Body Fat Calculator
Estimate body fat percentage from neck, waist and hip measurements with the US Navy equations, plus lean mass, fat mass and the ACE category.
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About this toolWhat is the Body Fat Calculator?
The Body Fat Calculator turns three or four tape measurements into an estimate of how much of your weight is fat. Men enter height, neck and waist at the navel; women add the hips at their widest point. The page runs the US Navy circumference equations, splits your body weight into fat mass and lean mass, and places the result in the American Council on Exercise (ACE) bands from essential fat to obese.
Two other estimates sit underneath for comparison. The Deurenberg equation guesses body fat from BMI, age and sex, and the 2023 US Army one-site equation uses only waist and weight. When the three numbers disagree, that disagreement is useful: it usually means your build is unusual for one of the methods, most often because you carry more muscle than the BMI-based formula expects.
Everything is calculated in your browser as you type. Measurements are not saved or sent anywhere; Copy link builds a URL with your inputs only when you click it.
- US Navy equations with the exact DoD inch constants
- Fat mass and lean body mass from your weight
- ACE category table with your position highlighted
- Deurenberg BMI-based and Army one-site comparisons
- Metric or US units with automatic conversion
- Runs in your browser; nothing is stored
How to use the Body Fat Calculator
- Choose Male or Female and Metric or US units. Switching units converts the numbers you already typed.
- Enter height and weight, and your age for the BMI-based comparison.
- Measure your neck just below the larynx with the tape sloping slightly down at the front, and enter it.
- Men: measure the waist horizontally at the navel after a normal breath out. Women: measure the narrowest part of the waist and the widest part of the hips.
- Read your body fat percentage, fat mass, lean mass and ACE category; open How this was calculated to see the formula with your numbers in it.
Worked examples
Man, metric
Input
Male, 35 years, 178 cm, 80 kg, neck 38 cm, waist 90 cm
Output
Body fat 20.3% (Average), fat mass 16.2 kg, lean mass 63.8 kg
The tape figure lands in the 18 to 24% average band. Deurenberg gives 22.1% from his BMI of 25.2 and age, and the Army one-site equation gives 22.4%, so all three agree within about 2 points.
Woman, metric
Input
Female, 35 years, 165 cm, 65 kg, neck 33 cm, waist 76 cm, hips 100 cm
Output
Body fat 30.2% (Average), fat mass 19.7 kg, lean mass 45.3 kg
Waist plus hips minus neck is 143 cm, which drives the result. Losing 3 cm from the waist alone would bring her to about 28.7%, still in the average band but moving toward the fitness band, which tops out at 24%.
Woman, US units
Input
Female, 5 ft 4 in, neck 13 in, waist 29 in, hips 39 in
Output
Body fat 29.2% (Average)
163.205 × log10(55) − 97.684 × log10(64) − 78.387 = 29.2. Entering the same body in centimetres gives the same answer because the page converts to inches before applying the DoD constants.
Background and insight
James Hodgdon and Marcie Beckett at the Naval Health Research Center in San Diego published the circumference equations in 1984, after measuring body density by underwater weighing in several hundred sailors and regressing it against tape sites. The inch form used by the US Department of Defense in Instruction 1308.3 is 86.010 × log10(waist − neck) − 70.041 × log10(height) + 36.76 for men and 163.205 × log10(waist + hip − neck) − 97.684 × log10(height) − 78.387 for women. This page converts centimetres to inches and uses those exact constants, so metric and US entries give identical answers.
A metric version built on body density (495 / density − 450, the Siri equation) circulates widely online. It agrees with the DoD version to within about half a percentage point over normal ranges, which is far smaller than the method's own error. Validation studies against DEXA put the Navy tape method at roughly 3 to 4 percentage points of standard error, with bigger misses for very lean and very heavy people.
The ACE ranges shown are the ones most fitness professionals quote: for women, essential fat 10 to 13%, athletes 14 to 20%, fitness 21 to 24%, average 25 to 31% and obese 32% or more; for men, 2 to 5%, 6 to 13%, 14 to 17%, 18 to 24% and 25% or more. They are descriptive bands, not clinical cut-offs. Women carry more essential fat for hormonal and reproductive reasons, which is why every female band starts higher.
Common use cases
Tracking a cut or bulk
Measure every two to four weeks under the same conditions and watch whether weight change is coming from fat or lean mass.
Second opinion on BMI
A muscular person flagged as overweight by BMI can check whether the tape method agrees.
Military fitness prep
Practise the tape test before an official weigh-in and see where the Navy and Army one-site methods put you.
Setting protein targets
Use lean body mass rather than total weight when a coach prescribes protein per kilogram of lean mass.
Tips and limitations
- Measure in the morning before eating, with the tape level and snug but not pressing into the skin. Take each site twice and average.
- A 1 cm error at the waist moves the male result by roughly 1 percentage point, so consistent technique matters more than the exact formula.
- The equations were built on adults. They are not meant for children, pregnancy, or people with large fluid shifts.
- Very lean athletes and people with obesity get the widest errors. If the number decides something important, use DEXA or hydrostatic weighing.
- The ACE bands describe populations. A reading in the average band is common and not a diagnosis; waist size and fitness tell you more about health risk.
Frequently asked questions
How accurate is the US Navy body fat method?
Against DEXA it is usually within 3 to 4 percentage points for most adults. It tends to overestimate very lean people and underestimate people with a lot of abdominal fat. Its strength is repeatability: the same person measured the same way will see real changes.
Where exactly do I measure my waist for the Navy formula?
Men measure horizontally at the navel, relaxed, at the end of a normal breath out. Women measure the narrowest part of the torso, usually a little above the navel. Hips are measured at the largest protrusion of the buttocks with feet together.
What is a healthy body fat percentage?
The ACE bands call 14 to 17% fit for men and 21 to 24% fit for women, with 18 to 24% and 25 to 31% as average. Below essential fat (about 2 to 5% for men and 10 to 13% for women) is unsafe to maintain. Health risk rises more with waist size and inactivity than with any single percentage.
Why is my body fat different from my smart scale?
Scales use bioelectrical impedance, which reads body water and shifts with hydration, meals and skin temperature by several points in a day. The tape method reads shape. Both are estimates; pick one, use it the same way each time, and compare trends rather than single numbers.
What is lean body mass?
Everything that is not fat: muscle, bone, organs, water and connective tissue. It is your weight minus fat mass. At 80 kg and 20% fat, lean mass is 64 kg. Some protein and drug-dosing rules use lean mass instead of total weight.
Does the Army still use the neck and waist tape test?
Since 2023 the US Army uses a one-site test: abdominal circumference at the navel plus body weight. The Navy-style multi-site equations are still widely used elsewhere. This page shows both so you can compare.
Can I calculate body fat from BMI alone?
You can estimate it with the Deurenberg equation, 1.2 × BMI + 0.23 × age − 10.8 × sex − 5.4, which this page shows as a comparison. It cannot tell muscle from fat, so it overestimates muscular people and can underestimate older adults who have lost muscle.
Is there an in-depth guide to the Body Fat Calculator?
Yes. “Body Fat, Ideal Weight and Macros: What the Formulas Can and Cannot Tell You” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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