Gas Partial Pressure Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/8/2026
Dalton's law gives a gas's partial pressure with P(gas) = (n(gas) ÷ n(total)) × P(total). For 2 mol of a gas out of 10 mol total, at a total pressure of 1 atm, that gas's partial pressure is 0.2 atm.
Explanation
In a mixture of several gases that do not chemically react with each other (like air, a mixture mainly composed of nitrogen and oxygen), each gas contributes to the mixture's total pressure in proportion to its relative amount of substance — this is Dalton's law of partial pressures, stated by John Dalton in the early 19th century. A gas's partial pressure corresponds to the pressure that gas would exert if it alone occupied the entire available volume, at the same temperature: it is calculated by multiplying the mixture's total pressure by that gas's mole fraction (its amount of substance divided by the mixture's total amount of substance). The sum of the partial pressures of all gases present in the mixture always gives back exactly the total pressure, a direct and verifiable consequence of this law. This concept is essential in respiratory physiology (the partial pressure of oxygen in inhaled air determines how much oxygen actually diffuses into the blood, a key factor in altitude medicine or scuba diving), in industrial chemistry, and more generally whenever a gas mixture is studied rather than a pure gas. For the general relationship between pressure, volume, temperature, and amount of substance of a pure gas, see our ideal gas law calculator; for the link between volume and amount of substance at fixed temperature and pressure, our Avogadro's law calculator.
Example: 2 mol of a gas out of 10 mol total, total pressure of 1 atm
Inputs
Amount of the gas: 2 mol. Total amount in the mixture: 10 mol. Total pressure: 1 atm.
Calculation
Mole fraction = 2 ÷ 10 = 0.2. Partial pressure = 0.2 × 1 = 0.2 atm.
Result
This gas's partial pressure in the mixture is 0.2 atm.
Frequently asked questions
What does partial pressure mean in practice?
A gas's partial pressure in a mixture corresponds to the pressure that gas would exert on its own if it occupied the entire available volume at the same temperature, in the absence of the other gases in the mixture. It is a useful theoretical quantity for reasoning about the individual behavior of each component of a gas mixture, even though in reality all the gases occupy the same volume simultaneously.
Does the sum of partial pressures always give the total pressure?
Yes, that is precisely what Dalton's law states: in a mixture of gases that do not chemically react with each other, the total pressure is exactly equal to the sum of the partial pressures of each gas making up the mixture. This is a direct consequence of the fact that the mole fractions of all gases present always add up to 1 (100% of the mixture).
Why is oxygen's partial pressure important at altitude?
Because even though the proportion of oxygen in the air stays roughly the same everywhere (about 21%), total atmospheric pressure decreases with altitude, which reduces oxygen's partial pressure accordingly — it is this drop in partial pressure, not a change in air composition, that makes breathing harder at high altitude and requires gradual acclimatization or supplemental oxygen beyond a certain altitude.