Body composition calculator
Body density calculator
Turn a body density into a body-fat percentage with both published equations at once — Siri and Brožek, side by side. Free, instant, no sign-up.
Add your bodyweight to split the estimate into fat mass and fat-free mass.
The number is settled — the coach handles the part that actually changes it.
What is body density?
Body density is your mass divided by your volume, in grams per cubic centimetre. It is the quantity underwater weighing and air-displacement plethysmography actually measure, and the quantity every skinfold equation actually predicts — Jackson & Pollock and Durnin & Womersley both stop at density. Fat is less dense than everything else in the body, at about 0.900 g/cm³ against roughly 1.100 for bone, muscle and organs, so the density of the whole tells you the proportions of the parts.
Siri and Brožek
Siri (1961) reasoned straight from those two densities and got %fat = 495/D − 450. Brožek and colleagues (1963) took a different route, anchoring the model on a chemically analysed "reference man" at about 15 % fat, and got %fat = 457/D − 414.2. The two lines cross at a density of 1.0615 — around 16 % body fat — and diverge either side of it, so at 30 % fat Siri reads roughly a point higher and at 6 % fat roughly half a point lower.
Which one should you use?
For most adults it barely matters — a point of disagreement is well inside the error of whatever produced the density in the first place. Siri is the default in sports science and the one nearly every calculator reports, so it is the right choice if you want your number to be comparable with anyone else's. Brožek is generally preferred at the extremes and for populations whose fat-free mass density is known to differ from the reference assumption: older adults with lower bone mineral density, and some ethnic groups where the two-compartment assumption is a poorer fit. The honest answer is to report which one you used.
How to use the number
Both equations are two-compartment models: they assume fat-free mass has a fixed density of 1.100 g/cm³. That assumption is what breaks in growing adolescents, in the very elderly, and in heavily dehydrated people, and no arithmetic downstream can repair it. Treat the percentage as an estimate with a real error bar, and track it against itself over months rather than against someone else's number.