Body Fat Calculator
The scale can't tell muscle from fat; a tape measure gets surprisingly close. The US Navy developed this circumference formula to field-assess recruits without lab equipment — neck and waist for men, plus hips for women — and validation studies put it within roughly ±3% of DEXA scans for most people. Enter four numbers and you get a body fat percentage, a fitness category, and the fat-versus-lean split of your current weight.
That lean-mass number is the one worth tracking: at a constant body weight, falling body fat means you are recomposing — gaining muscle while losing fat — something the scale alone would hide. Re-measure with the same tape at the same time of day, and the page remembers your measurements locally so the monthly comparison takes thirty seconds.
How to use
- Measure per the method: neck below the larynx, waist at the navel (men) or narrowest point (women), hips at the widest point for women.
- Enter height, circumference measurements and your weight — the estimate, category and mass split appear instantly.
- Share the estimate in one tap, or bookmark the page — measurements are remembered for your monthly recheck.
Frequently asked questions
How does the Navy body fat formula work?
It models the body as cylinders and logs: for men, 495 ÷ (1.0324 − 0.19077×log10(waist−neck) + 0.15456×log10(height)) − 450; women add hip circumference to the equation. The log terms capture how fat is distributed — exactly what a scale cannot see.
What is a healthy body fat percentage?
For men, roughly 14-18% is athletic-fitness range and 18-25% average; for women, 21-25% athletic-fitness and 25-32% average — women carry essential fat the male formula never sees. Below the essential floor is a health risk, not an achievement.
How accurate is the tape method?
Within about ±3% of DEXA for most body types when measured carefully, which is enough to track trends. Consistency beats precision: same tape tension, same time of day, same hydration state — the direction of change is the signal.
Why track body fat instead of just weight?
Weight cannot distinguish recomposition from stagnation. Two people at 80 kg can hold 15% or 30% body fat with opposite health profiles — and during a training block, stable weight with falling body fat is the win the scale would misread as failure.