The Navy formula
The U.S. Navy method estimates body fat from tape measurements instead of a lab scan. It compares the circumference where you store fat (waist, plus hips for women) against a frame reference (neck and height). Men: 495 / (1.0324 - 0.19077 x log10(waist - neck) + 0.15456 x log10(height)) - 450. Women: 495 / (1.29579 - 0.35004 x log10(waist + hip - neck) + 0.22100 x log10(height)) - 450. All measurements in centimeters.
How to measure
Measure in the morning, standing relaxed, the tape snug but not compressing the skin. Neck: just below the Adam's apple. Waist: at the narrowest point, usually around the navel for men. Hips (women): at the widest point. Take each measurement twice and use the average.
Worked example
Man, height 180 cm, neck 40 cm, waist 90 cm, weight 85 kg: waist - neck = 50, log10(50) = 1.699, log10(180) = 2.255. Density term = 1.0324 - 0.19077 x 1.699 + 0.15456 x 2.255 = 1.0569. Body fat = 495 / 1.0569 - 450 = 18.4%. Fat mass = 85 x 0.184 = 15.6 kg, lean mass = 69.4 kg, category Average.
What to do with the number
Body fat percentage tells you whether a change in weight is fat or muscle. Pair it with the TDEE calculator to set a calorie target, then the macro calculator to split those calories into protein, carbs, and fat. Tape-method estimates usually land within 3 to 4 percentage points of lab measurements, close enough to track trends over weeks.