BMI vs Body Fat % (US Navy & BMI/Deurenberg)

How we calculate

BMI

BMI = weight / height² (metric, height in metres). Imperial uses 703 × lb / in². Adult categories: Underweight <18.5; Normal 18.5–24.9; Overweight 25–29.9; Obese ≥30.

Body Fat % methods

• Deurenberg (BMI, age, sex): BF% ≈ 1.20·BMI + 0.23·Age − 10.8·S − 5.4, where S=1 for males, 0 for females.
• US Navy tape method: Men: 86.010·log10(waist − neck) − 70.041·log10(height) + 36.76; Women: 163.205·log10(waist + hip − neck) − 97.684·log10(height) − 78.387. (All in inches.)

Body fat categories (ACE)

Men: Essential 2–5%, Athletes 6–13%, Fitness 14–17%, Average 18–24%, Obese ≥25%
Women: Essential 10–13%, Athletes 14–20%, Fitness 21–24%, Average 25–31%, Obese ≥32%

Privacy: All calculations happen locally in your browser.

Disclaimer: Informational only; not medical advice.

BMI vs Body Fat % — what each number really means

Body Mass Index (BMI) is a height-adjusted weight ratio. It is quick to calculate and useful for population-level screening, but it does not distinguish between fat mass and lean mass. That’s why this page pairs BMI with estimated body fat percentage (BF%), which better reflects composition.

Two estimation approaches we show

• Deurenberg (BMI + age + sex): A research-based equation that uses your BMI together with age and sex to estimate BF%. It requires no tape measurements, so it’s convenient, but it inherits BMI’s limitations in very muscular or very lean individuals.
• US Navy tape method: Uses circumferences (waist and neck for men; waist, hip, and neck for women) plus height. Because it looks at where tissue is carried, it can better track fat change over time than BMI alone. Accuracy depends on consistent measuring technique.

How to measure for the Navy method

• Height: Barefoot, upright, head neutral.
• Waist: At the navel (men) or the narrowest natural waist (women if different), relaxed abdomen, after a normal exhale.
• Hip (women): Widest point over the buttocks.
• Neck: Just below the larynx (Adam’s apple), shoulders relaxed.

Use a flexible cloth tape, keep it level and snug (not compressing skin), and measure at the same time of day. Hydration, recent meals, and posture can shift readings by ~0.5–1.5%.

Interpreting categories (adults)

BMI bands: Underweight (<18.5), Normal (18.5–24.9), Overweight (25–29.9), Obese (≥30). Body fat % bands vary by sex (e.g., men: athletes ~6–13%, average ~18–24%; women: athletes ~14–20%, average ~25–31%).

When the numbers disagree

If BMI shows “overweight” but BF% is in a healthy or athletic range, lean mass is likely elevating BMI. Conversely, a “normal” BMI with higher BF% may indicate low muscle mass. In both cases, BF% and waist circumference usually provide the more actionable picture for training and nutrition.

What this tool can and cannot tell you

• Great for: Private, repeatable check-ins; trend tracking; comparing BMI to BF% context.
• Not for: Diagnosing disease; assessing fat distribution (visceral vs subcutaneous); children/teens; pregnancy; elite bodybuilding or very atypical physiques.

Privacy & disclaimer: All calculations run locally in your browser—nothing is uploaded. Results are estimates for educational use by adults. For personalised health decisions, consult a qualified clinician. If you’re actively training, track trends over weeks, not single-day readings.