Free Calculator

Body Composition Calculator

Estimate your body fat percentage, lean muscle mass, and FFMI using the US Navy formula. No equipment needed — just a tape measure.

Waist: measure at navel height, relaxed, after exhaling.

Neck: measure just below the larynx, slightly downward slope.

What is body composition and why does it matter?

Body composition describes the ratio of fat mass to lean mass (muscle, bone, and organs) in your body. Two people can weigh exactly the same yet look and perform completely differently depending on how much of that weight is fat versus muscle.

Unlike BMI, which only uses height and weight, body composition analysis separates fat from lean tissue. This matters because muscle is metabolically active — it burns calories at rest, supports joint stability, and drives long-term health. Tracking body composition over time is far more meaningful than tracking weight alone.

The US Navy method explained

The US Navy circumference method estimates body fat from waist, neck, and (for women) hip measurements combined with height. It uses a logarithmic formula validated against hydrostatic weighing and DEXA scanning, with a typical error margin of ±3–4 %. While not as precise as a lab test, it is free, reproducible, and far more useful than BMI.

For best results, always measure at the same time of day (morning, before eating) and under the same conditions. Measure 2–3 times and use the average. Track changes every 4–8 weeks rather than weekly to see meaningful trends rather than noise from daily hydration fluctuations.

Frequently asked questions

The US Navy formula is accurate to within ±3 % compared to DEXA scanning. It is significantly more informative than BMI alone because it accounts for body proportions rather than just weight relative to height.
Measure at navel height in a relaxed standing position, after breathing out. Do not pull the tape tight — it should lie flat against the skin without compressing it.
For men, 10–20 % is considered a healthy range; for women, 18–28 %. Fitness levels sit around 8–15 % for men and 15–22 % for women. Sustained body fat below 5 % (men) or 12 % (women) is not healthy long-term.
BMI cannot distinguish between muscle and fat. A muscular person may be classified as "overweight" by BMI despite having a very low body fat percentage. Body composition measurement gives a much more accurate picture of health and fitness.
FFMI (Fat-Free Mass Index) measures muscularity independently of body fat. An FFMI below 18 is average or below average; 20–22 is athletic; 22–25 is excellent for an advanced lifter; above 25 approaches or exceeds the natural genetic ceiling for most people.
Once every 4–8 weeks is sufficient for tracking progress. Daily fluctuations in water retention, food volume, and hydration can skew results, so measuring under consistent conditions (same time of day, same hydration state) gives the most meaningful trend data.

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