In short: BMI cannot tell muscle from fat, which is why two people with identical BMI can look completely different. Body fat percentage is the number that actually tracks fitness progress, and you can estimate it at home with a 2-dollar tape measure.
Updated August 28, 2026, after a fresh review, with updated visuals and links.
This tool uses the US Navy circumference method: neck, waist (and hip for women) plus height go through a formula developed for the military, with an error of roughly 3-4 percent against lab methods.
How to measure correctly
The formula leans on a simple idea: for a given height, a thicker neck means more lean mass and a thicker waist means more stored fat. Take neck just below the Adam's apple, waist at the navel with a relaxed belly, and hip at the widest point.
Example: a 175 cm man with an 85 cm waist and a 38 cm neck lands near 17 percent, solidly in the fitness band. A 165 cm woman with an 80 cm waist, 95 cm hip and 34 cm neck lands near 29 percent.
Categories follow ACE ranges: for men, 6-13 percent reads athletic, 14-17 fitness, 18-24 average. For women the bands sit about eight points higher because essential fat is higher, which the calculator handles automatically.
Worth remembering
- Measure first thing in the morning before eating, three times, and average the numbers. Post-meal bloating can add two phantom centimeters.
- Track the trend every two weeks, not daily. The tape is a compass, not a microscope.
Frequently asked
Is the Navy method accurate?
Within about 3-4 percentage points of a DEXA scan for most body types. That makes it the best zero-equipment estimate; for a precise number, DEXA or a Bod Pod remains the reference.
Why does it need the hip only for women?
Fat distribution differs by sex. The formula was fitted on Navy data where hip circumference strongly improved predictions for women but not for men, so it is included only there.
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