Body fat percentage by the US Navy tape method, Jackson-Pollock 3- or 7-site skinfolds, or from BMI, with fat and lean mass and the ACE category.
By the US Navy circumference method you carry about 16.9 % body fat — 11.9 kg of fat and 58.1 kg of lean mass — which ACE classes as “Fitness”.
This is the metric form; the original inch-based Hodgdon–Beckett regression gives results a few tenths of a percent different.
The ACE category is read from the value rounded to 0.1 %.
Body fat percentage is the share of body weight that is fat. The US Navy method (Hodgdon and Beckett, 1984) uses the logarithms of height and of waist minus neck, with the hip added for women. The Jackson-Pollock methods turn a sum of 3 or 7 caliper skinfolds and age into body density, which the Siri equation converts with %BF = 495 ÷ density − 450. The Deurenberg method estimates it from BMI, age and sex. Fat and lean mass follow from your weight.
People use it to follow changes in body composition that weight and BMI miss. With the defaults, a man 175 cm tall with a 38 cm neck and an 85 cm waist comes out at 16.9 % body fat, 11.9 kg of his 70 kg, which the American Council on Exercise (ACE) classes as fitness.
Each method is a regression fitted on adult samples, so a single reading can differ from a laboratory measurement by several percentage points. Repeat readings are comparable only when they use the same method and measurement sites.
Hodgdon and Beckett developed the Navy equations at the Naval Health Research Center in 1984 to predict the body fat of Navy personnel from height and a few circumferences. They use base-10 logarithms of measurements in centimetres:
The outer "495 ÷ … − 450" is the Siri equation, which turns body density into a fat percentage. Everything inside the brackets is a prediction of density.
A worked example for a woman who is 168 cm tall and weighs 64 kg, with a 33 cm neck, 76 cm waist and 100 cm hips:
Age does not enter the Navy equations. The calculator uses the metric form; the original inch-based regression gives results a few tenths of a percentage point different.
Skinfold equations predict body density from the thickness of pinched folds of skin and the fat under them. The calculator uses the Jackson-Pollock equations and then the same Siri step.
| Method | Men | Women |
|---|---|---|
| 3-site | Chest, abdomen, thigh | Triceps, suprailiac, thigh |
| 7-site | Chest, midaxillary, triceps, subscapular, abdomen, suprailiac, thigh | Same seven sites |
The men's equations were fitted on 308 men aged 18 to 61 and cross-checked on 95 more; the women's on 249 women aged 18 to 55, whose authors advised care with women over 40. The calculator warns above 61 for men and 55 for women.
A 35-year-old man with a 14 mm chest, 24 mm abdominal and 18 mm thigh skinfold has a sum of 56 mm. His density is 1.10938 − 0.0008267 × 56 + 0.0000016 × 56² − 0.0002574 × 35 = 1.10938 − 0.04630 + 0.00502 − 0.00901 = 1.05909, and 495 ÷ 1.05909 − 450 gives 17.4 % body fat.
For the woman in the Navy example, a 1 cm error at the waist, hip or neck shifts the result by about half a percentage point, and a 1 cm error in height by about a quarter:
| Measurement off by +1 cm | New body fat | Change |
|---|---|---|
| Waist | 29.62 % | +0.49 points |
| Hip | 29.62 % | +0.49 points |
| Neck | 28.63 % | −0.49 points |
| Height | 28.86 % | −0.26 points |
For skinfolds, reading each of the man's three folds 1 mm thicker raises his result from 17.4 % to 18.2 %, enough to move him from the ACE "Fitness" band to "Average".
With a tape, measure on bare skin, at the same time of day, after a normal breath out.
With calipers, the standards summarised in a 2026 review of skinfold protocols agree on six points:
The same review found that standards disagree on where some sites are. The abdominal fold, for example, is taken 2 cm, 3 cm or 5 cm to the side of the navel, vertical in some standards and horizontal in others. Pick one written protocol and keep to it, and have the same person take the measurements each time.
The calculator labels results with the American Council on Exercise (ACE) categories, which use the same bands at every age. The most widely used age-specific ranges come from Gallagher and colleagues (2000). They measured 1,626 adults with DXA and four-compartment models in the UK, US and Japan and worked out the body fat that corresponds to BMIs of 18.5, 25 and 30 at each age. For white adults, as tabulated by Colombo and colleagues (2008):
| Age | Women: healthy | Women: obese from | Men: healthy | Men: obese from |
|---|---|---|---|---|
| 20 to 39 | 21 to 33 % | 39 % | 8 to 21 % | 26 % |
| 40 to 59 | 23 to 35 % | 41 % | 11 to 23 % | 29 % |
| 60 to 79 | 25 to 38 % | 43 % | 13 to 25 % | 31 % |
Values between the healthy upper limit and the obese threshold correspond to a BMI of 25 to 29.9. The paper gives separate figures for African American and Asian adults, and the authors called the ranges provisional.
The two systems can disagree. The woman in the Navy example, at 29.1 %, is "Average" by ACE and inside the healthy range at any age by Gallagher. A 65-year-old woman at 36 % is "Obese" by ACE, whose obese band starts at 32 % for women, but inside Gallagher's healthy range for her age.
The calculator rejects any result at or below 0 % or at 75 % and above as a sign of a typing or unit error, and it picks the category from the result rounded to 0.1 %. Your body fat percentage is the input the Katch-McArdle equation needs in the BMR and TDEE calculator. For lean mass from height and weight formulas instead, use the lean body mass calculator.
Checked against: Python 3.8 decimal: 495 / (1.0324 − 0.19077·log10 48 + 0.15456·log10 178) − 450 = 17.2039; 80 × 17.2039 % = 13.763 kg; ACE men fitness 14–17 %
Checked against: Python 3.8 decimal: 495 / (1.29579 − 0.35004·log10 134 + 0.22100·log10 165) − 450 = 25.3755
Checked against: Python 3.8 decimal, metric form after ×2.54 conversion = 17.4347; Hodgdon–Beckett inch form gives 17.513
Checked against: Python 3.8 decimal: 1.10938 − 0.0008267·45 + 0.0000016·45² − 0.0002574·30 = 1.0676965; Siri 495/D − 450 = 13.6149
ACE's reference ranges put average at 18–24 % for men and 25–31 % for women, fitness at 14–17 % and 21–24 %, and athletes at 6–13 % and 14–20 %. Obese starts at 25 % for men and 32 % for women. Essential fat, the minimum needed for normal physiological function, is 2–5 % for men and 10–13 % for women. These are fitness-industry reference bands, not clinical diagnostic cut-offs.
Their standard errors are about 3 to 4 percentage points of body fat. Each formula was fitted against underwater weighing: the Jackson-Pollock men's equations had a standard error of 0.0073 g/ml in body density (about 3 points of body fat), the women's 3.6 to 3.8 points, and Deurenberg's BMI formula 4.1 points. Laboratory methods such as underwater weighing and DXA scans remain the reference.
Use a non-stretch tape that sits snug on the skin without compressing it. Measure the neck just below the larynx and the waist at the navel for men or at the narrowest point for women; women also measure the hips at the widest point of the buttocks. Averaging three readings per site helps, because at the defaults a 1 cm change in waist moves the result by about 0.8 percentage points.
BMI divides weight by height squared and says nothing about what the weight is made of, while body fat percentage estimates the share that is fat. An 80 kg, 178 cm man has a BMI of 25.2 whatever his build; Deurenberg's formula puts a 40-year-old at 23.3 % fat, but a muscular man of the same size measures far lower by tape or calipers. In Deurenberg's 1991 data, BMI with age and sex explained 79 % of the variation in measured body fat.
Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 8 worked examples whose answers come from independent sources; for example, “US Navy, man 178 cm, neck 38, waist 86” is checked against Python 3.8 decimal: 495 / (1.0324 − 0.19077·log10 48 + 0.15456·log10 178) − 450 = 17.2039; 80 × 17.2039 % = 13.763 kg; ACE men fitness 14–17 %.
Hodgdon JA, Beckett MB. Prediction of percent body fat for U.S. Navy men and women from body circumferences and height. Naval Health Research Center reports 84-11 and 84-29 (1984); Jackson AS, Pollock ML. Generalized equations for predicting body density of men. Br J Nutr 1978;40:497–504; Jackson AS, Pollock ML, Ward A. Generalized equations for predicting body density of women. Med Sci Sports Exerc 1980;12:175–181; Siri WE. Body composition from fluid spaces and density (1961); Deurenberg P, Weststrate JA, Seidell JC. Body mass index as a measure of body fatness. Br J Nutr 1991;65:105–114; American Council on Exercise — body fat percentage categories.
For information only, not medical advice. These formulas describe populations, not you; talk to a clinician before acting on a result.
8 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.
Lean body mass (LBM) from height, weight and sex by the Boer, James and Hume formulas, with the implied body fat percentage.
Body mass index (BMI) from height and weight in metric or imperial units, with the WHO adult category and the healthy weight range for your height.
Basal metabolic rate (BMR) by Mifflin-St Jeor, Harris-Benedict or Katch-McArdle, and daily calorie needs (TDEE) for five activity levels.
Waist-to-hip ratio and waist-to-height ratio from tape measurements, with the WHO risk cut-offs, the NICE bands and the WHO waist size category.
Ideal body weight (IBW) for your height and sex by the Devine, Robinson, Miller and Hamwi formulas, with the healthy BMI weight range.
Body surface area (BSA) in m² from height and weight by the Mosteller, DuBois and Haycock formulas, with the spread between them.
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