Body fat percentage tells you what your weight is made of, which is the question BMI cannot answer. Two people at identical height and weight can differ enormously in composition, and it is composition that drives both health risk and appearance.
The US Navy circumference method
This calculator uses the circumference method developed for the US Navy. It regresses body fat against the ratio of waist to neck (plus hips for women) relative to height, and needs nothing more than a tape measure.
Accuracy is roughly ±3–4 percentage points against underwater weighing — good enough to track direction reliably, which is what most people actually need.
- Measure on bare skin, first thing in the morning, before eating.
- Waist: at the navel for men, at the narrowest point for women.
- Neck: just below the larynx, tape sloping slightly downward at the front.
- Hips (women): at the widest point of the buttocks.
- Keep the tape snug but do not compress the skin, and stay relaxed — do not suck in.
Body fat categories
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25%+ | 32%+ |
Women require substantially more essential fat than men for normal hormonal and reproductive function. Sustained very low body fat in women commonly disrupts the menstrual cycle and bone density.
How the measurement methods compare
| Method | Typical error | Notes |
|---|---|---|
| DEXA scan | ±1–2% | Reference standard; also reports regional distribution and bone density |
| Hydrostatic weighing | ±2% | Accurate but impractical |
| Bod Pod | ±2–3% | Air displacement; quick but scarce |
| Skinfold calipers | ±3–4% | Cheap and good in trained hands; poor in untrained ones |
| Navy circumference | ±3–4% | Tape measure only; used here |
| Bioelectrical impedance | ±4–8% | Smart scales; swings heavily with hydration |
Lean body mass is the number to protect
Lean body mass is everything that is not fat — muscle, bone, organs and water. During weight loss it is the variable worth defending: losing fat while keeping lean mass is what changes how you look and how strong you stay.
Two things preserve it reliably: adequate protein (roughly 1.6–2.2 g per kg of bodyweight) and continued resistance training through the deficit.
Worked example
Using the values pre-loaded in the calculator above:
| Input | Value |
|---|---|
| Units | Metric (cm / kg) |
| Age (yrs) | 30 |
| Sex | Male |
| Height (cm or in) | 175 |
| Weight (kg or lb) | 75 |
| Neck circumference (cm or in) | 38 |
| Waist circumference (cm or in) | 85 |
| Hip circumference (female) (cm or in) | 95 |
| Output | Value |
|---|---|
| Body fat | 16.9% |
| Fat mass | 12.7 kg |
| Lean mass | 62.3 kg |
| BMI method estimate | 20.1% |
| BMI | 24.49 |
Frequently asked questions
What is a healthy body fat percentage?
Broadly 14–24% for men and 21–31% for women. Athletes typically sit below those ranges; sustained levels near the essential-fat floor are not healthy long term for either sex.
How accurate is the Navy method?
Within about 3–4 percentage points of a DEXA scan for most people. It is more consistent than smart scales and tracks change reliably if you measure the same way each time.
Why does my smart scale disagree?
Bioelectrical impedance passes a current through the body and infers composition from resistance, which changes with hydration, food, exercise and skin temperature. Readings can swing several percent within one day.
Can I lose fat from one specific area?
No. Spot reduction does not work — fat is mobilised systemically, and where you lose it first is largely genetic. Training a muscle builds that muscle without preferentially burning the fat above it.