What Is a Body Fat Calculator?
A body fat calculator estimates what share of your total weight comes from fat tissue versus lean mass — muscle, bone, organs, and water. While bathroom scales only report total pounds or kilograms, body fat percentage gives you a clearer picture of composition, which matters for fitness goals, health screening, and tracking progress during weight loss or strength training. Our free body fat calculator uses the US Navy circumference method, the same anthropometric approach used in military fitness assessments and many clinical field studies.
You enter height, weight, sex, age, and tape measurements at the neck and waist (plus hips for women). The calculator applies sex-specific logarithmic formulas to estimate body fat %, then derives fat mass and lean mass in kilograms or pounds. Results update instantly as you adjust inputs, so you can compare morning versus evening measurements or see how a few inches at the waist changes your estimate. For how we verify health formulas, see our methodology page.
How to Use This Body Fat Calculator
- Choose imperial (lb, in) or metric (kg, cm) units.
- Select your sex — the Navy formula differs for men and women.
- Enter age, height, and current body weight.
- Measure neck circumference just below the larynx, tape sloping slightly downward.
- Measure waist at the narrowest point (men) or at navel level (women).
- Women: add hip measurement at the widest part of the buttocks.
- Review body fat %, fat mass, and lean mass. Use the copy button to save results.
US Navy Body Fat Formula Explained
The US Navy method estimates body fat from circumference measurements and height using logarithmic regression equations developed from large military datasets. For men, the formula uses the log of (waist − neck) and the log of height. For women, it uses the log of (waist + hip − neck) and the log of height. Constants differ by sex because men and women store fat differently — men tend toward abdominal fat, while women carry more subcutaneous fat at hips and thighs.
The male equation published in Navy instruction OPNAVINST 6110.1J — Physical Readiness Program takes the form: %BF = 495 / (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450, with all circumferences and height in inches. The female equation substitutes (waist + hip − neck) and uses coefficients 1.29579, −0.35004, and 0.221. Our calculator converts metric inputs to inches internally so you can measure in either system.
After computing body fat percentage, we multiply by total weight to get fat mass, then subtract from total weight for lean mass. This decomposition helps athletes track muscle retention during a cut and helps anyone understand whether scale weight changes reflect fat loss or lean tissue changes. The CDC — About Adult BMI notes that BMI alone cannot distinguish fat from muscle; combining BMI with a circumference-based body fat estimate gives a more complete picture. Compare your BMI with our BMI calculator.
Body Fat vs. BMI and Weight
BMI (body mass index) divides weight by height squared and classifies results into underweight, normal, overweight, and obese categories. It is fast and population-level useful, but it misclassifies muscular people as overweight and can miss normal-weight individuals with high body fat — sometimes called "skinny fat." Body fat percentage directly targets adiposity rather than total mass.
Ideal weight ranges based on BMI 18.5–24.9 tell you what a healthy scale weight might look like for your height. Our ideal weight calculator shows that range alongside the Devine formula. Calorie and macro planning build on the same anthropometric inputs: use our calorie calculator, BMR calculator, and macro calculator to align nutrition with your composition goals.
Tips for Accurate Measurements
Consistency beats precision. Measure at the same time of day — morning before eating is common — with the same tape measure and the same posture. Stand relaxed, feet together, arms at sides. Do not suck in your stomach or flex muscles. Take each measurement twice and average if they differ by more than half an inch. Hydration, menstrual cycle, and recent meals can shift circumferences slightly; track trends over weeks rather than reacting to a single reading.
The Navy method is not appropriate for everyone. It was validated on adults and may be less accurate for very lean bodybuilders, people with significant abdominal distension, or those with mobility limitations that affect posture. DEXA scans, hydrostatic weighing, and bioelectrical impedance offer alternative estimates with their own trade-offs in cost, availability, and error. Treat this calculator as a practical field estimate, not a clinical diagnosis.
Frequently Asked Questions
What is body fat percentage?▼
Body fat percentage is the proportion of your total body weight that comes from fat tissue, as opposed to lean mass (muscle, bone, organs, and water). Unlike BMI, which only uses height and weight, body fat % attempts to separate adipose tissue from lean tissue. It is commonly used in fitness, clinical screening, and military fitness standards.
How accurate is the US Navy body fat method?▼
The US Navy circumference method is a field-tested anthropometric formula that correlates well with hydrostatic weighing and DEXA scans for most adults when measurements are taken consistently. Error margins of roughly 3–4 percentage points are typical. Accuracy depends on correct tape placement, relaxed posture, and measuring at the same time of day.
Where do I measure neck, waist, and hip?▼
For the Navy method, measure neck just below the larynx (Adam's apple), sloping slightly downward. Measure waist at the narrowest point for men, or at the navel level for women. Women also measure hips at the widest part of the buttocks. Keep the tape horizontal, snug but not compressing skin.
What is a healthy body fat percentage?▼
General fitness guidelines suggest roughly 10–20% for men and 18–28% for women, though athletes and older adults may fall outside these ranges while still being healthy. Essential fat minimums are about 2–5% for men and 10–13% for women. Context matters — muscle mass, age, and activity level all influence what is appropriate for you.
Why does this calculator ask for sex?▼
The US Navy publishes separate logarithmic formulas for men and women because fat distribution differs by sex. Men use height, neck, and waist only. Women add hip circumference because waist-to-hip ratio is a stronger predictor of body composition in female populations. Using the wrong formula produces misleading results.
Explore more tools in our health calculators category.