Partial Pressure Calculator (Dalton's Law)
Calculate the partial pressure of each gas in a mixture from the total pressure and each component's mole fraction (or moles), using Dalton's Law of Partial Pressures: P_i = x_i × P_total.
Partial pressure calculation and Dalton's law
Calculating partial pressures means finding the pressure that each component gas exerts on its own within a mixture of several gases. Under Dalton's law of partial pressures, put forward by the English chemist John Dalton, the total pressure of a mixture equals the sum of the partial pressures of all its components, and each component's partial pressure is its mole fraction multiplied by the total pressure (P_i = x_i × P_total). From an everyday mixture such as air to the breathing gas used in diving, there is no shortage of situations where you want the pressure of each component.
Enter the total pressure and the proportion of each component and this tool works out every partial pressure automatically from Dalton's law. You can give the proportions either as mole fractions or as numbers of moles, and if the mole fractions do not add up to exactly 1 the tool normalises them while preserving the ratios — so there is no need to fuss over rounding. The whole calculation is done inside your browser and the figures you enter are never sent anywhere.
How to calculate partial pressures
- Choose the input method Pick entry by mole fraction if you know the mole fractions, or entry by moles if you know the amount of substance of each component.
- Enter the total pressure Put in the pressure of the whole mixture (P_total). Keep the unit — atm, kPa, Torr — consistent with the unit you want the results in.
- Enter the value for each component Give the mole fraction or the number of moles for each component. You can add and remove components between two and five with the add component button.
- Check each partial pressure As you enter, the mole fraction and partial pressure of each component appear. If the sum of the partial pressures matches the total pressure, you can be confident the calculation is sound.
Tips for getting more out of it
- In moles mode, just enter each component's moles — mole fractions are computed automatically and always add up to 1.
- In mole-fraction mode, if the fractions don't add up to exactly 1, they're automatically normalized, so small input errors won't break the calculation.
- You can add or remove between 2 and 5 components. Try it with the main constituents of air (nitrogen, oxygen, argon, carbon dioxide).
- For a single gas, use the companion Ideal Gas Law calculator (PV=nRT) to explore how pressure, volume, temperature, and moles relate.
When partial pressure calculation is useful
Finding the oxygen partial pressure from the composition of air
From the mole fractions of nitrogen, oxygen, argon and carbon dioxide plus the atmospheric pressure, you get all the partial pressures at once. It suits background reading for discussions of altitude and mountain sickness, where oxygen partial pressure comes up.
Managing breathing gas for scuba diving
From the change in total pressure with depth and the mole fractions of a mixture such as nitrox, you can estimate the oxygen and nitrogen partial pressures as a reference when weighing the risk of oxygen toxicity or decompression sickness.
Confirming how pressure divides in a laboratory gas mixture
In an experiment sealing several gases into the same vessel, you can estimate in advance how much of the pressure each component bears, starting from the amounts of substance you charged.
Combining with the ideal gas law
If you want the relation between pressure, volume, temperature and amount of substance for a single component first, you can use the companion ideal gas law calculator (PV=nRT) to get the moles of each component, then convert to partial pressures here.
Comparing against saturated vapour pressure
When dealing with a moist gas, checking that the partial pressure of water vapour does not exceed the value from the saturated vapour pressure calculator gives you a guide to whether condensation or supersaturation is occurring.
Partial pressure terms explained
- Partial pressure
- The pressure a single component gas within a mixture would show if it were assumed to occupy the container on its own. By Dalton's law it is found as the component's mole fraction multiplied by the total pressure (P_i = x_i × P_total).
- Mole fraction
- The share of one component gas's amount of substance in the total amount of substance of the whole mixture. It takes a value between 0 and 1, and the mole fractions of all components always add up to 1.
- Dalton's law of partial pressures
- The law that the total pressure of a gas mixture equals the simple sum of the partial pressures of its component gases. The English chemist John Dalton put it forward in 1801.
- Total pressure
- The pressure that a mixture of several gases exerts as a whole on the walls of its container. It equals the sum of all the component partial pressures.
- Collection over water
- An experimental technique in which a generated gas is led into a vessel filled with water and collected by displacing the water. Because water vapour mixes into the collected gas, obtaining an accurate partial pressure for the gas requires subtracting the partial pressure of water vapour, its saturated vapour pressure, from the total.
- Ideal gas
- A hypothetical gas assumed to have negligible intermolecular forces and negligible molecular volume. Both Dalton's law of partial pressures and the ideal gas law (PV=nRT) are relations premised on the behaviour of such a gas.
Frequently Asked Questions
Side Note — When partial pressure becomes a matter of life and death
Dalton's Law of Partial Pressures was proposed in 1801 by the English chemist John Dalton, better known for atomic theory. Dalton had a strong interest in meteorology, and while studying how water vapor and other gases behave in the atmosphere, he found that even when several gases are mixed together, each behaves independently, and the total pressure is simply the sum of each gas's partial pressure.
The air we breathe is itself a mixture: roughly 78% nitrogen, 21% oxygen, 0.93% argon, and 0.04% carbon dioxide. At sea level, where total pressure is about 1 atmosphere, oxygen's partial pressure is roughly 0.21 atm — and it is this partial pressure, not the percentage alone, that governs how efficiently oxygen is absorbed into the bloodstream.
The concept of partial pressure is also central to medicine and sports science. Altitude sickness sets in as total atmospheric pressure drops with elevation, lowering oxygen's partial pressure, while scuba diving increases total pressure with depth, raising nitrogen's partial pressure and driving more of it into the bloodstream — the root cause of decompression sickness ("the bends"). Understanding partial pressure correctly is not just a chemistry exercise; it is directly tied to physical safety.