About the body fat calculator
This body fat calculator estimates your body fat percentage from tape measurements using the U.S. Navy method, and from BMI, age and sex using a second formula. It also splits your weight into fat mass and lean mass and places the result in a commonly used category range.
Neither method is a clinical measurement. They are practical ways to track relative changes when a scan is not available.
Who should use the body fat calculator
- Gym-goers and runners tracking body composition alongside weight.
- People who want a second view beyond BMI.
- Anyone comparing two estimation methods.
How to use the body fat calculator: what each input means
- Sex, age, height, weight
- Used by both methods.
- Neck and waist
- In the usual Navy technique, the neck is measured just below the larynx and the waist at the navel for men or at the narrowest part of the torso for women. Use a flexible tape, keep it level and snug without squeezing, and repeat the same way each time.
- Hips (women)
- Measured at the widest point of the buttocks.
- Units
- Metric or inches; US/UK users can choose their units.
How the body fat calculator works: method and formula
The Navy method uses logarithms of circumference differences and height to predict body density, and converts density to body fat percentage. It uses neck and waist for men, and neck, waist and hip for women.
The BMI method estimates body fat as 1.2 × BMI + 0.23 × age, minus 16.2 for men or 5.4 for women. It is less specific than tape measurements because BMI cannot tell muscle from fat.
- Navy (male, cm)
- BF% = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
- Navy (female, cm)
- BF% = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
- BMI method
- BF% = 1.2 × BMI + 0.23 × age − 16.2 (male) or − 5.4 (female)
How to read your body fat calculator result
Categories (essential fat, athletes, fitness, average, above average) come from commonly published ranges and differ between men and women. Essential fat is the minimum needed for normal body function, not a goal.
Measure at the same time of day with the same tape position and pressure. Track the trend over weeks rather than any single reading; small measurement errors move the estimate noticeably.
Body Fat Calculator worked example
A 30-year-old man, 180 cm tall, 80 kg, with a neck of 38 cm and a waist of 90 cm.
- Waist − neck = 52 cm.
- Navy method: 495 ÷ (1.0324 − 0.19077 × log10(52) + 0.15456 × log10(180)) − 450 = 19.8%.
- Fat mass = 80 × 19.8% = 15.8 kg, lean mass = 64.2 kg.
- BMI method: BMI is 24.7, so 1.2 × 24.7 + 0.23 × 30 − 16.2 = 20.3%.
About 19.8% body fat by the Navy method (average range) and 20.3% by the BMI method.
Practical uses of the body fat calculator
- Tracking body composition during a training block.
- Estimating lean mass for the Katch-McArdle BMR equation.
- Cross-checking the two methods to see how far apart they are.
Assumptions and limitations of this calculator
- Estimates only; typical errors of several percentage points are normal. DEXA scans, skinfold callipers and bioimpedance have their own accuracy and limits.
- Tape-measure accuracy depends on technique. Very muscular or very lean people may find the formulas unreliable.
- Adults only. Do not use these categories to judge children or as a goal for dieting.
This calculator is for general information and education only. It is not medical advice, a diagnosis or a treatment plan. Results are estimates based on population formulas and cannot account for your individual health. If you have a medical condition, are pregnant, are under 18, or are planning a major change to your diet or exercise, speak to a doctor or registered dietitian first.