How to use: Body fat percentage is a better health indicator than BMI because it distinguishes fat from muscle. This uses the US Navy method, which requires circumference measurements. Select your gender, enter your age, height (cm), and weight (kg). Measure your neck circumference at the thinnest point. Measure waist at navel level (widest part for women). Women also measure hip at the widest point. Take measurements in the morning, without clothes, using a flexible tape measure. Healthy ranges: Men 8-19%, Women 21-33%. Athletes are lower, essential fat minimum is 3-5% for men, 10-13% for women.