Health & fitness
Body fat calculator
A tape measure and four numbers. Less precise than a DEXA scan, but repeatable at home — which makes it useful for tracking change over time.
How to take the measurements
The formula is sensitive to measurement technique, so consistency matters more than precision. Measure in the morning before eating, with the tape snug but not compressing the skin, and take each measurement three times and average them.
- Neck — just below the larynx, with the tape sloping slightly downward at the front.
- Waist — at the navel for men; at the narrowest point for women. Measure at the end of a normal exhale, without pulling the stomach in.
- Hip (women) — around the widest part of the buttocks, feet together.
If you are tracking change over weeks, use the same tape, the same time of day and the same landmarks every time. A consistent method with a small systematic error will show a real trend; an inconsistent method with a smaller error will not.
What the number is worth
The US Navy method was developed to screen large populations cheaply, not to produce laboratory-grade results. Validation studies typically put it within about three to four percentage points of a DEXA scan for most people, with larger errors at the extremes and for very muscular or very lean individuals.
That accuracy is fine for its actual purpose: watching a trend. If the estimate moves from 24% to 21% over three months using the same method, the direction is almost certainly real even if neither absolute figure is exact. Comparing your tape reading against someone else's DEXA result is not meaningful.
Body fat versus BMI
BMI uses only height and weight, so it cannot distinguish muscle from fat. A rugby player and a sedentary person of the same height and weight get the same BMI and very different body compositions. Adding circumference measurements gives the calculation something to work with.
Waist-to-height ratio, shown above, is the simplest useful measure of all. It needs two numbers, has no sex-specific formula, and a ratio under 0.5 — keeping your waist under half your height — is a widely used rule of thumb associated with lower cardiometabolic risk.
Ranges and what they mean
| 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 for physiological reasons, which is why the ranges differ by roughly eight to ten points. Sustained levels below the essential threshold are associated with hormonal disruption, loss of menstrual function, bone density loss and impaired immune function — the very low figures seen in competitive bodybuilding are held briefly and are not a health target.
Common questions
Why does my result differ from a smart scale?
Bioelectrical impedance scales estimate body fat by passing a small current through the body, and the reading is strongly affected by hydration, recent meals and exercise. The same person can vary several percentage points across a day. Neither method is authoritative; pick one and use it consistently.
What is a healthy body fat percentage?
Broadly, 14–24% for men and 21–31% for women covers the fitness and average bands, but health is not determined by a single number. Fat distribution, fitness, blood markers and family history all matter, and a doctor can interpret those together in a way a calculator cannot.
Can I use this if I am very muscular?
The circumference method tends to overestimate body fat in people with thick necks or large waists from muscle rather than fat, and to underestimate it in people with little muscle. If you sit far outside typical proportions, treat the figure as a rough baseline for tracking rather than an accurate measurement.
Is it accurate during pregnancy?
No. Body composition formulas are not valid during pregnancy, and body fat tracking is not an appropriate goal in that period. Speak to a midwife or doctor about what to monitor instead.