Complete guide
Reviewed July 2026Body fat percentage tells you something the bathroom scale never can: how much of your weight is fat versus everything else - muscle, bone, water and organs. Two people can weigh the same and have completely different bodies, and body fat percentage is what separates 'lean and muscular' from 'skinny-fat'. It's a far better health and fitness marker than weight or BMI alone.
This calculator uses the US Navy circumference method, which estimates body fat from a few tape measurements (waist, neck, and hips for women) plus height. It's free, needs only a tape measure, and correlates reasonably well with more expensive methods when measured carefully.
Below: how the method works, healthy ranges by sex, worked examples, an honest comparison of measurement methods and their accuracy, and why body fat percentage succeeds where BMI misleads.
How the US Navy method works
Men: %BF = 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450 Women: %BF = 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450 (all measurements in cm)
The method uses the fact that fat tends to accumulate at specific sites - the waist for both sexes, and the hips for women - relative to the neck and overall height. It plugs these circumferences into a validated logarithmic formula. Accurate measurement matters more than anything: measure at the right spots, relaxed, without pulling the tape tight.
Healthy ranges
Women naturally and healthily carry more body fat than men, largely for hormonal and reproductive reasons - which is why the ranges differ and why comparing a woman's percentage to a man's is meaningless. Essential fat is the minimum needed for survival; going below it is dangerous.
| Category | Men | Women |
|---|---|---|
| Essential fat | 2-5% | 10-13% |
| Athletes | 6-13% | 14-20% |
| Fitness | 14-17% | 21-24% |
| Acceptable | 18-24% | 25-31% |
| Obese | 25%+ | 32%+ |
Worked example
- Man, height 178 cm, neck 38 cm, waist 90 cm.
- waist - neck = 52 cm; log10(52) = 1.716; log10(178) = 2.250.
- Denominator = 1.0324 - 0.19077 x 1.716 + 0.15456 x 2.250 = 1.0324 - 0.3274 + 0.3478 = 1.0528.
- %BF = 495 / 1.0528 - 450 = 470.1 - 450 = about 20.1% - in the 'acceptable' range, near the fitness border.
Method accuracy and why it beats BMI
| Method | Accuracy | Cost / access |
|---|---|---|
| Navy tape (this calculator) | Moderate (+/-3-4%) | Free, tape measure |
| Skinfold calipers | Moderate-good with skill | Cheap, needs technique |
| Bioelectrical impedance (scales) | Variable, hydration-sensitive | Cheap, home scales |
| DEXA scan | High (reference standard) | Expensive, clinic |
| Hydrostatic weighing | High | Specialist facility |
Measuring accurately
- Waist: measure at the narrowest point (men often at the navel), relaxed, not sucked in.
- Neck: just below the larynx, tape sloping slightly down to the front.
- Hips (women): at the widest point of the buttocks.
- Keep the tape snug but not compressing the skin, and measure at the same time of day for consistency.
Using this calculator
- Select your sex and enter height, neck and waist (plus hip for women), all in the same units.
- Read your estimated body fat percentage and compare it to the healthy ranges.
- Track the trend over weeks rather than obsessing over a single reading.
- For a fuller picture, pair it with a waist measurement and your fitness performance.
Common mistakes
- Measuring the waist while holding your breath or sucking in - inflates leanness.
- Placing the tape at the wrong site (waist at the belt line rather than the natural waist/navel).
- Comparing men's and women's percentages directly - the healthy ranges differ.
- Trusting a single reading; measure consistently and follow the trend.
- Expecting tape methods to match a DEXA scan exactly - they estimate within a few percent, not to the decimal.
Frequently asked questions
Glossary
- Body fat percentage
- The proportion of total body weight that is fat tissue.
- US Navy method
- A circumference-based body-fat estimate using tape measurements and height.
- Essential fat
- The minimum body fat needed for survival - ~2-5% men, ~10-13% women.
- Lean body mass
- Everything that isn't fat: muscle, bone, water and organs.
- DEXA
- Dual-energy X-ray absorptiometry - a reference-standard body-composition scan.
- Bioimpedance
- A body-fat method using a small electrical current; sensitive to hydration.
- Skinfold calipers
- A pinch-based method estimating fat from skin-fold thickness.
- Skinny-fat
- Normal weight with high body fat and low muscle - missed by BMI.
Key takeaways
Body fat percentage measures composition - fat versus lean mass - which the scale and BMI miss entirely. The free US Navy tape method estimates it within a few percent from neck, waist (and hip) circumferences and height. Healthy ranges differ by sex (roughly 14-24% men, 21-31% women), essential fat is the survival floor, and the goal is a healthy range, not the lowest number. Measure carefully, compare within your sex, and track the multi-week trend.
Enter your measurements above for your estimated body fat percentage; then track it monthly alongside your training to see composition change even when the scale doesn't.