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Formula
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
- BF%
- estimated body fat as a percentage of body weight
- waist
- waist circumference in inches (men at the navel, women at the narrowest point)
- neck
- neck circumference in inches, just below the larynx
- hip
- hip circumference in inches at the widest point of the buttocks (women only)
- height
- standing height in inches, without shoes
What the body fat calculator estimates
Body fat percentage is the share of your weight that is fat. It says more about health and fitness than weight alone, because two people with the same weight can carry very different amounts of fat and muscle.
This body fat calculator uses the US Navy circumference method, the tape-measure equations developed by James Hodgdon and Mary Beckett at the Naval Health Research Center in 1984 and later adopted across the US military. It returns:
- Estimated body fat percentage, to one decimal place.
- ACE category, from essential fat to obesity, using the American Council on Exercise norms for your sex.
- Fat mass and lean mass, if you add your weight.
- The working, so you can see each logarithm and coefficient.
You need only a tape measure and your height. Measurements can be in inches or centimeters, and you can mix them.
How to use the body fat calculator
- Choose your sex. The male and female equations use different measurements and coefficients.
- Enter your height without shoes.
- Measure your neck just below the larynx, with the tape sloping slightly down toward the front.
- Measure your waist. Men measure at the navel; women measure at the narrowest point between the ribs and hips.
- Women also enter the hips, measured around the widest part of the buttocks with feet together.
- Optionally add your body weight to see how many pounds or kilograms are fat and how many are lean tissue.
Take each measurement twice and use the average. Keep the tape level and snug without pressing into the skin, and measure after breathing out normally rather than holding in your stomach.
How to calculate body fat percentage with the Navy formula
The method rests on a simple observation: waist size rises as body fat rises, while neck size mostly reflects frame and muscle. Subtracting the neck from the waist (plus hips for women) gives a measure of abdominal fat that is then scaled by height. The relationship is logarithmic, so each extra inch at the waist adds a little less body fat than the one before.
Working it out by hand for a man
Take a man 72 in tall with a 16.5 in neck and a 38 in waist.
- Waist − neck = 38 − 16.5 = 21.5 in.
- log₁₀(21.5) ≈ 1.3324, and log₁₀(72) ≈ 1.8573.
- Body fat = 86.010 × 1.3324 − 70.041 × 1.8573 + 36.76 ≈ 114.60 − 130.09 + 36.76 ≈ 21.3%.
- At 210 lb, fat mass is 210 × 0.213 ≈ 44.7 lb, and lean mass is about 165.3 lb.
That puts him in the ACE acceptable band (18–24%) for men.
Working it out by hand for a woman
Take a woman 66 in tall with a 13.5 in neck, 31 in waist and 40 in hips.
- Waist + hip − neck = 31 + 40 − 13.5 = 57.5 in.
- log₁₀(57.5) ≈ 1.7597, and log₁₀(66) ≈ 1.8195.
- Body fat = 163.205 × 1.7597 − 97.684 × 1.8195 − 78.387 ≈ 287.19 − 177.74 − 78.39 ≈ 31.1%.
That is at the top of the ACE acceptable band (25–31%) for women.
Body fat by waist size
These figures were calculated with the same equations the calculator uses. They show how strongly the waist drives the result when height and neck stay the same.
| Waist | Man, 70 in tall, 15.5 in neck | ACE category |
|---|---|---|
| 30 in | 7.4% | Athletes |
| 32 in | 12.2% | Athletes |
| 34 in | 16.5% | Fitness |
| 36 in | 20.4% | Acceptable |
| 38 in | 23.8% | Acceptable |
| 40 in | 27.0% | Obesity |
| 42 in | 29.9% | Obesity |
| Waist | Woman, 65 in tall, 13 in neck, 39 in hips | ACE category |
|---|---|---|
| 26 in | 24.6% | Fitness |
| 28 in | 27.3% | Acceptable |
| 30 in | 29.8% | Acceptable |
| 32 in | 32.3% | Obesity |
| 34 in | 34.7% | Obesity |
| 36 in | 37.0% | Obesity |
Each extra 2 in of waist adds about 3 to 5 percentage points for men and about 2.5 for women, slightly less at larger waist sizes.
ACE body fat categories
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletes | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Acceptable | 25–31% | 18–24% |
| Obesity | 32% and above | 25% and above |
Women’s ranges are higher because they need more essential fat for reproductive and hormonal function. A result at or below essential fat is not a goal: it can disrupt hormones, bone health and, in women, menstrual cycles, so talk to a doctor if your measurements put you there.
When a body fat estimate is useful
- Tracking a weight-loss or training plan. Body fat can fall while weight stays flat if you are gaining muscle, which the scale alone would miss.
- Setting a target weight. If you know your lean mass, you can work out what you would weigh at a lower body fat, assuming lean mass stays the same. The ideal weight calculator gives another view based on height.
- Planning nutrition. Lean mass is a better base for protein targets than total weight. Pair the result with the TDEE calculator and the macro calculator.
- Checking a BMI result. A muscular person with a high BMI may have a normal body fat, and a slim person with a normal BMI may carry more fat than expected.
Common mistakes
- Pulling the tape too tight. Compressing the skin shrinks the waist and lowers the result.
- Measuring the waist in the wrong place. Men should measure at the navel, not at the belt line, which is often lower and smaller.
- Holding in the stomach. Measure at the end of a normal breath out.
- Rounding height down by an inch or two. Height enters the formula on a log scale, so it matters less than the waist, but it still shifts the result.
- Comparing results across methods. A tape-based body fat calculator, smart scales, skinfold calipers and DXA scans each have their own biases. Track changes with one method rather than mixing them.
Glossary
- Fat mass. The weight of all body fat, including essential and stored fat.
- Lean mass. Everything that is not fat: muscle, bone, organs and water. Also called fat-free mass.
- Essential fat. The minimum fat needed for normal body function, found in nerves, bone marrow and organs.
- DXA (dual-energy X-ray absorptiometry). A low-dose scan that measures fat, lean tissue and bone, often used as a reference method.
Worked examples
A man measured in inches
Male, 71 in tall, neck 16 in, waist 36 in at the navel, weighing 190 lb.
Inputs: Sex = Male; Height = 71; Height unit = Inches; Neck circumference = 16; Neck circumference unit = Inches; Waist circumference = 36; Waist circumference unit = Inches; Body weight (optional) = 190; Body weight (optional) unit = Pounds
Waist − neck = 36 − 16 = 20 in Body fat = 86.010 × log₁₀(20) − 70.041 × log₁₀(71) + 36.76 = 86.010 × 1.301 − 70.041 × 1.8513 + 36.76 = 19.0%
Answer: Estimated body fat (US Navy method): 19.0%, ACE category “Acceptable”. An estimate, not medical advice.
A woman measured in centimeters
Female, 165 cm tall, neck 33 cm, waist 76 cm, hips 100 cm, weighing 64 kg.
Inputs: Sex = Female; Height = 165; Height unit = Centimeters; Neck circumference = 33; Neck circumference unit = Centimeters; Waist circumference = 76; Waist circumference unit = Centimeters; Hip circumference = 100; Hip circumference unit = Centimeters; Body weight (optional) = 64; Body weight (optional) unit = Kilograms
Waist + hip − neck = 29.92 + 39.37 − 12.99 = 56.3 in (centimeters converted to inches) Body fat = 163.205 × log₁₀(56.3) − 97.684 × log₁₀(64.96) − 78.387 = 163.205 × 1.7505 − 97.684 × 1.8127 − 78.387 = 30.2%
Answer: Estimated body fat (US Navy method): 30.2%, ACE category “Acceptable”. An estimate, not medical advice.
A very lean man (edge of the athlete range)
Male, 69 in tall, a thick 16 in neck and a 30 in waist, no weight entered, so only the percentage is shown.
Inputs: Sex = Male; Height = 69; Height unit = Inches; Neck circumference = 16; Neck circumference unit = Inches; Waist circumference = 30; Waist circumference unit = Inches
Waist − neck = 30 − 16 = 14 in Body fat = 86.010 × log₁₀(14) − 70.041 × log₁₀(69) + 36.76 = 86.010 × 1.1461 − 70.041 × 1.8388 + 36.76 = 6.5%
Answer: Estimated body fat (US Navy method): 6.5%, ACE category “Athletes”. An estimate, not medical advice.
Assumptions and limitations
- This is an estimate, not medical advice. The US Navy equations predict body fat with a typical error of about 3 to 4 percentage points compared with laboratory methods, and the error can be larger for very muscular or very heavy people.
- The calculator uses the Hodgdon and Beckett equations in the inch form adopted by the US Department of Defense. Centimeter entries are converted to inches first (1 in = 2.54 cm), so both unit systems give the same result for the same body.
- The equations are for adults. They were developed on US Navy personnel and are not meant for children, pregnant women or people with conditions that change body shape, such as ascites or limb loss.
- Categories follow the American Council on Exercise (ACE) norms. ACE lists whole-number bands with small gaps between them (for example, men 13% and 14%), so a value in a gap is placed in the lower band.
- Fat mass is body weight × body fat %, and lean mass is the rest of your weight (muscle, bone, water and organs). Both are shown only when you enter your weight.
- Results below the ACE essential-fat range or in the obesity range carry a note suggesting you talk to a doctor or registered dietitian.
Frequently asked questions
Are body fat percentage calculators accurate?
Tape-measure calculators like this one are reasonable estimates, not measurements. In the studies it was built on, the US Navy method predicted body fat to within about 3 to 4 percentage points of underwater weighing for most people. Careful, repeated measurements matter more than the formula. A difference of half an inch at the waist changes a man's result by roughly one percentage point.
How do I measure my neck and waist correctly?
Use a flexible, non-stretch tape held snug but not compressing the skin. Measure the neck just below the Adam's apple with the tape sloping slightly downward to the front. Men measure the waist horizontally at the navel; women measure at the narrowest point. Breathe out normally and take each measurement twice.
Why do women need a hip measurement?
Women carry more fat on the hips and thighs, so the female equation adds the hip circumference to the waist before subtracting the neck. Without it, the formula would miss a large share of body fat in many women and underestimate the result.
What is a healthy body fat percentage?
ACE places 18–24% for men and 25–31% for women in its acceptable range, with lower bands for fitness and athletes. Essential fat, the minimum the body needs, is about 2–5% for men and 10–13% for women. A doctor can tell you what is healthy for you, taking age and health history into account.
Is body fat percentage better than BMI?
They measure different things. BMI only uses height and weight, so it cannot tell muscle from fat. Body fat percentage separates the two, but tape-based estimates have their own error. Using both, along with waist size, gives a fuller picture than either one alone.
Can I use this calculator if I am very muscular?
You can, but treat the result with caution. A thick, muscular neck lowers the estimate, so strength athletes can come out leaner than they are, while people with large muscular waists may come out higher. A lab method such as a DXA scan is more reliable for them.
How often should I measure my body fat?
Every two to four weeks is enough to see a trend. Measure at the same time of day, ideally in the morning before eating, because food, fluid and bloating can change your waist by half an inch or more from one day to the next.
Sources
- Circumference-Based Predictions of Body Fat Revisited: Preliminary Results From a US Marine Corps Body Composition Survey — Frontiers in Physiology (Potter et al., 2022), via PubMed Central. Checked October 10, 2026.
- Percent Body Fat Calculator — Percent Body Fat Norms for Men and Women (archived page) — American Council on Exercise. Checked October 10, 2026.
- How the Military Measures Body Composition — Human Performance Resources by CHAMP (HPRC), Uniformed Services University. Checked October 10, 2026.
- Adult BMI Categories — Centers for Disease Control and Prevention. Checked October 10, 2026.