Your scale tells you how much you weigh, not what that weight is made of. Body fat percentage answers the more useful question: how many of those kilos are fat? Here is how the number is calculated, how to turn it into kilos you can actually track, and what counts as a healthy range for your age and sex.
What body fat percentage actually measures
Body fat percentage is simply the share of your total body mass that is fat. The calculation is a plain division: total fat mass divided by total body mass. If 20 kg of an 80 kg body is fat, the figure is 25%.
That fat is not all the same. Part of it is essential fat, which your body needs for vital functions. The rest is storage fat, accumulated in adipose tissue. This split explains why the healthy floor is so different between men and women: women need roughly 9 percentage points more essential fat than men, for hormonal and childbearing reasons. For men, the minimum generally considered healthy sits around 4 to 6%.
How the percentage is calculated from body density
Nobody can weigh their fat directly, so the classic lab approach measures body density instead, usually by weighing you underwater (hydrostatic weighing). Fat is less dense than the rest of the body, so the lower your overall density, the higher your fat share. Two well known formulas turn that density into a percentage:
- Siri formula: body fat % = (4.95 ÷ density) − 4.50
- Brozek formula: body fat % = (4.57 ÷ density) − 4.142
In plain language: divide a fixed number by your measured density, then subtract a constant. Multiply the result by 100 to read it as a percentage.
A worked example, from density to kilos
Take a measured body density of 1.05 g/cm³:
- Siri: 4.95 ÷ 1.05 = 4.714, minus 4.50 = 0.214, so about 21.4% body fat
- Brozek: 4.57 ÷ 1.05 = 4.352, minus 4.142 = 0.210, so about 21.0%
The two formulas land within half a point of each other. Now make the number concrete. Suppose this person weighs 80 kg:
- Fat mass = 80 × 0.214 = about 17.1 kg
- Lean mass (everything else) = 80 − 17.1 = about 62.9 kg
This is where the percentage becomes genuinely useful. If the goal is 15% body fat, keep lean mass fixed and divide it by the lean share you want: 62.9 ÷ (1 − 0.15) = about 74 kg. In other words, reaching 15% would mean losing roughly 6 kg of pure fat, assuming no muscle is lost along the way (in practice some usually is, so treat this as a best case target, not a promise).
What is a good body fat percentage?
There is no single “good” number, because age and sex shift the picture. Two useful reference points:
- Population averages (US NHANES data, 1999 to 2004): men go from about 23% at ages 16 to 19 to 31% at ages 60 to 79. Women go from 32% at ages 8 to 11 to 42% at ages 60 to 79.
- Athletic performance ranges suggested by the University of Arizona: 6 to 15% for men, 12 to 18% for women.
So our 21.4% example sits below the average for men of any age group listed, but above the athletic range. Whether that matters depends entirely on the goal.
Common mistakes when reading your number
- Using the same target for men and women. Because women carry around 9 points more essential fat, a figure that is lean for a man can be unhealthily low for a woman.
- Trusting a single smart scale reading. Bathroom scales use bioelectrical impedance (BIA), which is sensitive to hydration and the time of day. Compare readings taken under the same conditions, and watch the trend rather than one value.
- Treating any method as exact. Even underwater weighing, considered the reference method, carries an uncertainty of about ±3.8%. Skinfold calipers depend heavily on who takes the measurement, while DEXA scans are among the most precise options.
- Over-trusting tape measure formulas. The US Navy method, based on neck, waist and hip circumferences, was designed for military use and is criticised for limited accuracy outside that context.
Body fat vs BMI
BMI only uses height and weight, so it cannot tell fat from muscle. Body fat percentage looks at what the weight is made of. If you only have a scale and a tape measure, BMI is still a quick first screen, and you can run yours with our BMI calculator. Just remember that two people with the same BMI can have very different body fat percentages.
