Free Body Fat Percentage Calculator
Estimate adult body fat percentage and fat and lean mass using the BMI-based Deurenberg equation. Use advanced options and copy or download your results. Browser-based and no signup required.
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How to use the body fat percentage calculator
Estimate adult body fat percentage and fat and lean mass using the BMI-based Deurenberg equation. No signup required.
- Choose metric or US measurements and enter adult age, weight and height. Unit changes convert the current measurements.
- Select the sex-specific option used by the Deurenberg population equation.
- Review estimated body-fat percentage alongside BMI, estimated fat mass and estimated lean mass.
- Read the limitations before copying or downloading; do not treat the estimate as a clinical measurement.
When a BMI-based body fat estimate is useful
This calculator suits adults who want a rough body fat figure without calipers, a tape measure or a scan. Height, weight, age and sex are things most people already know, so it gives a starting point in a few seconds. It can help you put a BMI number in context, since two people with the same BMI may differ in age.
It is also useful for tracking a general trend. If you recalculate every few months with the same inputs measured the same way, you can see whether the estimate is moving. Coaches and students sometimes use it to understand how the Deurenberg equation behaves before using more detailed methods.
It is general information only. For a health decision, a goal plan or a medical concern, talk with a doctor or a registered dietitian who can measure body composition directly.
Worked example with real numbers
Take a 40-year-old using the male formula who weighs 80 kg and is 180 cm tall. First the calculator finds BMI: 80 divided by 1.80 squared, which is 80 / 3.24 = 24.69 kg/m².
Next it applies the equation: body fat % = 1.20 x 24.69 + 0.23 x 40 - 10.8 x 1 - 5.4. That is 29.63 + 9.20 - 10.80 - 5.40 = 22.63%. Estimated fat mass is 80 x 0.2263 = 18.10 kg, and estimated lean mass is 80 - 18.10 = 61.90 kg.
For comparison, a 35-year-old using the female formula who weighs 62 kg and is 165 cm tall has a BMI of 22.77. The estimate is 1.20 x 22.77 + 0.23 x 35 - 5.4 = 29.98%, with about 18.59 kg of fat mass. The tool shows these results with more decimal places and lets you copy or download them.
How the Deurenberg equation works
The formula was published by Deurenberg and colleagues in 1991 after comparing BMI with body fat measured in a reference population. It adds an age term because body fat tends to rise with age at the same BMI, and it subtracts 10.8 points for the male formula because men on average carry less fat at the same BMI.
The tool accepts adults aged 18 to 100. In US mode it converts pounds and inches to kilograms and centimeters before calculating, and the mass results are shown in kilograms. If the inputs push the equation to zero or below, or to 100 or above, the tool shows an error instead of a misleading number.
Because it uses only BMI, age and sex, the equation cannot see where weight comes from. A muscular person may get an estimate that is too high, and someone with low muscle mass may get one that is too low.
Common mistakes to avoid
Do not compare this number directly with readings from a smart scale, calipers or a DEXA scan. Each method has its own error, and switching methods can look like a change that did not happen. Pick one method for tracking and stick with it.
Enter weight and height in the unit system selected. Typing centimeters while US mode is active, or pounds in metric mode, will produce a wildly wrong BMI. Also avoid using the result for children, teenagers or during pregnancy, since the equation was not built for them. Treat the output as a broad estimate, and ask a professional before setting targets based on it.
Useful tips and limitations
- This is the adult BMI-based method, not a Navy circumference tape test.
- Body build and population differences can change accuracy; professional measurements may be more appropriate.
- Weigh yourself at the same time of day, such as in the morning, so small fluid swings do not move the estimate.
- If you lift weights regularly, expect the estimate to read high and consider a direct measurement for a better picture.
Common questions
Which body-fat method does this percentage calculator use?
It uses the adult Deurenberg BMI-based equation with age and a sex-specific coefficient. It does not use neck, waist or hip circumference and is not a Navy tape-test tool.
Is BMI-derived body-fat percentage a direct measurement?
No. It is a population estimate and can differ from actual body composition, especially for unusual muscle mass or body build. It is not a diagnosis.
Who should avoid using this adult body-fat estimate?
It is not designed for children, pregnancy or clinical assessment. Results outside the usable equation range are rejected rather than being forced into a percentage.
Why does my estimate go up as I get older at the same weight?
The equation adds 0.23 percentage points for each year of age, reflecting the average shift in body composition with age in the study population.
Are the fat and lean mass results shown in pounds in US mode?
No. The calculator converts your inputs and reports fat mass and lean mass in kilograms. Multiply by about 2.2046 to get pounds.
Why do I get an error instead of a result?
Very low or very high BMI values can push the equation outside zero to 100 percent. The tool rejects those cases, so check that the units and numbers are correct.
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