What this calculator does
Find out roughly how much of your body is fat, using only a tape measure. This calculator uses the U.S. Navy method. You measure your height, neck and waist, plus your hips if you're female. The Navy designed it as a quick, cheap check that needs no special equipment. It's still one of the most widely used tape-measure methods.
Enter your measurements, weight and age. You'll see your estimated body fat percentage and how many pounds or kilograms of fat that is. You'll also see your lean mass, which is everything else: muscle, bone, organs and water. The calculator shows where you fall on the widely published American Council on Exercise (ACE) chart, plus a second estimate from BMI to compare.
These are estimates, not medical measurements or a diagnosis. For a check of your health, talk to a healthcare professional.
Where it's used
Why this math matters outside the classroom:
- Logarithms in practice. Students see base-10 logarithms at work, since the Navy equation takes log₁₀ of girth measurements and of height.
- Fat and lean mass. Applying a 20% estimate to a weight of 180 lb splits it into 36 lb of fat mass and 144 lb of lean mass.
- Tape-measure tracking. Gym members record waist and neck measurements every few weeks and follow how the estimate trends over time.
- Military fitness checks. The U.S. military has used tape-measure equations like this one in body composition checks for service members.
- Comparing methods. Coaches compare the tape estimate with a BMI-based estimate to see how two approaches give different numbers for one person.
How to use it
- Choose US units (inches, pounds) or metric units (centimeters, kilograms), and select your sex.
- Use a soft, flexible tape to measure your neck and waist, plus your hips if you're female. Enter them along with your height.
- Enter your weight to split it into fat and lean mass. Enter your age for the BMI-based comparison.
- To compare fairly over time, measure at the same time of day, in the same way, each time.
What the inputs mean
- Sex
- The method has one formula for men and another for women. The women's formula also uses a hip measurement.
- Age
- Whole years, 18 or over. The Navy formulas don't use age. Only the BMI-based comparison does.
- Neck circumference
- Measure just below your Adam's apple (the larynx), with the tape sloping slightly down toward the front. Keep the tape snug, but don't let it press into the skin.
- Waist circumference
- Men: measure straight around at the level of your belly button. Women: measure at the narrowest part of your waist. Relax and measure at the end of a normal breath out.
- Hip circumference
- Women only: measure around the widest part of your buttocks, with the tape level.
- Weight
- Weight doesn't change the Navy body fat percentage. It's used to split your weight into fat and lean mass, and for the BMI-based comparison.
Formula and method
Male: %BF = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76 Female: %BF = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387 Fat mass = weight × %BF ÷ 100; lean mass = weight − fat mass BMI-based comparison (Deurenberg): %BF = 1.2 × BMI + 0.23 × age − 10.8 × sex − 5.4 (sex = 1 male, 0 female)
In the formulas, %BF is your body fat percentage. Waist, hip, neck and height are your measurements in inches. These are the inch versions of the formulas Hodgdon and Beckett published for the U.S. Navy in 1984. If you enter centimeters, the calculator first divides each one by 2.54. So the same body gives exactly the same answer in either unit.
log₁₀ is the base-10 logarithm, the 'log' key on most calculators. It tells you what power of 10 makes a number, so log₁₀(100) = 2. It only works on numbers above zero. That's why the men's formula needs a waist bigger than the neck, and the women's formula needs waist plus hip bigger than the neck. If your measurements give 0% or less, the calculator asks you to check them, since that usually means a typing or measuring slip.
The ACE chart sorts body fat into five bands: essential fat, athletes, fitness, average and obese. For men the ranges are 2–5, 6–13, 14–17, 18–24, and 25 and above. For women they are 10–13, 14–20, 21–24, 25–31, and 32 and above. The chart uses whole percents, so your estimate is rounded to a whole percent before it's placed.
The Deurenberg formula (1991) estimates body fat from BMI, age and sex, with no tape needed. In it, sex is 1 for men and 0 for women. It was built from Dutch adults and children whose body fat was measured by underwater weighing. But BMI can't tell muscle from fat, so it's shown only as a cross-check.
Worked example
A man is 5 ft 10 in (70 in) tall and weighs 180 lb. His neck measures 15 in and his waist 34 in.
- Waist − neck = 34 − 15 = 19 in.
- %BF = 86.010 × log₁₀(19) − 70.041 × log₁₀(70) + 36.76 = 109.99 − 129.23 + 36.76 = 17.51%.
- Fat mass = 180 × 17.51 ÷ 100 = 31.5 lb.
- Lean mass = 180 − 31.5 = 148.5 lb.
- ACE chart: 17.51% rounds to 18%, the bottom of the 'average' band for men (18–24%).
- BMI-based comparison at age 35: BMI = 81.647 ÷ 1.778² = 25.83, so 1.2 × 25.83 + 0.23 × 35 − 10.8 − 5.4 = 22.8%.
Result: His estimated body fat is 17.5%, which is average on the ACE chart. That's about 31.5 lb of fat and 148.5 lb of lean mass. The BMI-based estimate of 22.8% is higher. That gap is common for someone with a muscular build.
Understanding the result
Body fat percentage tells you more about what your body is made of than weight or BMI does. Two people with the same weight can carry very different amounts of fat and muscle.
The ACE chart is a fitness guide, not a medical category, and it doesn't adjust for age. Other groups publish different charts, and body fat tends to rise with age in healthy adults. A doctor or other qualified professional can tell you what's right for you.
If the tape estimate and the BMI-based estimate differ by several points, the tape method is usually the better guide. It reflects where you carry fat, not just your total weight.
Where you place the tape makes a real difference. So the most useful thing this method gives you is often the trend. Measurements taken the same way over time can show whether your body is changing.
Assumptions and limitations
- You're an adult, and you measure at the standard spots described above.
- Your height and measurements are entered accurately. A half-inch error at the waist can move the result by about one percentage point.
- Results are shown rounded to one decimal place, but the math itself isn't rounded.
- The formulas were built from Navy personnel whose body fat was measured by underwater weighing. That group was mostly young and fit. The method is less accurate for older adults, very lean or very muscular people, and people with more body fat.
- For one person, the typical error is a few percentage points compared with lab methods such as DEXA scans or underwater (hydrostatic) weighing.
- It isn't valid for children, during pregnancy, or for anyone whose body shape is very different from the original study groups.
- The BMI-based estimate shares BMI's blind spot. It guesses too high for muscular people and can guess too low for people with little muscle.
- Tape measurements reflect where you carry fat. So two people with the same body fat can get different estimates.
Frequently asked questions
How accurate is the Navy body fat method?
For many adults it lands within a few percentage points of lab methods. For one person the error can be bigger, especially for very muscular or older people. Treat it as an estimate, and focus on changes over time measured the same way.
Why do I get an error saying my waist must be larger than my neck?
The men's formula takes the logarithm of waist minus neck, and that only works when the difference is above zero. If you see this, check both measurements and the units. A common cause is a neck entered in centimeters with a waist in inches.
Does it matter whether I use inches or centimeters?
No. The formulas are written in inches, and centimeters are converted exactly before the math starts. So both give the same result.
What is a normal body fat percentage?
On the ACE chart, 18–24% is called average for men and 25–31% for women. The fitness band is 14–17% for men and 21–24% for women. Women carry more essential fat than men, which is why their ranges are higher. These bands come from one fitness organization and aren't health targets. A tape-measure estimate isn't a diagnosis.
Why are the tape and BMI estimates different?
They use different information. The Navy method compares your waist with your neck and height. The BMI method only knows your weight for your height. A muscular person usually gets a higher BMI-based estimate. Someone who carries more weight around the waist may get a higher tape estimate.
How do I measure my waist and neck correctly?
Use a flexible tape and stand relaxed. Keep the tape level and snug without pressing into the skin. Measure your neck just below the Adam's apple. Men measure the waist at the belly button. Women measure the waist at its narrowest point and the hips at their widest. Take each measurement two or three times and use the average.
Are my measurements stored or shared?
No. The math runs in your browser, and this calculator doesn't create a shareable link with your measurements.
How this calculator is checked
Method: U.S. Navy circumference equations (Hodgdon JA, Beckett MB, Naval Health Research Center, 1984), inch form; metric inputs converted with 1 in = 2.54 cm. American Council on Exercise body fat chart for the bands. Deurenberg P, Weststrate JA, Seidell JC (1991, British Journal of Nutrition) BMI-based adult equation for the comparison. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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