Partial Pressure Calculator
The partial pressure of a gas in a mixture — from its mole fraction and the total pressure, or from moles, volume and temperature.
Dalton's law: each gas in a mixture exerts the pressure it would exert alone, and the total is their sum.
How the partial pressure calculator works
Dalton's law: each gas in a mixture exerts the pressure it would exert alone, and the total is their sum. So the partial pressure is the mole fraction times the total: P_i = χ_i P_total.
At sea level that makes oxygen's partial pressure 0.209 × 101.3 = 21.2 kPa. At 5,500 m the composition is unchanged but the total pressure has halved, and so has the oxygen — which is exactly why altitude is hard to breathe at.
Formula: P_i = χ_i × P_total = n_i RT / V
Worked examples
| Inputs | Partial pressure | Note |
|---|---|---|
| Oxygen at sea level | 21.17693 kPa | 21.2 kPa |
| Oxygen at 5,500 m | 10.5754 kPa | half as much — the composition has not changed |
| A mole in 22.4 L at STP | 101.38819 kPa | 101.4 kPa |
FAQFrequently asked questions
What is Dalton's law?
Each gas in a mixture exerts the pressure it would exert alone, and the total is the sum of those partial pressures.
What is oxygen's partial pressure at sea level?
About 21.2 kPa — 20.9% of 101.325 kPa.
Why is altitude hard to breathe at?
The composition of air does not change with height; the total pressure falls. At 5,500 m oxygen's partial pressure is half its sea-level value.
Does it apply to dissolved gases?
Indirectly, through Henry's law: how much dissolves is proportional to the partial pressure above the liquid.
Why do divers care?
Because the partial pressure of nitrogen and oxygen rises with depth. Oxygen becomes toxic above about 1.4 bar partial pressure, which is what limits gas mixes.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.