With a tape measure and your height, this body fat calculator estimates what share of your weight is fat, using the US Navy circumference method.
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Key points
- It is an estimate from tape measurements. The formula uses height plus neck and waist (and hips for women), and was built on US Navy personnel.
- Expect a margin of error of several percentage points, and a larger one in bigger, older or less muscular bodies than the original study groups.
- The real strength is tracking change. Measured the same way every month, the number shows whether you are losing fat even when the scale is slow.
- Body fat says more than BMI, but neither one alone tells you how healthy you are. Waist size, blood tests, fitness and how you feel all count.
- Other methods exist, such as DEXA scans and bioelectrical impedance scales. Each has its own limits, especially at a higher weight.
How to measure yourself for the calculator
Good measurements matter more than anything else here, because a 2 cm error on the waist shifts the result noticeably. You need a flexible tape measure, a mirror or a helper, and about five minutes.
- Height. Stand barefoot against a wall, heels together, and look straight ahead. Mark the top of your head with a flat object and measure to the floor.
- Neck. Place the tape just below the Adam’s apple or the bump of the voice box. Let it slope slightly downward towards the front. Keep your shoulders relaxed and do not flare your neck.
- Waist for men. Measure around the belly at the level of the navel, with the tape level all the way round.
- Waist for women. Measure at the narrowest part of the waist, usually a little above the navel. If there is no clear narrowing, measure halfway between the lowest rib and the top of the hip bone.
- Hips (women only). Stand with your feet together and measure around the widest part of the buttocks, seen from the side.
For every measurement, stand upright and breathe out gently before you read the tape. Hold it snug against the skin without pressing in, and keep it horizontal. Measure each site three times and use the average.
Tips for comfortable, consistent measuring
Measure on bare skin or over thin underwear, at the same time of day, ideally in the morning before eating. A mirror helps you check that the tape is level at the back, where it tends to slide down.
If reaching round is difficult, ask someone you trust, or use a tape with a lock button. Some people with a larger belly find it easier to measure while lying down, but always measure in the same position from then on.
Write the numbers down. Measuring again every four weeks gives the clearest picture of change.
What does your body fat percentage mean?
Your result is the estimated share of your body weight that is fat tissue. Women naturally carry more fat than men at the same weight and health, and most people gain fat with age, so the same number means different things for different people.
There is no single international table of healthy body fat ranges. The most widely used ranges come from a study by Gallagher and colleagues. They measured body fat precisely in more than 1,600 white, African American and Asian adults, and worked out which body fat levels matched the BMI cut-offs for healthy weight, overweight and obesity.
| Group | Age | Linked to healthy BMI | Linked to overweight | Linked to obesity |
|---|---|---|---|---|
| Women | 20–39 | 21–33% | 33–39% | 39% or more |
| Women | 40–59 | 23–34% | 34–40% | 40% or more |
| Women | 60–79 | 24–36% | 36–42% | 42% or more |
| Men | 20–39 | 8–20% | 20–25% | 25% or more |
| Men | 40–59 | 11–22% | 22–28% | 28% or more |
| Men | 60–79 | 13–25% | 25–30% | 30% or more |
Treat these ranges with care. The authors themselves called them provisional, they were derived from BMI rather than from health outcomes, and they differ by ethnic background. In that study, Asian adults tended to carry more body fat at the same BMI than white or African American adults.
What a high result does and does not mean
A result above these ranges suggests that you carry more fat than is usual for your age and sex. It does not tell you where that fat sits, how your organs are working, or what your blood pressure and blood sugar are.
Fat stored deep in the belly, around the organs, is more strongly linked to type 2 diabetes and heart disease than fat on the hips and thighs. That is why the waist measurement is useful on its own; our waist calculator turns it into a waist-to-height ratio.
If your result is high, take it as one reason to talk to your doctor about a fuller check, not as a label. Our page on what obesity is explains how doctors look at the whole picture.
How is body fat percentage calculated?
The calculator uses equations developed by James Hodgdon and Marcie Beckett at the Naval Health Research Center in San Diego. The US Navy needed a quick, cheap way to check body fat in thousands of service members without special equipment.
The researchers compared tape measurements with underwater weighing, a reference method that works out body density from how much a person weighs under water. They then searched for the simplest combination of measurements that best predicted the underwater result.
| Study | People measured | Measurements used | Typical error |
|---|---|---|---|
| Men (report 84-11) | 602 Navy men aged 18–56 | Height, neck, abdomen at the navel | About 3.5 percentage points |
| Women (report 84-29) | 214 Navy women aged 18–44 | Height, neck, narrowest waist, hips | About 3.7 percentage points |
Each equation first estimates body density from the logarithms of the measurements. A second step, the Siri equation, converts density into body fat percentage: 495 divided by density, minus 450.
For men, density is based on the difference between abdomen and neck, plus height. For women, it is based on waist plus hips minus neck, plus height. The neck is subtracted because it reflects body frame and muscle more than fat.
A worked example
Take a woman who is 165 cm tall, with a neck of 36 cm, a waist of 100 cm and hips of 124 cm. Waist plus hips minus neck is 188 cm. The equation gives a body density of about 0.990, which the Siri step converts to roughly 50% body fat.
Now imagine that, a few months later, her waist is 94 cm and her hips 120 cm, with the same neck and height. The result drops to about 46%. The exact number may be off by a few points, but the downward trend is real and meaningful.
If you see an error message, check that the waist is larger than the neck and that each value is in the right unit. Extreme results, under 2% or above 75%, are not shown because they point to a measuring mistake.
How accurate is the Navy method in larger bodies?
Less accurate than in the people it was built on. The original groups were mainly young, fit Navy personnel, and in cross-checks the error was already around 3 to 4 percentage points. In people with a high body fat level, the error is usually larger.
Several things contribute:
- The study groups. Few people in the original Navy samples had the body fat levels common in people living with obesity, so the equations are stretched beyond the range they were tested on.
- Soft tissue. The tape compresses soft tissue more in some places than others, and a small change in pressure can change the waist reading by a few centimetres.
- Finding the right spot. The navel and the narrowest waist can be harder to locate when the belly hangs lower, which makes repeat readings less consistent.
- Shape. Two people with the same three measurements can have very different amounts of fat stored deep inside the belly.
A study in US Marines compared a related abdominal tape method with DEXA scans. It tended to overestimate body fat in leaner people and underestimate it in people with more body fat. The authors concluded that tape methods are useful for screening groups, not for precise individual measurements.
The practical message: do not worry about whether your result is 41% or 44%. Use the same tape, the same spots and the same person measuring, and watch the direction over months.
Other ways to measure body fat
No method measures body fat perfectly in a living person. Each one estimates it from something else, such as X-ray absorption, electrical resistance or skin thickness. The table compares the options you are most likely to come across.
| Method | How it works | Strengths | Limits, especially at a higher weight |
|---|---|---|---|
| DEXA (DXA) scan | Low-dose X-rays distinguish fat, lean tissue and bone | Detailed, shows where fat is stored; often used as a reference in research | Costly, not widely available; very large bodies may not fit the scan area; small radiation dose |
| Bioelectrical impedance (BIA) | A weak current passes through the body; fat resists it more than water-rich tissue | Quick, found in many bathroom scales and clinics | Sensitive to hydration, meals and exercise; in obesity often underestimates fat mass |
| Skinfold calipers | Pinches of skin and fat at several sites are measured | Cheap; good in trained hands for leaner people | Hard to grasp thick folds; calipers may not open wide enough; depends on skill |
| Underwater weighing / air displacement | Body volume and density are measured in a tank or sealed pod | Long-standing reference methods | Specialist centres only; underwater weighing can be unpleasant |
| Tape methods (this calculator) | Circumferences and height are put into an equation | Free, private, can be repeated at home | Estimates with a margin of several points; less accurate in larger bodies |
A review of multi-frequency BIA in people with obesity found it reliable for comparing groups, but less trustworthy for one person’s exact number. Devices tended to report too little fat and too much lean mass, and algorithms adjusted for obesity did better.
If you use a smart scale at home, the same rule applies as for the tape: compare readings taken under the same conditions, such as first thing in the morning after using the toilet, and focus on the trend.
Is body fat percentage better than BMI?
BMI only compares weight with height, so it cannot tell fat from muscle. Body fat percentage looks at what the weight is made of, which is why experts increasingly want it measured alongside BMI.
The gap can be large. In a study of current and former US Navy men, a BMI of 30 or more labelled far fewer men as having obesity than a DEXA body fat measurement did. Many men with a BMI below 30 still had a high share of body fat.
An international commission in The Lancet Diabetes & Endocrinology has proposed that BMI alone should no longer be enough to diagnose obesity in an individual. Excess fat should be confirmed with a waist-based measure, or by measuring body fat directly, for example with DEXA.
Above a BMI of about 40, excess fat can be assumed. You can compare with your BMI using our BMI calculator, or read about clinical and preclinical obesity.
Limits of the calculator and who should be careful
This tool is designed for adults who are not pregnant. It is a rough, home-made estimate and cannot diagnose anything.
- Pregnancy. Body shape, fluid and the growing baby make the formula meaningless during pregnancy and for some months after birth. Follow your midwife or doctor.
- Children and teenagers. Their bodies change as they grow, and adult formulas do not apply.
- Swelling or fluid retention, for example from heart, kidney or liver disease or lymphoedema, changes measurements without changing fat.
- Lipoedema and other conditions that change fat distribution on the hips and legs can push a woman’s result up, because the hip measurement plays a large role.
- Eating disorders. If numbers about your body tend to trigger worry, restriction or bingeing, it is fine to skip this calculator. Talk to a professional; our page on psychological support can help.
- Very muscular bodies and older adults with muscle loss may be misjudged in either direction.
What to do with your result
Lowering body fat while keeping muscle comes down to steady habits in food and movement. Strength work deserves special attention, because it helps you keep the tissue you want while fat is lost. Talk to your doctor first before you start, especially if you have a heart condition or joint problems.
Build or keep muscle
Two short strength sessions a week are enough to start. The Start level of our free workout videos uses a chair and slow, controlled moves that are kind to joints.
Walk a little more each week
Regular walking uses energy, lifts mood and supports heart health at every body size. The walking plan starts with five minutes and builds from there.
Eat for fullness and protein
Meals with enough protein and fibre help you eat a little less without constant hunger, and protein supports muscle while you lose fat. Our free eating plan shows what that looks like on a normal day.
Track the trend, with support
- Log your monthly measurements in the free Obesity Foundation Club, next to your weight, walks and workouts.
- Book a free video meeting (in English) if you would like help making sense of your numbers and choosing a first step.
Related calculators
- Waist calculator: waist-to-height and waist-to-hip ratios from the same tape measure.
- BMI calculator: the quick screening number that body fat helps to put in context.
- Weight loss percentage calculator: see how much of your starting weight you have lost.
Frequently asked questions
What is a healthy body fat percentage?
There is no single agreed standard. Widely used provisional ranges suggest roughly 21 to 33% for women and 8 to 20% for men aged 20 to 39, with slightly higher ranges at older ages. These ranges were derived from BMI and differ by ethnic background, so treat them as a rough guide rather than a target.
Why does the calculator need my neck measurement?
The neck reflects body frame and muscle more than fat. By subtracting it from the waist (and hips for women), the US Navy formula separates people with a broad, muscular build from people whose larger waist is mainly due to fat.
Are smart scales more accurate than a tape measure?
Not necessarily. Bathroom scales use bioelectrical impedance, which is affected by how much you have drunk, eaten or exercised. In people with obesity, these devices often report too little fat. Both scales and tape methods work best for following trends under the same conditions.
How often should I measure my body fat?
Once every four weeks is enough. Body fat changes slowly, and measuring more often mostly shows measuring noise. Use the same tape, the same spots and, if possible, the same helper each time.
Can my body fat percentage go down while my weight stays the same?
Yes. If you start strength exercises and walk more, you may gain some muscle while losing fat, so the scale barely moves. A smaller waist or a lower body fat estimate can then show the progress the scale misses.
Sources
- Hodgdon JA, Beckett MB – Prediction of percent body fat for U.S. Navy men from body circumferences and height, Naval Health Research Center report 84-11 (1984)
- Hodgdon JA, Beckett MB – Prediction of percent body fat for U.S. Navy women from body circumferences and height, Naval Health Research Center report 84-29 (1984)
- Gallagher D et al. – Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index, American Journal of Clinical Nutrition (2000)
- Potter AW et al. – Circumference-based predictions of body fat revisited: preliminary results from a US Marine Corps body composition survey, Frontiers in Physiology (2022)
- Becroft L et al. – Validity of multi-frequency bioelectric impedance methods to measure body composition in obese patients: a systematic review, International Journal of Obesity (2019)
- Gasier HG et al. – Comparison of body composition assessed by dual-energy X-ray absorptiometry and BMI in current and former U.S. Navy service members, PLoS ONE (2015)
- Rubino F et al. – Definition and diagnostic criteria of clinical obesity, The Lancet Diabetes & Endocrinology Commission (2025)