Body Fat Percentage: How to Measure It and What It Means
Body fat percentage tells you what fraction of your total weight is actually fat tissue. Unlike BMI, it accounts for muscle mass, making it a far more meaningful fitness and health metric. The body fat calculator uses the US Navy circumference method — all you need is a tape measure.
The US Navy Body Fat Formula
The Navy method was developed by the US Department of Defense as a practical, no-equipment-needed assessment for military fitness. It uses body circumference measurements as a proxy for fat distribution:
- Men: %BF = 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76
- Women: %BF = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
All measurements in centimetres. The formula is accurate to within 3–4 percentage points of DEXA scan results for most people, making it suitable for tracking fitness progress.
How to Take Measurements Correctly
- Use a flexible tape measure. Measure each site twice and average if readings differ.
- Height: stand flat against a wall without shoes.
- Neck: just below the larynx (Adam's apple), tape horizontal.
- Waist (men): at the navel, after a normal exhale.
- Waist (women): at the narrowest point, just above the navel.
- Hip (women only): at the widest point around the buttocks.
Healthy Body Fat Ranges by Gender
| Category | Men | Women | What it means |
|---|---|---|---|
| Essential fat | 2–5% | 10–13% | Minimum for organ function; not sustainable long-term |
| Athletic | 6–13% | 14–20% | Typical for competitive athletes and very active individuals |
| Fitness | 14–17% | 21–24% | Lean, active, above-average fitness level |
| Acceptable | 18–24% | 25–31% | Average population range; generally low health risk |
| Obese | 25%+ | 32%+ | Associated with elevated cardiovascular and metabolic risk |
Why Women Have Higher Body Fat Than Men
Women naturally carry more body fat due to hormonal differences (primarily oestrogen), and biological requirements for reproductive function. The fat stored around the hips, thighs, and breasts is largely sex-specific and hormonally regulated. This is why women's essential fat threshold (10–13%) is higher than men's (2–5%) and why the fitness categories use different thresholds for each gender.
How Body Fat Changes with Age
Muscle mass declines with age (sarcopenia) even when weight stays the same, which means body fat percentage tends to rise with age even without dietary changes. A 45-year-old at 20% body fat may be in similar health shape to a 25-year-old at 16–17%. Some practitioners add 1–2 percentage points to the upper bound of each category per decade after age 40.
Measuring Progress Over Time
A single measurement has inherent variability of ±1–2 percentage points due to hydration, timing, and measurement technique. Take three readings on three consecutive mornings and average them. Track monthly rather than weekly — meaningful fat loss shows up as a consistent downward trend over 4–8 weeks, not day-to-day fluctuations.
Calculate Your Body Fat Percentage
US Navy method. Just a tape measure. Instant results — no signup, no data stored.
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