When two or more gases share the same container, each one contributes its own share of the total pressure — as if the others weren't there. That simple idea, formalized by John Dalton in 1801, is the backbone of everything from scuba gas blending to atmospheric science.
How the Partial Pressure Calculator works
The calculator applies Dalton's law of partial pressures: Pᵢ = Xᵢ × Ptotal, where Xᵢ is the mole fraction of gas i (Xᵢ = nᵢ / ntotal) and Ptotal is the total pressure of the mixture. This relationship holds because, in an ideal gas mixture, each gas behaves as though it alone filled the container — its pressure depends only on its own amount, the container's volume, and the temperature, not on what other gases are present.
The calculator supports three complementary views: computing partial pressures from moles or mole fractions (Partial Pressure tab), computing just the mole fractions of a mixture (Mole Fraction tab), and summing already-known partial pressures back into a total (Total tab). All three implement the same underlying law from different starting points.
Inputs and what they mean
Total pressure is the combined pressure of the whole mixture, in kPa, atm, bar, mmHg, or psi — for example, standard atmospheric pressure at sea level is about 101.3 kPa (1 atm). Each gas row takes a name and an amount: moles (any positive number) when you know how much of each gas is present, or a mole fraction (as a decimal like 0.4 or a percentage like 40%) when you already know each gas's share.
The mole-fraction input accepts either form — the calculator detects percentages automatically (any value above 1) and normalizes all fractions so they sum to exactly 100%, even if your entered values are slightly off due to rounding.
Limits and edge cases
Dalton's law assumes ideal-gas behavior: negligible intermolecular attraction and molecular volume. Real gas mixtures deviate from it at very high pressure or very low temperature, where molecules interact more strongly — professional applications (deep-sea diving, industrial gas storage) apply correction factors the calculator does not model. It also assumes the gases don't chemically react with each other; reacting mixtures (like combustion products) require a different approach. For everyday chemistry, atmospheric science, and gas-blending calculations at normal pressures and temperatures, the ideal-gas approximation used here is accurate to within a fraction of a percent.