Partial Pressure Calculator
Find a gas's partial pressure with Dalton's law.
Results update as you type.
How it works
In a mixture of gases, each gas contributes its own share of the total pressure, and this calculator finds that partial pressure using Dalton's law. You enter the moles of the component gas, the total moles of all gases present, and the total pressure of the mixture, and it returns the component's partial pressure along with its mole fraction. Dalton's law states that each gas in a mixture behaves independently and exerts the pressure it would if it occupied the whole volume alone, so a gas's partial pressure is simply its mole fraction of the mixture times the total pressure. The partial pressures of all the components add up to the total pressure. This concept is essential across chemistry and physiology: it governs how oxygen and carbon dioxide move between the lungs, blood, and tissues, why divers must manage the partial pressures of gases to avoid nitrogen narcosis and oxygen toxicity, and how gas reactions and equilibria are analysed. It works because, for ideal gases, pressure depends on the number of particles, not their type, so mole fraction directly determines the pressure share. Enter the component moles, total moles, and total pressure to find the partial pressure.
p = (moles of component / total moles) × total pressure
Dalton's law of partial pressures states that each gas in a mixture exerts the pressure it would if it alone occupied the container. A component's partial pressure equals its mole fraction, the moles of that gas divided by the total moles, multiplied by the total pressure of the mixture. The partial pressures of all components add up to the total pressure, since together they make up the whole gas.
Source:Chemistry (Dalton's law of partial pressures)
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Worked example
How to use this calculator
- 1
Enter your values
Enter moles of component, total moles of gas and total pressure.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Partial pressure updates instantly as you type — no button to press.
Frequently asked questions
What is partial pressure?
Partial pressure is the pressure that a single gas in a mixture would exert if it alone occupied the entire volume. In a gas mixture, each component contributes its partial pressure, and together they sum to the total pressure. Partial pressure reflects how much of the mixture that gas makes up, and it drives how gases dissolve and diffuse.
What is Dalton's law of partial pressures?
Dalton's law states that the total pressure of a mixture of non-reacting gases equals the sum of the partial pressures of the individual gases. Each gas acts independently, exerting pressure as if the others were not present. This lets you find any gas's partial pressure by multiplying its mole fraction by the total pressure of the mixture.
How is partial pressure related to mole fraction?
A gas's partial pressure equals its mole fraction multiplied by the total pressure. The mole fraction is the moles of that gas divided by the total moles of all gases. Because ideal gas pressure depends on the number of particles rather than their identity, the fraction of particles a gas represents equals the fraction of the total pressure it contributes.
Why does partial pressure matter in the body?
Gases move between air, blood, and tissues by diffusing from higher to lower partial pressure. Oxygen has a high partial pressure in the lungs, so it diffuses into the blood, while carbon dioxide diffuses out. These partial pressure gradients drive respiration. Understanding them is essential in medicine, high-altitude physiology, and scuba diving safety.
Do partial pressures add up to the total?
Yes. By Dalton's law, the partial pressures of all the gases in a mixture add up exactly to the total pressure. This is a useful check: if you calculate each component's partial pressure and sum them, the result should equal the total pressure you started with, since together the components make up the entire gas mixture.