Partial Pressure Calculator
Enter a mole fraction and the total pressure to get the partial pressure of a gas in kilopascals — and see how each gas in a mixture contributes its own share of the pressure.
Dalton's law in one step
Enter the mole fraction and the total pressure and the calculator returns the partial pressure (P_i = x × P_total) of that gas in kilopascals.
Keep x between 0 and 1
The mole fraction is a share of the whole mixture, so it always lies between 0 and 1 — the fractions of every gas add up to 1.
What is partial pressure?
One gas's share of the total
The partial pressure calculator uses Dalton's law to find how much of a gas mixture's total pressure comes from a single component. The partial pressure of a gas is the pressure it would exert if it alone filled the whole container at the same temperature, and it equals that gas's mole fraction multiplied by the total pressure. Enter two numbers — the mole fraction (a value from 0 to 1) and the total pressure in kilopascals — and the calculator returns the partial pressure in kilopascals. It is the number behind how much oxygen your lungs actually receive, how gases dissolve into liquids, and how chemical equilibria shift in a mixture.
Enter a mole fraction between 0 and 1 and a total pressure in kilopascals to get the partial pressure of that gas instantly.
The partial pressure of a gas is its mole fraction multiplied by the total pressure of the mixture.
P_i = x × P_totalThe mole fraction (x) is the proportion of the mixture made up by this one gas, a value from 0 to 1. Because it is dimensionless, the partial pressure comes out in the same unit as the total pressure — here, kilopascals. Add up the partial pressures of every gas in the mixture and you get the total pressure back, which is exactly what Dalton's law promises.
Suppose you want the partial pressure of oxygen in dry air at standard atmospheric pressure (101.325 kPa), where oxygen makes up a mole fraction of 0.21.
Identify the mole fraction
Oxygen is 0.21 of dry air, so x = 0.21 — its share of all the gas molecules present.
Take the total pressure
Standard atmospheric pressure is 101.325 kPa, the combined pressure of all the gases.
Multiply
0.21 × 101.325 = 21.28 kPa — the partial pressure of oxygen in the air around you.
The result tells you how much of the total pressure one gas is responsible for. In the example, oxygen's 21.28 kPa is its slice of the 101.325 kPa atmosphere — the pressure it would still exert if every other gas were removed. The key idea behind Dalton's law is that each gas contributes pressure in direct proportion to its mole fraction, and the partial pressures of all the components sum to the total pressure. So nitrogen, with a mole fraction of about 0.78, contributes roughly 79 kPa, and the small remainder comes from argon, carbon dioxide, and trace gases — together they add back up to the full 101.325 kPa. Doubling a gas's mole fraction doubles its partial pressure, and at a fixed mole fraction a higher total pressure raises every component's partial pressure proportionally. That is why climbers struggle at altitude: the mole fraction of oxygen stays at 0.21, but the lower total pressure drags its partial pressure down with it.
The formula is simple and exact for the ideal gas model, but a couple of practical points are worth keeping in mind.
Ideal gases and a mole fraction from 0 to 1
Dalton's law assumes the gases behave ideally and do not react with one another, which holds well at ordinary temperatures and pressures but breaks down for real gases under high pressure or near condensation. The mole fraction must lie between 0 and 1 — it is a share of the whole mixture, never larger than the total — and the partial pressure comes out in whatever unit you use for the total pressure, kilopascals in this calculator.