Estimate Your Body Fat Percentage (No Calipers Needed)

The Deurenberg Formula

Body fat percentage tells you how much of your body weight is fat, which BMI alone cannot reveal. Measuring it precisely usually needs special equipment, but the Deurenberg formula gives a useful estimate from numbers you already have — your BMI, your age and your sex. The app uses this formula, so you get a body fat estimate automatically alongside your BMI, with nothing extra to buy or measure.

The formula is:

Body fat % = (1.20 × BMI) + (0.23 × age) − (10.8 × sex) − 5.4

where sex is 1 for men and 0 for women. Worked example: a 30-year-old woman with a BMI of 23. That is (1.20 × 23) + (0.23 × 30) − (10.8 × 0) − 5.4 = 27.6 + 6.9 − 0 − 5.4 = about 29% body fat. For a 30-year-old man with the same BMI of 23, the sex term subtracts 10.8, giving roughly 18%. This is why body fat, unlike BMI, genuinely differs by sex.

Typical Body Fat Ranges

Body fat percentage is interpreted differently for women and men, because women naturally carry more essential fat. Widely cited general reference ranges for adults are approximately:

• Women: essential fat around 10–13%, fit/athletic roughly 14–24%, average/acceptable around 25–31%, higher above that.
• Men: essential fat around 2–5%, fit/athletic roughly 6–17%, average/acceptable around 18–24%, higher above that.

These bands shift with age — it is normal for body fat to rise gradually over the years — and they vary between reference sources, so treat them as a rough map rather than strict lines. What matters more than a single figure is the trend over time and how you feel and function. The app's estimate helps you see roughly where you sit and whether your composition is moving in the direction you want.

How Accurate Is the Estimate?

Honesty matters here. The Deurenberg formula is a statistical estimate, not a direct measurement. Because it is derived from BMI, it shares BMI's core blind spot: it cannot see your actual body composition, so it can be off for very muscular people, athletes, older adults and some ethnic groups. For an individual, the estimate can differ from a true measurement by several percentage points in either direction.

More precise methods exist and are more accurate: DEXA scans (considered a gold standard), hydrostatic (underwater) weighing, air-displacement plethysmography (Bod Pod), and, less precisely, skinfold calipers and bioelectrical impedance scales. These require equipment, cost or training. The value of the Deurenberg estimate is convenience — it gives you a reasonable ballpark instantly, at no cost, which is genuinely useful for tracking change over time even if the absolute number is not exact.

Use the estimate as a directional guide, not a diagnosis. This guide is for general information only and is not medical advice — for an accurate body composition assessment or any health decision, consult a doctor or a qualified professional.