Partial Pressure Calculator
In a mixture of non-reacting ideal gases, each component contributes a share of the total pressure proportional to its mole fraction. This share is the partial pressure, and Dalton's law states that all the partial pressures sum to the total pressure of the mixture. This calculator finds the partial pressure of one gas from the moles of that gas, the total moles in the mixture, and the total pressure. It also reports the mole fraction. Enter moles in any consistent unit and the pressure in whatever unit you prefer; the partial pressure comes out in that same pressure unit.
Partial pressure formula
Mole fraction x_i = moles of gas i / total moles
Partial pressure P_i = x_i * P_total
Dalton's law: P_total = sum of all P_i
Total moles must be greater than zero and at least equal to the moles of the gas of interest. The partial pressure is returned in the same unit you used for total pressure.
Partial pressure context
- Dalton's law applies to mixtures of non-reacting ideal gases.
- Mole fraction is dimensionless and always lies between 0 and 1.
- Dry air is about 78 percent nitrogen and 21 percent oxygen by mole fraction at sea level.
- Partial pressure drives gas exchange in the lungs and gas solubility under Henry's law.
- At high pressure or low temperature, real-gas behavior makes the law approximate rather than exact.
Partial pressure: frequently asked questions
What is partial pressure?
Partial pressure is the pressure a single gas in a mixture would exert if it alone occupied the entire volume at the same temperature. By Dalton's law, the total pressure of an ideal gas mixture is the sum of the partial pressures of all its components.
How do I calculate partial pressure from mole fraction?
Multiply the mole fraction of the gas by the total pressure: partial pressure = mole fraction x total pressure. The mole fraction is the moles of that gas divided by the total moles of all gases in the mixture.
What is Dalton's law of partial pressures?
Dalton's law states that for a mixture of non-reacting ideal gases, the total pressure equals the sum of the individual partial pressures. Each gas behaves independently, so its partial pressure depends only on its own mole fraction and the total pressure.
Can I find partial pressure from moles directly?
Yes. This calculator lets you enter moles of the gas of interest and total moles in the mixture. It computes the mole fraction as the ratio, then multiplies by total pressure. You can also enter a mole fraction directly if you already know it.
Does Dalton's law work for real gases?
Dalton's law is exact for ideal gases and is a very good approximation for real gases at moderate pressures and temperatures. It becomes less accurate at high pressures or low temperatures where intermolecular forces and finite molecular volume matter.
Official sources
- NIST Chemistry WebBook: gas-phase thermochemical reference data.
- BIPM: SI units of pressure and amount of substance.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.