Common Measurement Mistakes
The Navy method requires measuring neck and waist circumference (plus hips for women), and small errors compound significantly. A 1-centimetre difference in waist measurement can shift results by 1–2 percentage points. Always measure at the same time of day, ideally in the morning before eating, using a flexible tape measure pulled snug but not compressing skin. Measure at the narrowest part of the neck, at the navel for waist, and at the widest point of the hips. Take three measurements at each site and use the average. The BMI-based Deurenberg method is convenient but less accurate for athletes with high muscle mass — BMI cannot distinguish between fat and lean tissue, so muscular individuals will get falsely high body fat estimates. Neither method accounts for body fat distribution, which matters for health risk assessment. Visceral fat (around organs) carries more health risk than subcutaneous fat (under skin), but circumference measurements can't differentiate between them. For tracking trends over time, consistency matters more than absolute accuracy — use the same method, same time of day, and same measuring technique every time.
How Body Fat Percentage is Calculated
US Navy Method
The Navy method uses simple circumference measurements and is accurate to within 1–3% of DEXA scan results for most people. The formulas differ by sex:
Men: %BF = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
Women: %BF = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
BMI (Deurenberg) Method
The Deurenberg formula estimates body fat from BMI, age, and sex. It's less accurate than the Navy method but requires only weight, height, and age.
%BF = 1.20 × BMI + 0.23 × age − 10.8 × (1 if male, else 0) − 5.4
Body Fat Categories
Categories follow the American Council on Exercise (ACE) guidelines. Essential fat is required for bodily functions; athletic ranges are typical of competitive athletes; fitness indicates a lean physique; average is health-neutral; obese indicates elevated health risk.