Health & Wellness

BMI and Body Fat Estimator

Go beyond BMI with U.S. Navy circumference body-fat estimates.

BMI was invented in 1832 by a Belgian statistician who explicitly said it should not be used to assess individuals. It mislabels muscular athletes as obese and misses "skinny fat" bodies with normal BMI but high body fat. This calculator computes both BMI (because doctors still use it) and the U.S. Navy circumference method for body fat percentage, which is more accurate for individuals.

Your measurements

Body fat percentage
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Enter your measurements to see BMI and body fat percentage.

Note: All calculations run in your browser. Nothing is sent to a server, stored, or tracked.

How this calculator works

The math, in plain English

BMI (Body Mass Index) = weight(kg) ÷ height(m)². It is a population-level statistic that works tolerably for sedentary non-athletes between ages 20-65, but it fails for athletes (muscle weighs more than fat), the elderly (who lose muscle and gain fat at the same BMI), and people under 5' or over 6'6".

The U.S. Navy body fat formula

Developed by the U.S. Naval Health Research Center, this method uses neck and waist (plus hip for women) circumferences to estimate body fat with surprising accuracy — typically within 3-4% of DEXA scans. The formula:

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 measurements in inches.)

A worked example
35-year-old male, 5'10", 185 lbs, 15.5" neck, 36" waist. BMI: 26.5 (overweight). Navy body fat: ~21.5% — in the "acceptable" range for men (18-25%). This is a case where BMI misleads: the man has a normal body fat level despite "overweight" BMI.

Healthy body fat ranges

Men: Essential 2-5%, Athletic 6-13%, Fitness 14-17%, Acceptable 18-25%, Obese 25%+.

Women: Essential 10-13%, Athletic 14-20%, Fitness 21-24%, Acceptable 25-31%, Obese 32%+.

Women carry essential fat in breasts, hips, and thighs — this is biology, not a flaw. The "essential" minimum is higher because reproductive health depends on it.

The waist-to-height ratio

A newer metric gaining favor: waist circumference should be less than half your height. A 5'10" (70") man should have a waist under 35". This ratio predicts cardiovascular risk better than BMI because abdominal fat (visceral fat around the organs) is the metabolically dangerous kind.

FAQ

Common questions

Why is BMI still used if it is inaccurate?
Because it is free, fast, and correlates with body fat at the population level. For insurance underwriting, military fitness standards, and large epidemiological studies, BMI is good enough. For individuals — especially athletes, the elderly, and people of Asian or Pacific Islander descent — body fat percentage is far more meaningful.
How accurate is the Navy body fat method?
Typically within 3-4% of DEXA scans (the gold standard). It is more accurate than bioelectrical impedance scales (which can vary by 5-8% based on hydration) and skinfold calipers (which require trained technicians). For home use, the Navy method is the best combination of accuracy and convenience. Measure in the morning, before eating, for consistency.
How do I measure my waist correctly?
Stand relaxed, exhale normally, and measure at the level of your navel (belly button) — not at the "narrowest point," which varies. Keep the tape horizontal and snug but not compressing skin. Do not "suck in" — measure your natural waist. For neck, measure just below the larynx (Adam's apple) with tape flat against skin.
What body fat percentage should I aim for?
For health, the "acceptable" range (men 18-25%, women 25-31%) is fine. For aesthetics, the "fitness" range (men 14-17%, women 21-24%) is achievable for most people with consistent training. The "athletic" range (men 6-13%, women 14-20%) requires dedicated training and nutrition — and is hard to sustain. Below the "essential" minimum is dangerous.
I have a normal BMI but high body fat — what should I do?
This is called "normal weight obesity" or "skinny fat," and it is increasingly common in sedentary populations. The fix is resistance training: build muscle to lower body fat percentage, even if your weight stays the same. Cardio alone will not fix it — you need to add muscle, not just lose fat. Aim for 2-3 strength training sessions per week.
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Disclaimer: This calculator is provided for educational and informational purposes only. It does not constitute financial, tax, legal, medical, or professional advice. Results depend on the accuracy of the inputs you provide and the assumptions documented above. Always consult a qualified professional before making decisions based on these calculations.