Body fat percentage is what BMI tries and fails to estimate. Two adults at the same BMI can have wildly different metabolic risk, aesthetics and athletic capacity depending on where their weight lives — muscle or fat. Getting body fat within ±3–4% of true value takes 5 minutes with the right method and turns nutrition and training from guesswork into engineering.

The gold standard is DEXA (dual-energy X-ray absorptiometry) at ±1% accuracy, followed by hydrostatic weighing and BOD POD. At home, the best free options are Jackson-Pollock 7-site skinfold (±3% with practice), US Navy tape method (±3–4%), Jackson-Pollock 3-site, and YMCA. Bioelectrical impedance scales are cheap but unreliable — hydration alone can swing them 5–8% within a day.

The US Navy method deserves its popularity: only a tape measure needed, calibrated against real body composition data since 1984, and hard to fake. Measure neck at the narrowest point below the larynx, waist at the navel (do not suck in), and for women add hip circumference at the widest point. Take three measurements per site and average them. The formula uses log ratios of these against height. Do it fasted first thing in the morning for consistency.

Healthy ranges depend on sex and goals. Essential fat (needed for hormones and organ protection) is 3–5% for men, 10–13% for women — women's higher baseline is biological, not fat gain. Athletes typically 6–13% (men) or 14–20% (women). General fitness: 14–17% and 21–24%. Aesthetic 'lean' ranges are lower but usually unsustainable long-term without extreme discipline or PEDs.

Track body fat monthly, not daily. Aim for 0.5–1% loss per month during a cut to preserve muscle. Faster drops usually mean muscle loss — visible on the mirror as flat instead of lean. Combine body fat % with waist measurement, strength numbers and weekly progress photos. Any single measurement can lie; the trend across four data streams cannot.