Two ways to estimate body fat at home
Your body fat percentage is the share of your weight that is fat. It tells you more than weight or BMI alone, because two people with the same weight can carry very different amounts of muscle. This calculator gives two estimates.
- The US Navy method uses a tape measure. It was developed by James Hodgdon and Marcie Beckett at the Naval Health Research Center in 1984, by comparing circumference measurements with underwater weighing.
- The BMI-based estimate uses only height, weight, age and sex. It comes from a 1991 study by Paul Deurenberg and colleagues that compared BMI with measured body fat in 1,229 adults.
The US Navy formulas
The equations use measurements in inches. If you enter centimetres, the calculator converts them first (1 inch = 2.54 cm), so both unit systems give the same answer.
Women: % fat = 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387
For men, “waist” means the abdomen measured at the navel. For women it is the natural waist, the narrowest point between the ribs and the hips. In the original studies, the equations matched underwater weighing with a standard error of about 3.5 percentage points.
The BMI-based formula
It is quick, but it cannot tell muscle from fat. A muscular person with a high BMI will get an estimate that is too high.
Worked examples
A man, 70 in (5 ft 10 in) tall, neck 15 in, waist 34 in. Waist minus neck is 19, and log10(19) = 1.2788. Height gives log10(70) = 1.8451. 86.010 × 1.2788 − 70.041 × 1.8451 + 36.76 = 109.98 − 129.23 + 36.76 = 17.5% body fat. Rounded to 18%, that is the bottom of the ACE “average” band for men (18–24%), right next to “fitness”.
A woman, 65 in (5 ft 5 in) tall, neck 13 in, waist 28 in, hips 38 in. Waist plus hips minus neck is 53, and log10(53) = 1.7243. log10(65) = 1.8129. 163.205 × 1.7243 − 97.684 × 1.8129 − 78.387 = 281.41 − 177.09 − 78.39 = 25.9% body fat, in the ACE “average” band for women.
Fat and lean mass. If that woman weighs 60 kg, her fat mass is 60 × 0.259 = 15.6 kg and her lean (fat-free) mass is 44.4 kg. Lean mass includes muscle, bone, organs and water.
Body fat categories
There is no official medical cut-off for body fat. A widely used guide comes from the American Council on Exercise (ACE):
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletes | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Average | 25–31% | 18–24% |
| Obese | 32% and above | 25% and above |
Women carry more essential fat than men for hormonal and reproductive health, which is why their ranges are higher. Going below the essential range is not a goal; it can harm health.
How to measure well
- Use a soft, non-stretch tape. Keep it level and snug, but do not squeeze the skin.
- Measure on bare skin, standing relaxed, after breathing out normally. Do not suck in.
- Take each measurement two or three times and use the average.
- Measure at the same time of day, ideally in the morning, when you track progress.
Small errors matter. On the men’s formula, measuring the waist 1 inch too large raises the result by about 2 percentage points. That is why tracking the trend over several weeks is more useful than any single reading.
When the results differ
If the Navy and BMI-based numbers are far apart, think about your build. Muscular people usually get a higher BMI-based estimate, because BMI counts muscle as if it were fat. People who have lost muscle, often older adults, can get a lower BMI-based estimate than the tape suggests.
For a lab-grade number, a DEXA scan or air-displacement test is far more precise. You can use your lean mass from this page in the Katch–McArdle formula on the BMR calculator, compare it with your BMI, and plan intake with the TDEE calculator.