Partial Pressure Calculator
Apply Dalton's law of partial pressures to any gas mixture: find each gas's partial pressure from mole fractions or volume percent, convert partial pressures back to mole fractions, or add partial pressures to get the total pressure. Five pressure units (atm, kPa, mmHg, bar, psi) with automatic conversion — everything runs in your browser, nothing is uploaded.
Partial pressure from mole fractions
Enter each gas and its mole fraction (xi). Fractions should sum to 1 — if they do not, you can normalize them with one click.
| Gas | Mole fraction xi |
|---|
Mole fractions from partial pressures
Enter each gas's partial pressure; the tool divides each by their sum to get the mole fractions.
| Gas | Partial pressure Pi |
|---|
Total pressure from partial pressures
Dalton's law in its simplest form: Ptotal = ΣPi.
| Gas | Partial pressure Pi |
|---|
Partial pressures from volume percent (air preset)
For ideal gases, volume percent = mole percent. Preset uses dry-air composition (NASA OCHMO-TB-003 sea-level: 78.08% N2, 20.95% O2, 0.93% Ar, 0.04% CO2 — CO2 rounded from 0.038% to close the table at 100.00%). Edit any row for a custom mixture.
| Gas | Volume % |
|---|
Worked examples (recomputed from first principles)
- Mole fractions → partial pressures: x = 0.5 / 0.3 / 0.2 at Ptotal = 2.0 atm gives P = 1.0 / 0.6 / 0.4 atm.
- Reverse: P = 1.0 / 0.6 / 0.4 atm gives x = 0.5 / 0.3 / 0.2 (sum = 1).
- Dry air at 1 atm: pN2 = 0.7808 atm (593.408 mmHg), pO2 = 0.2095 atm (159.22 mmHg), pAr = 0.0093 atm (7.068 mmHg), pCO2 = 0.0004 atm (0.304 mmHg).
- Dry air at 2 atm: pO2 = 0.419 atm (318.44 mmHg).
- Unit reference: 1 atm = 101.325 kPa = 760 mmHg = 1.01325 bar = 14.6959 psi.
- Altitude: at Ptotal = 0.5 atm, pO2 = 0.10475 atm (79.61 mmHg) — about half the sea-level value.
Is this the right calculator? (boundary)
- Scuba diving / nitrox: this page covers Dalton's law for general gas mixtures. For ppO2 at depth, MOD, nitrox blending, CNS and OTU tracking, use our nitrox lab instead.
- Water vapor in humid air: the partial pressure of water vapor (dew point, humidity ratio) is computed from temperature and humidity in our psychrometric calculator.
- One gas alone: for a single gas, PV = nRT in our ideal gas law calculator is the right tool.
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 whole volume at the same temperature. In air at sea level (1 atm total), oxygen has a partial pressure of 0.2095 atm (159.22 mmHg), and nitrogen has 0.7808 atm (593.408 mmHg).
What is Dalton's law of partial pressures?
Dalton's law states that the total pressure of a gas mixture equals the sum of the partial pressures of each component: P_total = P_1 + P_2 + ... + P_n. Each gas behaves as if the others were not there, so the partial pressures simply add up. Example: partial pressures of 1.0, 0.6 and 0.4 atm add to a total of 2.0 atm.
How do you calculate partial pressure from mole fraction?
Multiply the mole fraction by the total pressure: P_i = x_i x P_total. Example: a mixture with mole fractions 0.5, 0.3 and 0.2 at a total pressure of 2.0 atm has partial pressures 1.0, 0.6 and 0.4 atm.
How do you find mole fractions from partial pressures?
Divide each partial pressure by the total pressure: x_i = P_i / P_total. Example: partial pressures of 1.0, 0.6 and 0.4 atm give mole fractions 0.5, 0.3 and 0.2, and they sum to 1.
How do you calculate partial pressure from volume percent?
For ideal gases, volume percent equals mole percent, so P_i = (vol% / 100) x P_total. Example: oxygen is 20.95% of dry air by volume, so at 1 atm its partial pressure is 0.2095 x 1 = 0.2095 atm. At 2 atm total, pO2 = 0.419 atm (318.44 mmHg).
What are the partial pressures of the gases in air at 1 atm?
Using dry-air composition (78.08% N2, 20.95% O2, 0.93% Ar, 0.04% CO2): pN2 = 0.7808 atm (593.408 mmHg), pO2 = 0.2095 atm (159.22 mmHg), pAr = 0.0093 atm (7.068 mmHg), pCO2 = 0.0004 atm (0.304 mmHg).
What units can partial pressure be expressed in?
Any pressure unit works: atm, kPa, mmHg (torr), bar or psi. 1 atm = 101.325 kPa = 760 mmHg = 1.01325 bar = 14.6959 psi. The calculator above converts between all five units automatically.
Do mole fractions have to add up to 1?
Yes. Mole fractions are defined as x_i = n_i / n_total, so they always sum to exactly 1 (100%). If your fractions sum to something else, the calculator shows the sum and offers a normalize button, but check your inputs first: a wrong sum usually means a missing gas or a typo.
Can a partial pressure be larger than the total pressure?
No. Because x_i is never larger than 1, P_i = x_i x P_total can never exceed P_total. If you get a partial pressure larger than the total, your mole fractions sum to more than 1, or a partial pressure was entered in the wrong unit.
Does temperature change partial pressure?
If the volume and the amount of each gas are fixed, every partial pressure scales with absolute temperature, so all P_i values rise and fall together with the total pressure while the mole fractions stay unchanged. Doubling the absolute temperature doubles each partial pressure and the total.
When does Dalton's law break down?
Dalton's law is exact for ideal gases. It becomes only approximate at high pressures (roughly above 10 atm), for strongly non-ideal gases, and when the gases react with each other, for example ammonia and hydrogen chloride forming solid ammonium chloride. Near 1 atm it is very accurate.
What is the difference between partial pressure and vapor pressure?
Partial pressure is one gas's contribution to a mixture (Dalton's law). Vapor pressure is the pressure of a vapor in equilibrium with its liquid or solid phase. Water vapor in humid air does have a partial pressure, and our psychrometric calculator computes it from temperature and humidity.
Is this the same as the ppO2 used by scuba divers?
Related, but this tool is not a dive planner. Divers use the partial pressure of oxygen at depth, where total pressure equals the ambient pressure (about 1 atm per 10 m of seawater). For dive planning, including MOD, nitrox blending and oxygen toxicity tracking, use our nitrox-lab tool instead.
Why use mole fraction instead of mass fraction?
Gas pressure depends on the number of molecules, not their mass. A heavy gas such as argon contributes less pressure per gram than a light gas such as helium. Mole fraction counts molecules directly, which is why P_i = x_i x P_total works, while mass fraction would give wrong answers.
How does altitude affect the partial pressure of oxygen?
The oxygen fraction of air stays near 20.95%, but total pressure falls with altitude, so pO2 = 0.2095 x P_total falls too. At a total pressure of 0.5 atm, pO2 = 0.10475 atm (79.61 mmHg), roughly half the sea-level value, which is why breathing feels harder at altitude.
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