What a body fat percentage calculator does, and what it cannot do
Body fat percentage is the share of your total weight that is fat, as opposed to muscle, bone, organs and water. It answers the question the bathroom scale cannot: two people of the same height and weight can be built very differently, and the scale reports one identical number for both. This page estimates that share with the Deurenberg formula, published in 1991, which predicts body fat from three things you already know:
body fat % = 1.20 × BMI + 0.23 × age − 10.8 × sex − 5.4
(sex: 1 for men, 0 for women)
Age and sex are in the equation because body composition is not the same at every BMI. At the same height and weight, an adult in their fifties usually carries more fat and less muscle than a twenty year old, and women carry more essential fat than men for reproductive and hormonal reasons. The formula was fitted to adults whose fat had been measured properly, so it returns the typical value for people with your BMI, age and sex.
That word typical is the whole caveat. This is an estimate, not a measurement: for any individual it can be off by several percentage points in either direction, and it is furthest off for people whose build is unusual for their BMI, such as a trained lifter or an older adult with fluid retention. Use it to see which broad range you are in and which way the number is moving, not to settle a figure to one decimal place.
Which method to use for which question
- This calculator (BMI, age and sex): you want a free ballpark figure today and a trend line over the coming months. No equipment beyond a scale and a tape for your height.
- The US Navy tape method: you want an estimate that reacts to your shape rather than your weight. It measures height plus the circumference of the neck and waist, with the hips added for women, and ignores weight entirely.
- Skinfold calipers: you have someone who knows how to use them and will pinch the same sites every time. Cheap, sensitive to change, and heavily dependent on the tester.
- A bioimpedance scale or gym body composition machine: you want a number at home several times a week and understand that hydration moves it. Best read as a weekly average.
- A DEXA scan, air displacement or underwater weighing: you want the closest thing to a real measurement for a baseline or a before and after. Costs money and an appointment, so once or twice a year is realistic.
- BMI on its own: population screening and forms that ask for it. It says nothing about composition, which is why you are on this page.
How to get your estimate
- Pick your units. The calculator opens in feet, inches and pounds; the Metric option switches to centimeters and kilograms and converts what you already typed.
- Enter height, weight and age, then choose male or female. The formula has no third option, which is a limitation of the equation rather than a statement about people.
- Read the three figures that appear: the estimated body fat percentage, the BMI it was derived from, and the category that percentage falls into.
- Check the colored bar underneath. It shows where the estimate sits across the whole range for your sex, which matters more than the number when you are close to a boundary.
- The reference table highlights the row you are in, and the panel below shows your FFMI, an index of how much lean mass you carry for your height.
How to track the number over months
A single reading is close to meaningless. A line of readings taken the same way is genuinely useful, and this is where a formula beats a gadget: it has no hydration noise, because it only knows what you typed. Weigh yourself first thing in the morning, after the bathroom and before breakfast, on the same scale. Record the weight once a week, and come back here once a month to recompute. A common guideline is to aim for roughly half a kilogram to one kilogram of body weight a week, which for most people works out at about one percentage point of body fat a month.
Know what the line can and cannot see. Age and sex barely move, so month to month the estimate is really your weight change dressed up as a percentage. Swap two kilograms of fat for two kilograms of muscle and your weight is unchanged, so this page reports no progress even though your composition improved. Keep a tape measure reading of your waist in the same note, because that number does move when the formula stays still. To plan the calorie side, the TDEE calculator estimates what you burn in a day.
How to keep, print or share the result
- Screenshot the result card: on iPhone press the side button and volume up together, on Android the power and volume down buttons, on Windows use Win+Shift+S, and on a Mac Cmd+Shift+4. The category, the bar and the table all fit in one capture.
- Write it into a log: four short fields are enough, namely date, weight, estimated body fat and waist. A note on your phone or a spreadsheet column makes the trend readable a year later.
- Print it for an appointment: Ctrl+P on Windows or Cmd+P on a Mac prints the page as it stands, ready to bring to a trainer or a clinician.
- Share the tool, not your data: your entries never appear in the address bar, so sending someone the link sends them an empty calculator.
Why every method and chart gives a different number
- The ranges themselves: the categories on this page follow the widely reproduced chart from the American Council on Exercise. Other organizations publish their own bands, and no body fat cutoff carries the official standing that the BMI thresholds do. Treat the labels as commonly used ranges rather than a medical standard.
- The US Navy tape method: circumference based, so a thick neck lowers the estimate and a wide waist raises it. It can disagree with a BMI based estimate by several percentage points on the same person, especially for muscular builds.
- Smart scales and handheld devices: these send a small current through the body and infer fat from the resistance. Readings move with hydration, food and alcohol, a foot to foot scale mostly senses the lower body, and each manufacturer applies its own correction. Two scales disagreeing is normal.
- DEXA scans: the closest thing to a reference in routine use, and repeatable on the same machine. Different manufacturers and software versions do not always agree with each other, so compare scans from the same clinic rather than across clinics.
- Calipers: the result depends on the equation chosen, the sites pinched and the hands holding them. Same tester, useful trend. Different tester, different number.
- Country to country: national health checks usually record BMI and waist circumference rather than body fat, and some countries apply lower BMI thresholds for Asian populations. Body fat targets set by sports bodies and armed forces are their own rules, not general health advice.
Limits, health notes and privacy
This page is general information, not medical advice. It does not diagnose anything and it is not a substitute for an assessment by a doctor, a registered dietitian or another qualified professional. If you are worried about your weight or planning a significant change to how you eat or train, take that conversation to a clinician who can look at your history rather than to a formula that knows four numbers.
The equation was developed on adults, so it should not be used for children or teenagers, during pregnancy, or for people with significant fluid retention, and it is unreliable for amputees and for very muscular or very old bodies. Lower is also not automatically better: essential fat exists for a reason, and dropping below it disrupts hormones, bone density and, in women, the menstrual cycle. If watching this number starts to feel compulsive, or is driving how much you eat, that is worth raising with someone rather than optimizing further.
Finally, remember what the number is: a prediction with a spread of several percentage points around it. A change from 21 to 20 percent on this page is a weight change, not a verified change in body composition. If you need a figure you can defend, book a scan.
Frequently asked questions
How accurate is a body fat percentage calculator?
Treat the number as an estimate, not a measurement. The Deurenberg formula was fitted to a group of adults whose body fat had been measured properly, so it describes the average person of your BMI, age and sex. For any one individual the prediction can be off by several percentage points in either direction, and it is furthest off for people whose build is unusual for their BMI. It is best used to watch a trend over months, not to settle an argument about a single figure.
What is a healthy body fat percentage for men?
The categories most often quoted come from the American Council on Exercise: roughly 2 to 5 percent is essential fat, 6 to 13 percent is the athlete range, 14 to 17 percent is described as fitness, 18 to 24 percent is average, and 25 percent or more is classed as obese. These are commonly used ranges rather than a medical standard, and no single number is a diagnosis on its own.
What is a healthy body fat percentage for women?
Using the same widely quoted ACE categories: roughly 10 to 13 percent is essential fat, 14 to 20 percent is the athlete range, 21 to 24 percent is fitness, 25 to 31 percent is average, and 32 percent or more is classed as obese. Women carry more essential fat than men because of reproductive and hormonal function, so a target borrowed from a male chart is usually too low to be healthy.
Why does my smart scale give a different number?
Bathroom scales that report body fat use bioelectrical impedance: they pass a tiny current through the body and infer fat from the resistance. The reading moves with hydration, food, alcohol, skin temperature and how damp your feet are, and a foot-to-foot scale mostly senses the lower body. Each manufacturer also applies its own correction, so two scales can disagree with each other and with this page. Weigh at the same time of day under the same conditions and read the direction of travel, not the absolute figure.
Is this the same as the US Navy body fat calculator?
No. The Navy method is a tape measure method: it uses height plus the circumference of the neck and waist for men, with the hips added for women, and it never looks at your weight. This page uses the Deurenberg formula, which starts from BMI, age and sex. The two methods can disagree by several percentage points on the same person, which is a fair reminder that both are estimates of the same underlying quantity.
How is body fat actually measured?
A DEXA scan uses low dose X-rays to separate fat, lean tissue and bone, and it is the reference most research compares against. Air displacement in a sealed chamber and underwater weighing work from body density. Skinfold calipers pinch fat at several sites and feed the thickness into an equation, which is cheap but depends heavily on the skill of whoever holds the calipers. All of these need equipment or a trained tester, which is why formula based estimates exist.
Can I have a normal BMI and still have high body fat?
Yes, and it is common enough to have a name in the literature: normal weight obesity. BMI only knows your height and weight, so it cannot tell muscle from fat. Someone who has lost muscle through years of inactivity can sit in the healthy BMI band with a body fat percentage well into the obese range, while a trained athlete can be flagged as overweight by BMI at a very low body fat. Since this calculator is built on BMI, it inherits that blind spot.
Why does the estimate look wrong for a very muscular person?
Because muscle raises BMI exactly the way fat does, and the formula sees only the BMI. A lifter or a rugby player will usually be told they have several percentage points more fat than they really carry. The same goes in reverse for someone with little muscle. If you train seriously, use calipers or a scan for the absolute number and keep this page for tracking direction between measurements.
How fast can body fat realistically come down?
A commonly used guideline is to aim for about half a kilogram to one kilogram of body weight per week, which usually works out at roughly one percentage point of body fat per month for most people. Going faster tends to cost muscle along with fat, which lowers the amount you burn at rest. Pair a modest calorie deficit with resistance training, and check progress every few weeks rather than every day.
Does this work for children, older adults or during pregnancy?
No. The prediction formula was developed on adults, so it should not be applied to children or teenagers, whose body composition is read from growth charts instead. It also has no meaning during pregnancy, when weight gain is expected and is not fat gain. Results for people over about 70, for anyone with a large amount of fluid retention, and for amputees are unreliable too. Ask a clinician for an assessment that fits the situation.
Sources
- The prediction formula comes from Deurenberg, Weststrate and Seidell, “Body mass index as a measure of body fatness: age- and sex-specific prediction formulas”, published in the British Journal of Nutrition in 1991. The paper fitted body fat measured by body density against BMI, age and sex in a group of adults, and reported a prediction error of several percentage points for individuals, which is why this page calls the output an estimate.
- The category names and ranges follow the body fat chart published by the American Council on Exercise, which is reproduced very widely in fitness writing. It is a commonly used reference rather than a clinical standard, and other bodies publish different bands.
- FFMI, the fat-free mass index shown under the main result, follows the definition used by Kouri and colleagues in a 1995 study of lean mass in drug-free and steroid-using athletes, published in the Clinical Journal of Sport Medicine. The height correction term and the figure of about 25 as a natural ceiling come from that work.
- The tape method described on this page is the circumference based equation used by the US armed forces for body composition screening. Descriptions of DEXA, air displacement, underwater weighing, calipers and bioimpedance reflect how those methods work in general practice; individual devices and clinics vary.
More health calculators
The BMI this estimate is built on has its own page, with the full range chart, at the BMI calculator. To turn a fat loss goal into a daily calorie figure, start with the BMR calculator for what your body burns at rest and the TDEE calculator for what you burn in a full day. The rest of the set is listed on the ToolMaple home page.
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Privacy: the calculation runs entirely in your browser. Your height, weight, age and sex are never sent to a server, never stored, and never added to the page address. Site analytics records the page view only; the calculator itself sends no event and never sees your numbers.