Gas Mixture & Partial Pressure Calculator

Dry air at sea level. By volume dry air is 78.08 % N₂, 20.95 % O₂, 0.93 % Ar and about 0.042 % CO₂. At 1 atm the partial pressures are 0.7808, 0.2095, 0.0093 and 0.00042 atm; the oxygen alone is 0.2095 × 101.325 = 21.23 kPa. The mole-weighted molar mass is 28.97 g/mol, so at 20 °C the density is 101,325 × 0.028966 ÷ (8.314 × 293.15) = 1.204 kg/m³.

Nitrox 32 for diving. At 30 m in sea water the pressure is 1.013 + 3 = 4.013 bar, so pO₂ = 0.32 × 4.013 = 1.284 bar. The depth where it reaches the 1.4 bar working limit is (1.4 ÷ 0.32 − 1.013) × 10 = 33.6 m, the gas’s maximum operating depth. Its nitrogen load at 30 m equals that of air at only 24.4 m (the equivalent air depth).

The common mistake: leaving out the atmosphere above the water. Taking 30 m as 3 bar gives air a pO₂ of 0.2095 × 3 = 0.63 bar, but the real pressure is 4.013 bar and the real pO₂ is 0.841 bar, a third higher. The mirror image in Graham’s law is turning the square root upside down: hydrogen effuses √(31.998 ÷ 2.016) = 3.984 times as fast as oxygen, not 0.251 times.

John Dalton, working on the water vapour in the Manchester air, read his law of partial pressures to the Manchester Literary and Philosophical Society in 1801: each gas in a mixture behaves as if it were alone in the container, and the total pressure is the sum of the parts. The same thinking about independent particles led him to his atomic theory a few years later.

The Scottish chemist Thomas Graham measured how fast gases escaped through tiny holes and porous plugs, and in 1848 stated that the rate of effusion is inversely proportional to the square root of the density, or of the molar mass. Its most famous use came a century later: the K-25 gaseous diffusion plant at Oak Ridge separated uranium-235 from uranium-238 as UF₆, where the rate difference is only 0.43 % per stage.

Partial pressure is what matters to a diver’s body. Paul Bert showed in 1878 that oxygen at high pressure is poisonous to the nervous system, and the usual limits today are 1.4 bar of oxygen for the working part of a dive and 1.6 bar as the absolute maximum. NOAA’s Nitrox I mixture, 32 % oxygen, made enriched air a standard recreational gas. Air composition here is the standard dry-air make-up by volume; carbon dioxide (about 0.042 %, or 420 ppm) is the one figure that keeps rising.

This calculator applies Dalton’s law to any mixture of up to five gases. Give the amounts as volume percent, moles, grams or partial pressures, and it returns each gas’s mole fraction, partial pressure and mass percent, the mixture’s average molar mass, and its density from the ideal gas law. Molar masses come from the formulas you type.

The diving mode turns depth into pressure and gives the oxygen partial pressure, the maximum operating depth for your pO₂ limit and the equivalent air depth, with warnings above 1.4 and 1.6 bar; it is a teaching aid, not a dive planner. The Graham’s law mode compares two gases’ effusion rates and shows where they meet in a tube. Everything runs in your browser.

It is aimed at chemistry and physics students, divers learning gas theory, and anyone working with gas cylinders or leaks.

Related tools: Ideal Gas Law Calculator, Molar Mass Calculator, and Boiling & Freezing Point Calculator.