Partial Pressure Calculator
Apply Dalton's law to find each gas's partial pressure from its mole fraction and the mixture's total pressure, or sum measured pressures to find the total.
🌬️ What is Partial Pressure?
Partial pressure is the pressure that one gas in a mixture would exert if it alone occupied the entire volume at the same temperature. Every gas in a mixture, like the nitrogen and oxygen in the air around you, contributes its own partial pressure, and Dalton's law says those individual pressures simply add up to the total pressure of the mixture. This calculator applies that law in both directions: finding each gas's partial pressure from its mole fraction and the mixture total, or summing known partial pressures to find the total.
Partial pressure appears constantly outside the classroom. Scuba divers and hyperbaric medicine specialists track the partial pressure of oxygen and nitrogen in breathing gas, since it is partial pressure, not total pressure alone, that determines how much gas dissolves in the bloodstream and drives risks like decompression sickness or oxygen toxicity. Atmospheric scientists describe air composition in terms of each gas's partial pressure at sea level. Industrial gas blending, from welding gas mixtures to nitrox scuba tanks, is specified and measured using partial pressures of each component.
A common misconception is that mole fraction and partial pressure are two unrelated numbers you have to look up separately. For an ideal gas mixture they are directly proportional, a gas's partial pressure is simply its mole fraction times the total pressure, so knowing any two of mole fraction, partial pressure, and total pressure lets you find the third with simple algebra.
This calculator is useful because most partial pressure problems in a course or lab either give you mole fractions and a total pressure, or give you several measured partial pressures and ask for the total, and doing that arithmetic by hand across several gas species is tedious and error-prone. This tool handles up to 4 species at once, with a full breakdown table and an automatic check that flags mole fractions that do not add up to 100%.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Composition of Dry Air
Total pressure = 1 atm. Species 1 (nitrogen) = 78%, Species 2 (oxygen) = 21%, Species 3 (argon) = 1%.
Example 2 — Two-Gas Lab Mixture
Total pressure = 2 atm. Species 1 (helium) = 60%, Species 2 (neon) = 40%.
Example 3 — Finding Total From Measured Partial Pressures
A gas analyzer reports three measured partial pressures: Species 1 = 0.5 atm, Species 2 = 0.3 atm, Species 3 = 0.2 atm.
Example 4 — Mole Fractions That Do Not Sum to 100%
Total pressure = 3 atm. Species 1 = 50%, Species 2 = 40% (a student forgot to record a third component).
❓ Frequently Asked Questions
🔗 Related Calculators
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 each gas would exert if it alone occupied the same volume. In formula terms, Ptotal = P1 + P2 + P3 + ... and each individual partial pressure equals Pi = xi times Ptotal, where xi is that gas's mole fraction.
How do you calculate partial pressure from mole fraction?
Multiply the gas's mole fraction (as a decimal, or divide a percentage by 100) by the total pressure of the mixture: Pi = xi times Ptotal. For example, if oxygen makes up 21% of a mixture at a total pressure of 1 atm, oxygen's partial pressure is 0.21 times 1 atm = 0.21 atm.
How do you find total pressure from individual partial pressures?
Add up every gas's partial pressure: Ptotal = P1 + P2 + P3 + ... For example, if a mixture contains gases with partial pressures of 0.5 atm, 0.3 atm, and 0.2 atm, the total pressure is 0.5 + 0.3 + 0.2 = 1.0 atm.
What is mole fraction and how does it relate to partial pressure?
Mole fraction (xi) is the fraction of total moles in a mixture contributed by one component, always between 0 and 1 (or 0% and 100%). For an ideal gas mixture, mole fraction and partial pressure fraction are identical, so xi = Pi divided by Ptotal, which is exactly how the 'From Partial Pressures' mode on this calculator finds each species' mole fraction.
Why do my mole fractions need to sum to 100%?
Dalton's law assumes every component of the mixture is accounted for, so all the mole fractions together must represent the whole mixture, summing to exactly 100%. If your fractions only sum to, say, 90%, either a component was left out or there was a measurement error, this calculator flags that mismatch rather than silently producing an inconsistent answer.
Does Dalton's law work for any gas mixture?
Dalton's law is exact for ideal gases, and it is an excellent approximation for real gas mixtures at ordinary pressures and temperatures, including air, breathing gas mixtures, and most laboratory gas handling. It becomes less accurate at very high pressures or very low temperatures, where gases start deviating noticeably from ideal behavior.
What is a real-world example of partial pressure?
Air at sea level is about 1 atm total pressure, made of roughly 78% nitrogen, 21% oxygen, and 1% argon and other trace gases. That means nitrogen's partial pressure is about 0.78 atm, oxygen's is about 0.21 atm, and the small remainder is shared among argon, carbon dioxide, and other trace gases, all summing back to the 1 atm total.
Why is partial pressure important in scuba diving and medicine?
Partial pressure, not just total pressure, determines how much of a gas dissolves into the bloodstream or affects the body. Divers breathing compressed air at depth face a higher partial pressure of nitrogen and oxygen than at the surface, which drives decompression sickness risk and oxygen toxicity limits. Anesthesiologists and respiratory therapists use partial pressure of oxygen (like in nitrox or medical oxygen mixtures) to control exactly how much oxygen reaches a patient.
Can partial pressure ever be negative or greater than the total pressure?
No. A valid partial pressure is always between 0 and the total pressure of the mixture, since it represents one component's share of the whole. A calculated partial pressure outside that range signals invalid input, such as a mole fraction above 100% or a negative measured pressure.
What is the difference between this calculator and the Mole Fraction Calculator?
The Mole Fraction Calculator focuses on finding a mole fraction from moles or masses of each component. This Partial Pressure Calculator instead goes directly between mole fraction (or measured partial pressure) and pressure, using Dalton's law, and produces a full multi-species breakdown table plus a consistency check against the total pressure.