What your result means
Every figure this calculator produces is an estimate of how much of your body weight is fat. None of the three methods can actually see fat. Each takes something easy to record — a few tape measurements, your BMI and age, or three pinches of skin — and feeds it into an equation that researchers fitted to a group of people whose body fat was measured in a laboratory.
So read the result as the middle of a range, not as an exact value. A reading of 23.4% isn't meaningfully different from 22% or 25%. The number earns its keep over time: taken the same way, with the same method, a shift of several points across a few months says more than any single reading can.
The category under the result comes from the American Council on Exercise bands further down. It labels the number. It doesn't judge your health.
U.S. Navy method
J. A. Hodgdon and M. B. Beckett at the Naval Health Research Center published these equations in 1984, so that body fat could be screened with nothing more than a tape. The men's version uses height, neck and waist. The women's version adds the hips.
The logic is easy to follow. A larger waist for a given height pushes the estimate up. Neck size is subtracted because it tracks frame and muscle more than fat, which pulls the estimate back down for broader builds. Many women store a good share of fat around the hips and thighs, and that is why hip circumference enters the women's equation.
Navy formula for men
| Body fat % | = 495 ÷ (1.0324 − 0.19077 × log₁₀(waist − neck) + 0.15456 × log₁₀(height)) − 450 |
Navy formula for women
| Body fat % | = 495 ÷ (1.29579 − 0.35004 × log₁₀(waist + hip − neck) + 0.22100 × log₁₀(height)) − 450 |
All lengths go in as centimeters; if you enter inches, the calculator converts them first. Both equations use a base-10 logarithm. Swapping in a natural log — an easy slip in a spreadsheet — produces a very different and wrong answer.
BMI-based estimate
Pick this one when you have a scale but no tape. It starts from body mass index — weight in kilograms divided by height in meters squared — and then adjusts for age and sex using an equation published by Paul Deurenberg and colleagues in the British Journal of Nutrition in 1991.
| Men | Body fat % = 1.20 × BMI + 0.23 × age − 16.2 |
| Women | Body fat % = 1.20 × BMI + 0.23 × age − 5.4 |
The age term is there because, at the same BMI, older adults in the study group tended to carry more fat. The different constants for men and women reflect the higher essential fat women carry. What the equation can't do is tell a heavy, muscular build from a heavy, higher-fat one, because BMI never looks at shape. A weightlifter and an inactive person with the same height, weight, age and sex get exactly the same estimate.
Jackson-Pollock 3-site skinfold method
Skinfold testing measures the thickness of a double layer of skin and the fat just under it, using a caliper. Andrew Jackson and Michael Pollock built their 3-site equations from skinfold readings paired with underwater weighing: the men's version appeared in 1978, and the women's followed in 1980, written with Ann Ward.
| Skinfold sites | |
|---|---|
| Men | Chest, abdomen, thigh |
| Women | Triceps, suprailiac, thigh |
The equations don't give body fat directly. They predict body density from the sum of the three skinfolds (S, in millimeters) and your age. A second equation, from W. E. Siri, converts density into a percentage.
| Men | Density = 1.10938 − 0.0008267 × S + 0.0000016 × S² − 0.0002574 × age |
| Women | Density = 1.0994921 − 0.0009929 × S + 0.0000023 × S² − 0.0001392 × age |
| Siri | Body fat % = 495 ÷ density − 450 |
The men's equation was developed on men aged 18 to 61, and the women's on women aged 18 to 55. You'll still get a number outside those ages, but the calculator notes it, since the original data can't say how well the equation holds there.
Compare methods
Choose Compare methods above and the calculator runs the Navy equation and the BMI-based equation on one set of entries. You get both percentages and the gap between them in percentage points. Skinfolds stay out of the comparison because they need a caliper and their own readings; if you have those, run the skinfold method separately and set the result beside the other two.
A gap of two or three points is ordinary. When the gap reaches eight or ten, one of two things is usually going on: a measurement went wrong, or your build is far from the typical person the equations were built around. Plenty of muscle, for instance, tends to push the BMI-based figure up.
Why methods can differ
The three methods aren't reading the same thing. The Navy equation reads shape — your waist against your neck and height. The BMI-based equation reads size, age and sex, and nothing else. Skinfolds read fat directly under the skin at three spots and assume that deeper fat, packed around the organs, follows along in a typical proportion.
They were also calibrated on different people: Navy personnel for one, Dutch adults for another, American volunteers for the third. If your body resembles one of those groups more closely, that method will probably suit you better. No single method is right for everyone, and averaging all three doesn't cancel their errors out.
- Very muscular builds: the BMI-based figure tends to come out high.
- Fat carried mostly around the middle: the Navy figure reacts strongly to waist size, while skinfolds can miss fat stored deep in the abdomen.
- Older adults: more fat tends to sit internally with age, which may leave skinfold results on the low side.
How to measure correctly
Tape measurements (Navy method)
Use a flexible tape that doesn't stretch. Stand relaxed with your weight on both feet, and breathe out gently before you read the number. The tape should lie flat on bare skin, level all the way around, snug without denting it.
- Neck: just below the Adam's apple, with the tape sloping slightly downward at the front.
- Waist: level with the navel for men; at the narrowest point of the torso for women.
- Hip (women): around the widest part of the buttocks, feet together.
Measure each spot twice. If the two readings differ by more than about a quarter of an inch (half a centimeter), take a third. The Average two readings helper under each field does the arithmetic and places the result in the calculator for you.
Skinfold sites (Jackson-Pollock 3-site)
Filled dots: men's sites. Open circles: women's sites. Thigh is used by both.
Take skinfolds on the right side of the body, standing. Pinch a fold between thumb and forefinger, set the caliper jaws about a centimeter from your fingers, and read the dial a second or two later while still holding the fold.
- Chest (men): diagonal fold halfway between the front crease of the armpit and the nipple.
- Abdomen (men): vertical fold about 2 cm to the right of the navel.
- Triceps (women): vertical fold on the back of the upper arm, midway between shoulder tip and elbow, arm hanging loose.
- Suprailiac (women): diagonal fold just above the top of the hip bone, in line with the front of the armpit.
- Thigh (both): vertical fold on the front of the thigh, midway between the kneecap and the hip crease.
Reaching your own chest or triceps with a caliper is awkward, so skinfolds go better when someone else takes them. Measure each site at least twice, cycling through all three sites instead of repeating one back to back, and use the average.
Body fat percentage categories
The American Council on Exercise groups body fat into five bands, with separate cutoffs for men and women because women carry more essential fat. After you calculate, the matching row is highlighted.
| Category | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25%+ | 32%+ |
The published bands use whole numbers, so a result such as 24.5% for a man falls between two rows; the calculator keeps it in the lower one, because it has not yet reached the 25% where the next row begins. These labels describe where a number sits in a general population. They aren't a diagnosis or a goal set for you, and a result near a boundary could belong in either neighboring row once measurement error is taken into account.
U.S. Navy military standards
These are U.S. Navy maximum-allowable standards for military fitness requirements, not general health targets or medical recommendations. They set the most body fat a sailor may carry in each age band, which answers a different question from the category table above.
| Age | Men (maximum) | Women (maximum) |
|---|---|---|
| 18–21 | 22% | 33% |
| 22–29 | 23% | 34% |
| 30–39 | 24% | 35% |
| 40+ | 26% | 36% |
The Navy's youngest band begins at 17; this calculator accepts adults from 18, so its first row reads 18–21. The Navy has revised its body composition screening in recent years, so anyone serving or enlisting should go by current official Navy guidance, not this table.
Accuracy and limitations
Everything on this page is a field estimate. Studies that check the Navy and skinfold equations against laboratory methods usually report typical errors of about three to four percentage points, with bigger misses for people far from the average. For any one person, the BMI-based estimate is generally the roughest of the three, because it knows nothing about body shape.
- The equations weren't built for pregnancy, and the results shouldn't be used during it.
- Very lean athletes, bodybuilders and people carrying a lot of excess weight tend to see larger errors.
- Skinfold results depend a great deal on the person holding the caliper and how consistently they find each site.
- Anyone under 18 is outside what these equations were built for, so the calculator doesn't accept those ages.
No method here is the most accurate one available, and the calculator makes no such claim. Its value is that you can see exactly how each figure was produced.
Estimate vs specialized measurement vs medical assessment
- Field estimate
- What this page produces: an equation applied to measurements you took yourself.
- Specialized measurement
- DEXA scanning, hydrostatic (underwater) weighing and air-displacement plethysmography (Bod Pod) assess body composition with dedicated equipment. They are more controlled, though each still has its own error, and their results aren't interchangeable with one another.
- Medical assessment
- A doctor or other qualified clinician reads body composition alongside blood pressure, blood tests, history and more. No calculator can do that, and nothing here is a diagnosis.
If a result worries you, take the number — and a note of how you measured it — to a healthcare professional rather than acting on it alone.
Worked example
A fictional 38-year-old woman stands 168 cm tall and weighs 64 kg. Her neck measures 33 cm, waist 76 cm and hips 100 cm. A friend also takes her skinfolds: triceps 20 mm, suprailiac 16 mm, thigh 26 mm.
Navy method
- Waist + hip − neck = 76 + 100 − 33 = 143 cm
- log₁₀(143) = 2.1553 and log₁₀(168) = 2.2253
- 1.29579 − 0.35004 × 2.1553 + 0.22100 × 2.2253 = 1.29579 − 0.75445 + 0.49179 = 1.03313
- 495 ÷ 1.03313 − 450 = 29.1%
- Fat mass: 64 × 0.2913 = 18.6 kg; lean mass: 64 − 18.6 = 45.4 kg
BMI-based estimate
- BMI = 64 ÷ 1.68² = 64 ÷ 2.8224 = 22.68
- 1.20 × 22.68 + 0.23 × 38 − 5.4 = 27.21 + 8.74 − 5.4 = 30.6%
Skinfold method
- Sum = 20 + 16 + 26 = 62 mm
- Density = 1.0994921 − 0.0009929 × 62 + 0.0000023 × 62² − 0.0001392 × 38
- = 1.0994921 − 0.0615598 + 0.0088412 − 0.0052896 = 1.0414839
- Siri: 495 ÷ 1.0414839 − 450 = 25.3%
- Fat mass: 16.2 kg; lean mass: 47.8 kg
One made-up person, three answers: 29.1% from the tape, 30.6% from BMI and 25.3% from the caliper. Nothing went wrong. All three land in the ACE "Average" band for women, and the fair way to describe her is "somewhere in the mid-to-high twenties," not any single decimal. If she wanted to track change, she would pick one of the three and repeat it the same way each time.
Common measurement mistakes
- Pulling the stomach in for the waist reading, or measuring straight after a big meal.
- Letting the tape drop at the back so it isn't level.
- Pulling the tape tight enough to dent the skin.
- Wrapping the neck tape over the Adam's apple instead of just below it.
- Switching units after typing measurements — the calculator keeps what you typed, so check the unit shown beside each box.
- Grabbing muscle along with the fat in a skinfold, or reading the caliper long after the jaws close.
- Comparing skinfolds taken by two different people as if they were one series.
- Treating results from different methods as measurements of the same thing.
Sources for the formulas
Each equation on this page is used exactly as published; none has been adjusted or blended.
- U.S. Navy method: Hodgdon, J.A. and Beckett, M.B. (1984), 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. The Navy's own review of the method and its error is in Body Composition in Military Services: Standards & Methods (DTIC).
- BMI-based estimate: Deurenberg, Weststrate and Seidell (1991), Body mass index as a measure of body fatness: age- and sex-specific prediction formulas, British Journal of Nutrition 65:105–114.
- Skinfold method: Jackson and Pollock (1978), Generalized equations for predicting body density of men, and Jackson, Pollock and Ward (1980), Generalized equations for predicting body density of women. Density is converted to percent fat with Siri's (1961) equation, 495 ÷ density − 450.
- Categories: the body fat bands published by the American Council on Exercise. Navy maximums: the U.S. Navy's physical readiness program standards, which the Navy revises — go by its current instruction for anything official.
Frequently asked questions
What is a healthy body fat percentage?
There isn't one number that suits everyone. The American Council on Exercise puts its "fitness" band at 14–17% for men and 21–24% for women, and its "average" band runs up to 24% and 31%. Those bands describe populations. Whether a figure is reasonable for you depends on your age, health and history, which is a question for a clinician rather than a formula.
How accurate is the Navy method?
For most adults it usually lands within about three to four percentage points of a DEXA scan. Individual misses can be larger, especially for people who are very lean, very muscular or carrying a lot of excess weight, and a sloppy tape measurement adds its own error on top of the equation’s.
Why are male and female formulas different?
Men and women store fat in different places and carry different amounts of essential fat. The women’s Navy equation therefore includes hip circumference, and the Jackson-Pollock equations use different skinfold sites and coefficients for each sex, because each version was fitted to measurements from that sex.
What is the difference between body fat percentage and BMI?
BMI compares weight with height and stops there. Body fat percentage estimates how much of that weight is fat. Two people with an identical BMI can be far apart in body fat when one of them carries a lot more muscle.
What is essential body fat?
It is the minimum fat the body needs to work normally — in nerves, cell membranes, bone marrow and around organs, plus sex-specific stores in women. The American Council on Exercise puts it at roughly 2–5% for men and 10–13% for women. Dropping below that range is harmful, not an achievement.
Can you have a normal BMI but higher body fat?
Yes. Someone with little muscle can weigh a “normal” amount for their height and still carry a relatively high share of fat; researchers sometimes call this normal-weight obesity. A tape or skinfold method will usually pick it up better than BMI alone.
Are body fat scales accurate?
Bathroom body fat scales use bioelectrical impedance: a weak current passes through you and the scale estimates fat from the resistance. Hydration, a recent meal, exercise and skin temperature all shift that reading, so one number can be off by several points. They are more useful for spotting a trend over weeks, measured under the same conditions each time.
How does the Jackson-Pollock skinfold method work?
You take three skinfold readings with a caliper — chest, abdomen and thigh for men, or triceps, suprailiac and thigh for women — and add them together. The sum and your age go into an equation that predicts body density, and Siri’s equation then converts density into a body fat percentage. The calculator shows each of those steps with your numbers.
Why do Navy, BMI and skinfold results differ?
Each method reads something different — overall shape, size alone, or fat just under the skin — and each was built from a different group of people. Their errors lean in different directions for different builds, so a disagreement of a few points is normal. When tracking change over time, stick with one method.
About these results
The body fat figures on this page are estimates from published equations, not a clinical measurement of body composition. Each equation came from a particular study group — U.S. Navy personnel, Dutch adults, and American men and women tested with underwater weighing — and is less reliable during pregnancy, for competitive athletes and bodybuilders, for very lean people, and for anyone whose build differs a lot from those groups. Nothing here diagnoses obesity or any other condition, or replaces advice from a healthcare professional.
Sources: Hodgdon, J.A. & Beckett, M.B., Naval Health Research Center, 1984 (Navy circumference equations); Deurenberg, P., Weststrate, J.A. & Seidell, J.C., Body mass index as a measure of body fatness: age- and sex-specific prediction formulas, British Journal of Nutrition, 1991; Jackson, A.S. & Pollock, M.L., Generalized equations for predicting body density of men, British Journal of Nutrition, 1978; Jackson, A.S., Pollock, M.L. & Ward, A., Generalized equations for predicting body density of women, Medicine and Science in Sports and Exercise, 1980; Siri, W.E., 1961 (density to percent fat); Kouri, E.M. et al., Clinical Journal of Sport Medicine, 1995 (fat-free mass index); American Council on Exercise, body fat categories.