Use this body composition calculator to estimate your body fat percentage from simple tape measurements. It applies the U.S. Navy circumference method, a widely used field technique that needs only your height plus neck, waist, and (for women) hip circumferences. Along with body fat percentage, you get estimated fat mass, lean body mass, fat mass index (FMI), and lean mass index (FFMI) normalized to 1.8 m.
Body composition matters more than body weight alone: two people at the same weight can have very different health risk profiles. Athletes, coaches, dietitians, and anyone tracking fitness progress can use these results as a convenient baseline. The Navy equation is validated against hydrostatic weighing and is generally accurate within about ±3–4% for most adults when measurements are taken consistently.
This tool supports both metric and imperial units, includes sex- and age-based reference ranges, and provides a step-by-step breakdown so you can verify every number.
🧮 Body Composition Calculator
U.S. Navy circumference method — estimate body fat, fat mass, lean mass, FMI and FFMI from simple tape measurements.
Step-by-Step Guide
All you need is a flexible tape measure, a scale, and about five minutes. Take measurements in light clothing or without clothing for best accuracy, and keep the tape snug but not tight enough to indent the skin.
- Height: Stand barefoot against a wall with heels together and look straight ahead. Record the distance from the floor to the top of the head.
- Neck: Place the tape just below the larynx (Adam's apple), sloping slightly downward at the front. Keep the tape horizontal all the way around.
- Waist: For men, measure at the level of the navel. For women, measure at the smallest point between the ribs and the hips. Exhale normally and read the value without pulling the tape tight.
- Hips (women only): Measure at the widest part of the buttocks, with feet together.
- Weight: Weigh yourself in the morning, ideally after emptying your bladder and before eating or drinking.
Enter the values, choose your sex and preferred units, then press Calculate Body Fat. Age is optional but enables age-adjusted healthy range notes.
Formula Explained
The U.S. Navy circumference method was developed from studies comparing tape measurements with hydrostatic (underwater) weighing. It assumes that most body fat is stored around the waist, neck, and hips, so a small set of circumferences can predict overall body density. The original equations use inches:
Women: BF% = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
Using your body fat percentage, the calculator then derives fat mass (weight × BF% / 100), lean mass (weight − fat mass), Fat Mass Index (fat mass ÷ height²), and Fat-Free Mass Index (lean mass ÷ height²). FFMI is also normalized to a reference height of 1.8 m with the correction 6.1 × (1.8 − height in m), which makes comparisons between people of different heights fairer.
Worked Examples
- Convert to inches: height 70.9 in, neck 15.4 in, waist 34.6 in
- BF% = 86.010 × log₁₀(19.2) − 70.041 × log₁₀(70.9) + 36.76 ≈ 17.7%
- Fat mass ≈ 14.2 kg, lean mass ≈ 65.8 kg
- FMI ≈ 4.4 kg/m², normalized FFMI ≈ 20.3 kg/m²
- Category: Fitness range — within the healthy 8–20% band for his age
- Convert to inches: height 65.0 in, neck 12.6 in, waist 29.1 in, hip 37.0 in
- BF% = 163.205 × log₁₀(53.5) − 97.684 × log₁₀(65.0) − 78.387 ≈ 26.6%
- Fat mass ≈ 17.3 kg, lean mass ≈ 47.7 kg
- FMI ≈ 6.4 kg/m², normalized FFMI ≈ 18.4 kg/m²
- Category: Acceptable range — within the healthy 21–33% band for her age
Body Fat Reference Table
The American Council on Exercise and other organizations commonly cite these ranges for adults. They are general guidelines, not diagnostic thresholds.
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Acceptable | 18–24% | 25–31% |
| Obese | 25% and above | 32% and above |
Age-adjusted healthy ranges used in this calculator are derived from the Gallagher et al. reference data: for men, roughly 8–20% (20–39 y), 11–22% (40–59 y), and 13–25% (60+ y); for women, 21–33%, 24–35%, and 26–37% across the same age bands.
Common Mistakes to Avoid
- Measuring the waist at the belt line instead of at the navel or smallest point — this is the most common source of error.
- Pulling the tape too tight or letting it sag, which respectively under- or over-estimates body fat.
- Using a metal tape or a worn-out cloth tape that has stretched over time.
- Measuring at different times of day — water shifts make circumference readings vary by several centimeters.
- Comparing your result to reference tables that use different measurement protocols.
Frequently Asked Questions
Why does the formula not include age?
The original Navy equation uses only circumferences and height. Age-related shifts in fat distribution are handled here through age-adjusted healthy ranges, which is the approach most practitioners use when interpreting results.
What is the difference between FMI and FFMI?
FMI and FFMI break your body mass index (BMI) into fat and fat-free components. For example, a BMI of 25 could be an athlete with very low FMI and high FFMI, or a sedentary person with high FMI and low FFMI — the health implications are very different. This is why body composition is more informative than BMI alone.
Is this calculator suitable for children or pregnant women?
No. The Navy equation was developed for healthy adults and does not account for the body composition changes of growth or pregnancy. For children and adolescents, use pediatric growth charts; during pregnancy, follow your healthcare provider's guidance.
For more background, see the Wikipedia article on body fat percentage or the CDC body weight assessment resources.