What this estimator works out
This estimator applies the Deurenberg equation, which predicts body fat percentage from BMI, age and sex. It is one of the simplest published methods and requires no equipment beyond a set of scales and a tape measure.
It is also, unavoidably, imprecise. The equation was derived by fitting a line through population data, and any individual can sit several percentage points either side of it. That limitation is not a caveat to skim past — it is the main thing to understand about the result.
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Before you rely on the figure
- Calculate your BMI first, using the BMI Calculator, and enter that figure rather than your weight.
- The equation was derived from adult populations. It is not valid for children, and it performs poorly for athletes and very muscular people.
- It is not appropriate during pregnancy.
- Treat the result as a broad band rather than a number — plus or minus four percentage points is a realistic error range.
Accurate methods — DEXA scanning, hydrostatic weighing, air displacement — measure the body directly and are expensive. Everything cheap, including bathroom scales that claim to measure body fat, is estimating from something else. Bioelectrical impedance scales are particularly sensitive to hydration and can vary by several percentage points across a single day.
How this calculator works
The Deurenberg equation, applied directly:
Body fat % = (1.2 × BMI) + (0.23 × age) − (10.8 × sex) − 5.4
where sex is 1 for male and 0 for femaleThe sex term is a constant offset reflecting the fact that women typically carry a higher proportion of body fat than men at the same BMI. The age term reflects the tendency for fat to increase and muscle to decrease with age at a stable weight.
Because everything is derived from BMI, the equation inherits BMI's central weakness: it cannot distinguish muscle from fat. A very muscular person will get a substantially overstated result.
Worked example: BMI 24.5 at age 40
Using the default figures — a BMI of 24.5, age 40, male:
- 1.2 × 24.5 = 29.4
- 0.23 × 40 = 9.2
- 10.8 × 1 = 10.8, subtracted
- Less the constant 5.4
- Estimated body fat: 22.4%
Change nothing but the sex to female and the estimate rises to 33.2%, which is the 10.8 point offset applied in full. That single term does more work than a decade of age, and it is worth understanding that the difference is built into the equation rather than measured from you.
Common mistakes
- Entering weight instead of BMI. The first field expects a BMI figure.
- Reading the result as a measurement. It is a prediction from population data.
- Using it if you train regularly with weights. It will substantially overstate body fat.
- Comparing it against a scale reading. Impedance scales use a different method and will not agree.
- Tracking small changes. The error range is wider than any change worth acting on.
Frequently asked questions
How accurate is it?
For an individual, roughly plus or minus four percentage points against a reference method, and worse at the extremes of body composition. It performs reasonably across a population and poorly for anyone unusual — which includes athletes, very lean people, and those with high or low muscle mass for their weight.
What is a healthy body fat percentage?
There is no single agreed figure, and published ranges vary between sources and by age. Broadly, essential fat is higher in women than men, and both very high and very low body fat carry health consequences. Because this estimate carries several percentage points of uncertainty, using it to aim at a specific target is not sensible.
Are body fat scales better?
Not necessarily, though they measure something different. Bioelectrical impedance scales pass a small current through the body and infer composition from resistance, which is strongly affected by hydration, recent food and exercise. They are reasonably good at showing a trend if used consistently at the same time of day, and poor at giving an accurate absolute figure.
What is measured accurately?
DEXA scanning is the usual practical reference, and is sometimes available privately. Hydrostatic weighing and air displacement plethysmography are more accurate again but rarely accessible. All of them cost money, and for most people the practical measures — waist measurement, how clothes fit, blood pressure and blood tests — are more useful than a precise composition figure.
Is what I enter stored?
No. Your figures are processed in your browser and never transmitted or retained. If thinking about weight, food or body shape is causing you distress, these tools are unlikely to help and may make things harder. Beat, the UK eating disorder charity, runs a free helpline and online support, and a GP can refer you to specialist services.
Related tools
References
- NHS — healthy weight, body composition and general health guidance
- National Institute for Health and Care Excellence — guidance on assessing weight and health risk in adults
Sources are checked at publication and can change — how I choose and check references.