the fuller health picture, combine with BMI and waist circumference.