How Is Body Fat Percentage Calculated?
You can estimate body fat at home with nothing but a tape measure. This guide explains the US Navy circumference method used by our calculator — the exact formula, a worked example, what counts as a healthy range, and the measuring mistakes that throw the number off.
Body fat percentage is most easily estimated at home with the US Navy method, which uses your height plus tape-measure readings of your neck and waist (and hips for women). It is plugged into a logarithmic formula. A healthy range is roughly 14–24% for men and 21–31% for women; athletes sit lower.
The Short Answer
Body fat percentage is the share of your total body weight that is fat. The most practical at-home estimate is the US Navy circumference method: you measure your height, neck, and waist (women also measure hips), then feed those numbers into a formula based on the logarithm of those circumferences. It is free, repeatable, and accurate to within about 3–4% for most people.
What You Measure
The Navy method needs just a soft tape measure and a flat surface to read the numbers:
- Height — standing, without shoes.
- Neck — just below the larynx, tape sloping slightly down to the front.
- Waist — men measure at the navel; women measure at the narrowest point.
- Hips (women only) — at the widest point.
The Formula
The method uses these logarithmic equations (measurements in inches):
- Men: %BF = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
- Women: %BF = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
The logarithms are why doing this by hand is painful — it is exactly the kind of repetitive arithmetic a calculator is built for.
A Worked Example
A man who is 70 in tall, with a 38 in waist and a 16 in neck:
- waist − neck = 22 → log₁₀(22) ≈ 1.342
- log₁₀(70) ≈ 1.845
- %BF = 86.010 × 1.342 − 70.041 × 1.845 + 36.76 ≈ 22.2%
Healthy Body Fat Ranges
The American Council on Exercise classifies body fat roughly like this:
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Above average | 25%+ | 32%+ |
How to Measure Accurately
- Measure in the morning, before eating, over bare skin.
- Keep the tape snug but not compressing the skin.
- Stand relaxed — do not suck in your stomach.
- Take each measurement twice and average them.
The Most Common Mistake
The biggest error is treating any single reading as gospel. The Navy method is an estimate, not a DEXA scan — it can be off by a few points, especially for very lean or very muscular people. What it does well is track change. Measure the same way, at the same time of day, every couple of weeks, and watch the trend rather than the absolute number. A second mistake is confusing body fat with BMI: BMI only uses height and weight, so a muscular person can have a high BMI but low body fat. They answer different questions.
Related Guides & Calculators
- Body Fat Calculator
- BMI Calculator
- Ideal Weight Calculator
- Calorie & Macro Calculator
- What Is a Good BMI?
Frequently Asked Questions
What is the most accurate way to measure body fat at home?
The US Navy circumference method is the most practical at-home option — it needs only a tape measure and is accurate to within about 3–4% for most people. Skinfold calipers can be slightly more precise but require practice and a second person.
What is a healthy body fat percentage?
Roughly 14–24% for men and 21–31% for women is considered the fitness-to-average range. Athletes are often lower (6–13% for men, 14–20% for women), and dropping below the essential-fat level is unhealthy.
Is the US Navy body fat method accurate?
It is a reliable estimate for the general population, typically within a few percentage points of more advanced methods like DEXA. It is less accurate for very lean or very muscular bodies, but it is excellent for tracking change over time.
What is the difference between body fat percentage and BMI?
BMI uses only height and weight, so it cannot tell muscle from fat. Body fat percentage estimates how much of your weight is actually fat, which is why a muscular person can have a high BMI but low body fat.
How can I lower my body fat percentage?
Combine a moderate calorie deficit with strength training to preserve muscle while losing fat. Recalculate every few weeks and judge progress by the trend, not a single reading.