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Body Fat Calculator.
A tape measure and three numbers get you within a few points of a lab scan. This runs the U.S. Navy formula and shows your fat mass, lean mass and category.
Estimates for healthy adults, not a diagnosis or medical advice. Formulas like these are population averages and can be off for athletes, older adults, pregnant people and anyone with a medical condition. Talk to a clinician before changing your diet or training. Nothing you type here leaves your browser. Terms.
Body fat categories (American Council on Exercise)
Guide
How to use the Body Fat Calculator
This calculator estimates what percentage of your weight is fat using nothing but a tape measure. It uses the U.S. Navy circumference method, developed to screen large numbers of people quickly without lab equipment. Measuring carefully is most of the job.
- Choose your Units, US (ft, in, lb) or Metric (cm, kg), and your Sex. Men and women use different measurements and formulas.
- Enter your Age, Height and Weight. Height goes into the main formula; weight turns the percentage into fat and lean mass, and weight and age feed the BMI comparison.
- Measure your Neck just below the larynx, tape angled slightly downward toward the front. Snug, not compressing the skin.
- Measure your Waist, at the navel for men and at the narrowest point of the torso for women. Stand relaxed, breathe out normally, keep the tape level.
- Women also fill in the Hip (women) field, measured at the widest point, tape level, feet together.
- Read the result: body fat percentage with its category, fat and lean mass, a BMI-method estimate, and a category table.
How the math works
The Navy method rests on a simple observation: for a given height, the difference between your waist and your neck tracks how much fat you carry around your middle. The neck is included because it scales with your frame, so subtracting it strips out some of the "big person" effect and leaves mostly the "fat around the middle" effect. Women add the hip because they store proportionally more fat below the waist.
The formula takes logarithms of those measurements, which lets one equation fit lean and heavy people reasonably well, using constants fit against underwater weighing, a lab method for measuring body density. Fat mass is weight times the percentage; lean mass, meaning muscle, bone, organs and water, is the rest. The BMI comparison, the Deurenberg equation, needs no tape.
The formula
Men: Body fat % = 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
Women: Body fat % = 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.22100 × log10(height)) − 450
Fat mass = Weight × Body fat % ÷ 100 Lean mass = Weight − Fat mass
All lengths are in centimeters (2.54 cm per inch); log10 is the base-10 logarithm. The BMI comparison uses Body fat % = 1.20 × BMI + 0.23 × age − 5.4, minus a further 10.8 for men.
Worked example
Take a 30-year-old man, 5 feet 11 inches, 185 pounds, with a 15.5-inch neck and a 34-inch waist. In centimeters: height 180.34, neck 39.37, waist 86.36, so waist minus neck is 46.99.
The logarithms are log10(46.99) = 1.67201 and log10(180.34) = 2.25609. The denominator is 1.0324 − 0.19077 × 1.67201 + 0.15456 × 2.25609 = 1.06213. Then 495 ÷ 1.06213 − 450 = 16.0 percent, "Fitness" for a man.
| Step | Calculation | Result |
|---|---|---|
| Waist − neck | 86.36 − 39.37 | 46.99 cm |
| Denominator | 1.0324 − 0.31897 + 0.34870 | 1.06213 |
| Body fat | 495 ÷ 1.06213 − 450 | 16.0% |
| Fat mass | 185 × 0.1604 | 29.7 lb |
| Lean mass | 185 − 29.7 | 155.3 lb |
| BMI method | 1.20 × 25.8 + 0.23 × 30 − 10.8 − 5.4 | 21.7% |
Notice the gap. The BMI method says 21.7 percent, nearly six points higher. That is typical: BMI cannot tell muscle from fat, so for anyone carrying more muscle than average it reads high. The tape looks at the waist, where fat shows up, and is usually closer for muscular people.
A woman's example: 165 cm tall, neck 33 cm, waist 76 cm, hip 96 cm. Waist plus hip minus neck is 139 cm, the denominator is 1.29579 − 0.75014 + 0.49006 = 1.03571, and 495 ÷ 1.03571 − 450 = 27.9 percent, "Average."
How to measure so the number means something
The formula is only as good as the tape work. Half an inch of sloppiness at the waist moves the estimate about a point.
- Snug, not tight. The tape should touch skin all the way around without denting it. Pulling it half an inch tighter in the example drops the reading from 16.0 to 15.0 percent, flattering and wrong.
- Level and relaxed. A tape that dips at the back reads short. Breathe out normally and let your stomach sit where it sits; sucking in lowers the number.
- Morning. Measure before eating and after using the bathroom. Your waist can grow an inch over a day from food and water.
- Same conditions every time. Same tape, same spots, same time of day. The absolute number carries a few points of error, but month-to-month change under identical conditions is far more trustworthy.
How to read the result
The Navy method is typically within three to four percentage points of a lab scan such as DEXA. So 16.0 percent really means "somewhere around 12 to 20, probably close to 16." That is good enough to place you in a category and track change, not good enough to argue over a single point.
The categories in the table come from the American Council on Exercise. For men, 14 to 17 percent is the fitness range and 25 and above is obese; for women, fitness is 21 to 24 and obese is 32 and above. Women's numbers run higher because essential fat, the amount needed for hormones and basic function, is higher. Essential is a floor, not a target.
Lean mass is the most useful number on the page if you are trying to change your body. When you lose weight you want lean mass to stay put and fat mass to fall; if both fall, you are losing muscle, and more protein plus resistance training is the usual fix. The body fat percentage also feeds the Katch-McArdle option in the Calorie Calculator, which works from lean mass; the BMI Calculator gives a quicker read.
These results are estimates for healthy adults and are not a diagnosis. If you have a medical condition, are pregnant, are an athlete, or are under 18, the formulas were not built for you; talk to a clinician about what your body composition means for you.
What this calculator doesn't do
It does not measure body fat. It predicts it from three or four circumferences using an equation fit to a specific group decades ago, and any individual can fall several points outside the prediction, especially the very lean, very heavy or very muscular. It cannot see visceral fat separately from fat under the skin, and the BMI comparison is rougher still.
If you need an accurate figure, a DEXA scan, hydrostatic weighing or a professional skinfold test does what a tape cannot.
Frequently asked questions
How accurate is the U.S. Navy body fat formula?
For most adults it lands within about three to four percentage points of a lab method like a DEXA scan. That is accurate enough to place you in a category and track change over time, but not precise enough to argue over a single point, and careless tape work widens the error.
What is a healthy body fat percentage?
Using the American Council on Exercise ranges, men are in the fitness range at 14 to 17 percent and average at 18 to 24, with 25 and above considered obese. Women are in the fitness range at 21 to 24 percent and average at 25 to 31, with 32 and above considered obese. Women's healthy ranges run higher because they carry more essential fat.
Where do I measure my waist for the body fat calculator?
Men measure at the navel. Women measure at the narrowest point of the torso, usually just above the navel, and also measure the hips at their widest point. Keep the tape level, snug but not tight, and breathe out normally without pulling your stomach in.
Why does the body fat calculator give two different numbers?
The headline uses the Navy tape-measure method; the second number estimates body fat from BMI, age and sex. They often differ by several points because BMI cannot tell muscle from fat. In the example a 185-pound man reads 16.0 percent by tape and 21.7 percent by BMI, and the tape is usually closer for muscular people.
How do I calculate my lean body mass?
Multiply your weight by your body fat percentage to get fat mass, then subtract that from your weight. At 185 pounds and 16.0 percent body fat, fat mass is about 29.7 pounds and lean mass is about 155.3 pounds. Lean mass includes muscle, bone, organs and water.
When is the best time to measure body fat with a tape?
First thing in the morning, before eating and after using the bathroom, since your waist can grow by an inch during the day from food and water. Use the same tape, the same spots and the same conditions every time so that changes between readings reflect your body and not your technique.
Can I use the Navy body fat method if I am very muscular or very lean?
You can, but expect more error at the extremes. The formula was fit to a general population, and people who are very lean, very heavy or unusually muscular can fall several points outside its prediction. A DEXA scan or professional skinfold test is the better option when you need a precise number.
Estimates for healthy adults, not a diagnosis or medical advice. Formulas like these are population averages and can be off for athletes, older adults, pregnant people and anyone with a medical condition. Talk to a clinician before changing your diet or training. Nothing you type here leaves your browser. See the terms of use.
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