Hydrostatic Weighing Body Fat Calculator
Turn underwater (hydrostatic) weighing results into body density and body-fat percentage with the Siri and Brozek equations, correcting for water temperature and residual lung volume.
How it works
body volume (L) = (mass on land − mass under water) ÷ water density − (residual volume + 0.1 L gut gas) · body density = mass on land ÷ body volume · Siri: %BF = 495 ÷ BD − 450 · Brozek: %BF = 457 ÷ BD − 414.2
Hydrostatic (underwater) weighing applies Archimedes' principle: the weight lost under water equals the weight of water displaced, which gives body volume. Air left in the lungs (residual volume) and about 100 mL of gas in the gut make you more buoyant, so they are subtracted. Fat is less dense (0.90 g/cm³) than lean tissue (1.10 g/cm³), so body density converts to fat percentage through the two-compartment Siri or Brozek equations. Water density is computed from temperature (Tilton–Taylor equation).
Worked example
A person weighing 80 kg on land and 3.5 kg fully submerged in 34 °C water (density 0.99440), with a residual volume of 1.3 L: volume = 76.5 ÷ 0.99440 − 1.4 = 75.53 L, density = 80 ÷ 75.53 = 1.0592 g/cm³, so body fat = 495 ÷ 1.0592 − 450 = 17.3% (Siri).
Sources: ACSM — Guidelines for Exercise Testing and Prescription
Frequently asked questions
How accurate is hydrostatic weighing?
It was long the reference method, with an error of about 1.5–2.5% body fat when residual volume is measured. Estimating residual volume instead of measuring it adds roughly another 2–3%.
How do I estimate residual volume?
Labs measure it by gas dilution. Without that, common estimates are about 24% of vital capacity for men and 28% for women, or 1.2–1.5 L for men and 1.0–1.2 L for women.
Why does water temperature matter?
Warm water is slightly less dense, so the same weight loss under water means a slightly larger body volume. At 34 °C water is about 0.6% less dense than at 4 °C.
Is a DEXA scan better than underwater weighing?
DEXA separates fat, lean and bone and shows where fat is, and it needs no breath-holding. Hydrostatic weighing gives a single total, but both are accurate when done well.
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This calculator gives an estimate for general fitness and education only. It is not a diagnosis — results from field tests and prediction equations can differ from laboratory measurements, so talk to a doctor or qualified exercise professional before acting on them.