Body Fat Calculator: How It Works
Body fat percentage answers the question BMI cannot: how much of your weight is fat rather than muscle, bone and water. This calculator uses the US Navy circumference method, which needs only a tape measure and gets within a few points of far more expensive equipment.
The Navy method
It uses circumference measurements, exploiting the fact that fat distributes predictably around the waist, neck and hips.
Men: 495 ÷ [1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)] − 450
Women: 495 ÷ [1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)] − 450
All measurements in centimetres. Measurement technique matters more than the arithmetic: measure at the navel for waist, below the larynx for neck, at the widest point for hips, at the same time of day, without pulling the tape tight, and ideally before eating.
Reading the result
| Category | Men | Women |
|---|---|---|
| Essential fat | 2 – 5% | 10 – 13% |
| Athletic | 6 – 13% | 14 – 20% |
| Fitness | 14 – 17% | 21 – 24% |
| Average | 18 – 24% | 25 – 31% |
| Above average | 25%+ | 32%+ |
Women carry more essential fat than men — this is physiological, related to hormonal and reproductive function, and not a difference to be trained away. Sustained very low body fat is associated with hormonal disruption, loss of menstrual function, poor bone density and impaired immunity in both sexes.
How the methods compare
| Method | Typical error | Practicality |
|---|---|---|
| DEXA scan | ±1–2% | Clinical, costly, considered the reference |
| Hydrostatic weighing | ±2% | Specialist facilities only |
| Skinfold calipers (trained) | ±3% | Cheap; highly dependent on the operator |
| Navy circumference | ±3–4% | Free, repeatable, needs only a tape |
| Bioelectrical impedance scales | ±4–8% | Convenient; strongly affected by hydration |
The impedance scales sold for home use pass a small current through the body and infer composition from resistance. Because water conducts, the reading swings with hydration, recent meals, exercise and time of day — sometimes by several percentage points within a single day. They are best used at a fixed time under fixed conditions, and read as a trend rather than a value.
Why the trend matters more than the number
Every accessible method carries a few points of error. What none of them have is random error — each is consistently biased in the same direction for the same person. So while your absolute figure may be off by three points, a fall from 26% to 22% measured the same way is real information. Measure monthly, under identical conditions, and pay attention to the direction.
Lean body mass
Once you know body fat percentage, lean mass follows: LBM = weight × (1 − body fat %). Someone at 80 kg and 20% fat carries 64 kg of lean mass. This is a more useful figure than weight for two purposes: setting protein intake, and checking during weight loss whether you are losing fat or muscle. Weight falling while lean mass holds steady is exactly what you want; both falling together means the deficit is too aggressive, protein is too low, or resistance training is missing.
This page is general information, not medical advice. Very low body fat carries genuine health risks. If you have concerns about your body composition or your relationship with food or exercise, speak to a doctor or registered dietitian.