Body Fat Calculator
This body fat calculator estimates your body fat percentage from weight, height, age, and sex. It first calculates your BMI, then applies the Deurenberg formula to convert that number into an estimated body fat percentage. The result is a quick reference figure, not a lab measurement, so it works best alongside other tracking methods rather than as a standalone diagnosis.
Quick answer
Body fat percentage is estimated from BMI, age, and sex using the Deurenberg formula.
What this tells you
- •Body fat percentage is estimated from BMI, age, and sex using the Deurenberg formula.
- •The calculator first works out BMI from your weight and height, then adjusts that figure for age and sex.
- •This is a population-based estimate, not a direct measurement of fat mass.
- •Results are most useful when tracked over time rather than judged from a single reading.
- •Athletes and people with high muscle mass may see estimates that read higher than their actual body fat.
How to Use
- 1Enter your weight in kilograms.
- 2Enter your height in centimeters.
- 3Enter your age in years.
- 4Select your sex (male or female), since the formula applies a different adjustment for each.
- 5Select Calculate to see your estimated body fat percentage and BMI.
How It Works
Formula
Body Fat % = 1.20 × BMI + 0.23 × Age − 10.8 × Sex − 5.4
(Sex = 1 for male, 0 for female. BMI = weight in kg ÷ (height in m)²)This is the Deurenberg formula, a widely cited equation that estimates body fat percentage from BMI rather than from skinfold thickness or bioelectrical signals. The calculator first computes BMI from your weight and height, then multiplies that number by 1.20 and adds a small upward adjustment for age (0.23 points per year). It then applies a downward adjustment for male sex (10.8 points, since men typically carry a lower body fat percentage than women at the same BMI) and subtracts a constant of 5.4. Because the formula relies on BMI, it inherits BMI's main weakness: it cannot tell the difference between muscle and fat, so it estimates body composition indirectly rather than measuring it.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Adult male, average build
BMI = 70 ÷ (1.75 × 1.75) = 22.9. Body fat % = (1.20 × 22.9) + (0.23 × 30) − (10.8 × 1) − 5.4 = 27.4 + 6.9 − 10.8 − 5.4 = 18.1%. This falls in a typical healthy range for an adult male of this age.
Adult female, average build
BMI = 60 ÷ (1.65 × 1.65) = 22.0. Body fat % = (1.20 × 22.0) + (0.23 × 28) − 0 − 5.4 = 26.4 + 6.4 − 5.4 = 27.5%. The estimate is higher than the male example at a similar BMI, because the formula does not subtract the 10.8-point male adjustment for female inputs.
Older male, higher BMI
BMI = 85 ÷ (1.80 × 1.80) = 26.2. Body fat % = (1.20 × 26.2) + (0.23 × 50) − 10.8 − 5.4 = 31.5 + 11.5 − 10.8 − 5.4 = 26.8%. Both the higher BMI and the higher age push the estimate up compared with the first example.
Younger female, lower BMI
BMI = 50 ÷ (1.60 × 1.60) = 19.5. Body fat % = (1.20 × 19.5) + (0.23 × 22) − 0 − 5.4 = 23.4 + 5.1 − 5.4 = 23.1%. Even at a lower BMI, the estimate stays a fair distance above the male examples because of how the formula treats sex.
Active male, higher weight
BMI = 95 ÷ (1.85 × 1.85) = 27.8. Body fat % = (1.20 × 27.8) + (0.23 × 35) − 10.8 − 5.4 = 33.3 + 8.1 − 10.8 − 5.4 = 25.2%. If most of that weight is muscle rather than fat, this estimate can read higher than a direct measurement such as a DEXA scan would show.
Linda, 52, checks the estimate after losing 6 kg
Linda runs the calculator in January at 78 kg and again in May at 72 kg. Her BMI falls from 78 ÷ (1.65 × 1.65) = 28.65 to 72 ÷ 2.7225 = 26.45, and the estimate falls from (1.20 × 28.65) + (0.23 × 52) − 5.4 = 40.9% to 38.3%. At 165 cm every kilogram is worth about 0.44 points, so 6 kg moves the estimate by 2.6 points. The formula cannot tell what those 6 kg were made of. If she kept lifting and held on to her muscle, most of the loss was fat and a DEXA scan would likely show a bigger drop than 2.6 points. If she lost muscle too, the real drop is smaller. A waist measurement taken on the same mornings is a cheap cross-check.
Estimated body fat by BMI, age, and sex
Each cell is what this calculator returns for that BMI, age, and sex. Read your BMI from the result card, then read across. For ages between the columns, add or subtract 0.23 points per year.
| BMI (CDC adult class) | Man, 30 | Man, 50 | Man, 70 | Woman, 30 | Woman, 50 | Woman, 70 |
|---|---|---|---|---|---|---|
| 18.5 (healthy weight starts) | 12.9% | 17.5% | 22.1% | 23.7% | 28.3% | 32.9% |
| 20 (healthy weight) | 14.7% | 19.3% | 23.9% | 25.5% | 30.1% | 34.7% |
| 22.5 (healthy weight) | 17.7% | 22.3% | 26.9% | 28.5% | 33.1% | 37.7% |
| 25 (overweight starts) | 20.7% | 25.3% | 29.9% | 31.5% | 36.1% | 40.7% |
| 27.5 (overweight) | 23.7% | 28.3% | 32.9% | 34.5% | 39.1% | 43.7% |
| 30 (obesity class 1 starts) | 26.7% | 31.3% | 35.9% | 37.5% | 42.1% | 46.7% |
| 35 (obesity class 2 starts) | 32.7% | 37.3% | 41.9% | 43.5% | 48.1% | 52.7% |
| 40 (obesity class 3 starts) | 38.7% | 43.3% | 47.9% | 49.5% | 54.1% | 58.7% |
Computed with the live formula: body fat % = 1.20 × BMI + 0.23 × age - 10.8 × sex - 5.4 (sex = 1 for men, 0 for women), rounded to one decimal as the result card rounds it. Each cell was produced by running the calculator at 170 cm with the weight that gives that exact BMI. The equation is the adult formula from Deurenberg, Weststrate and Seidell (British Journal of Nutrition, 1991), whose study of 1,229 people reported a standard error of 4.1 body fat points. Their sample spanned BMI 13.9 to 40.9, so the 40 row sits at the edge of the data. BMI classes follow the CDC adult categories.
What the Deurenberg equation measured, and where it misses
Deurenberg, Weststrate and Seidell published this equation in the British Journal of Nutrition in 1991. They measured body density by densitometry in 1,229 people aged 7 to 83 (521 male, 708 female), converted density to fat percentage with Siri's equation, and then fitted that result against BMI, age, and sex. For adults the fit was body fat % = 1.20 × BMI + 0.23 × age - 10.8 × sex - 5.4, which explained 79 percent of the variation with a standard error of estimate of 4.1 percentage points. Children 15 and under needed their own equation (1.51 × BMI - 0.70 × age - 3.6 × sex + 1.4) because BMI rises with height while children grow. This calculator uses only the adult version.
A standard error of 4.1 points means roughly two people in three land within 4 points of their measured value, and about one in twenty misses by more than 8 points if the errors follow a normal spread. In the table above, a 50-year-old man at BMI 25 reads 25.3 percent, so his lab result could plausibly sit anywhere from about 17 to 33.5 percent. The authors noted that this error is close to what skinfold calipers and bioelectrical impedance give. In their cross-validation the equation held up across ages but slightly overestimated body fat in people with obesity.
BMI is the only body input, and the CDC is explicit that BMI cannot separate fat from muscle and bone. A 30-year-old man at BMI 27.5 reads 23.7 percent here whether the extra weight is muscle or fat. The link between BMI and fat also shifts by ethnic group. In a 2002 review in Obesity Reviews, Deurenberg's group found that Asian populations carried 3 to 5 points more body fat than Caucasians at the same BMI. Their 1998 meta-analysis in the International Journal of Obesity found BMI 1.9 units lower in Chinese, 4.6 in Ethiopian, 3.2 in Indonesian and 2.9 in Thai groups than in Caucasians with the same body fat, age, and sex. For those groups this formula is likely to read low.
A widely reproduced body fat chart comes from the American Council on Exercise. Its band for typical non-athletes runs 18 to 24 percent for men and 25 to 31 percent for women, and its obese band starts at 25 percent for men and 32 percent for women. That chart has no age adjustment, while this equation adds 0.23 points per year, or 4.6 points every 20 years at the same BMI. So a 70-year-old woman at BMI 22.5, well inside the CDC healthy-weight range, reads 37.7 percent here and lands above the ACE obese line, while the same BMI at 30 reads 28.5 percent. Gallagher and colleagues took an age-aware route in the American Journal of Clinical Nutrition in 2000. They measured 1,626 adults with four-compartment models and DXA, predicted body fat from BMI, sex, age, and ethnic group, and proposed fat ranges matched to the BMI cutoffs of 18.5, 25, and 30, which they described as groundwork rather than final standards.
Between two readings only weight and age can change, so the estimate moves 1.2 points per BMI unit, about 0.39 points per kilogram at 175 cm, and it cannot register fat lost while muscle is gained at the same weight. Tape methods add information that BMI lacks. The circumference equations Hodgdon and Beckett developed at the Naval Health Research Center in 1984, tested on 602 Navy men, use neck and waist girths, so a shrinking waist at a steady weight changes their result. DXA or a four-compartment model in a lab gives a direct reading when a decision depends on the number.
Common mistakes
- Entering pounds in the weight field when the calculator expects kilograms, which throws off both BMI and the body fat estimate.
- Mixing up height units, such as entering total inches as centimeters.
- Treating the result as a clinical diagnosis rather than a general estimate.
- Comparing this formula's output directly against a caliper, DEXA scan, or bioelectrical impedance reading, which can use different methods and give different numbers.
- Expecting the estimate to shift meaningfully from small day-to-day changes in weight, when body composition typically changes gradually over weeks or months.
- Ignoring that the formula was built from population averages, so it can be less accurate for athletes, older adults, or people outside typical body proportions.
Limitations
This calculator uses the adult Deurenberg equation (Deurenberg, Weststrate and Seidell 1991), which predicts body fat from BMI, age, and sex instead of measuring fat mass. It was fitted against densitometry with a standard error of 4.1 percentage points, so about one person in three lands more than 4 points from a measured value. Because it relies on BMI, it cannot distinguish muscle from fat or bone, which the CDC names as BMI's main limit, so it tends to read high in muscular people, and the authors found it slightly overestimates body fat in people with obesity. The BMI and body fat relationship also differs by ethnic group. Asian populations studied by Deurenberg's group carried 3 to 5 points more fat than Caucasians at the same BMI (Obesity Reviews 2002), so the estimate may read low for them. The 1991 paper fitted a separate equation for children 15 and under, so the tool accepts ages 16 and up and gives no estimate for younger ages. It is also not designed for pregnant people or anyone with atypical body proportions. Results can differ from skinfold calipers, bioelectrical impedance scales, or DEXA scans, since each method measures body composition differently. Use this estimate as a general reference point, not a substitute for a direct assessment.
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