Partial Pressure Calculator — Dalton's Law
Find the partial pressure of each component in a gas mixture. Enter the total pressure, choose units, and supply mole fractions for up to 4 gases — defaults show dry air (N₂, O₂, Ar, CO₂) at 1 atm.
Pressure unit
Number of gas components
N2: x₁ × P = 0.7808 × 101.325
- 1
Mole fraction of gas 1 (x₁)
0.7808 - 2
Partial pressure of gas 1 (x₁ × P)
0.7808 × 101.325 = 79.1146Dalton's Law: each component's partial pressure equals its mole fraction times the total pressure.
How does this calculator work?
Dalton's Law: P_i = x_i × P_total, where x_i is the mole fraction of gas i. Mole fractions must sum to 1. Works for ideal gases at moderate pressures. Dry air at 1 atm: P(O₂) ≈ 21.2 kPa (159 mmHg), P(N₂) ≈ 79.1 kPa (593 mmHg). Partial pressures add up to the total pressure.
Formula
How this is calculated
Dalton's Law of Partial Pressures states that in an ideal gas mixture each component exerts the pressure it would exert if it alone occupied the container: P_i = x_i × P_total. The mole fraction x_i = n_i / n_total is the fraction of all moles belonging to species i; all mole fractions in the mixture must sum to exactly 1.
The law holds precisely for ideal gases and is an excellent approximation for real gases at pressures below a few tens of atmospheres and temperatures well above the dew point. It underpins many practical fields: respiratory physiology (PO₂ of inspired air at altitude), scuba diving (nitrogen narcosis, oxygen toxicity limits), anaesthesia gas blending, industrial process streams, and atmospheric science. Dry air at sea level (101.325 kPa) has P(O₂) ≈ 21.2 kPa and P(N₂) ≈ 79.1 kPa; at 5,500 m altitude (P ≈ 50 kPa), P(O₂) drops to ≈ 10.5 kPa — sufficient to cause altitude sickness in many people.
At very high pressures or for polar/associating molecules near saturation, the ideal mole-fraction relationship breaks down and fugacity-based models are needed. This calculator does not account for non-ideal behaviour.
Frequently asked questions
A mole fraction x_i is the number of moles of species i divided by the total moles of all species. For ideal gases the mole fraction equals the volume fraction, which is the same as the percentage by volume divided by 100. For example, atmospheric O₂ is about 20.95% by volume, so x_O₂ = 0.2095.
P(O₂) = 0.2095 × P_total. At sea level (101.325 kPa) this is ≈ 21.2 kPa. At 3,000 m altitude (P ≈ 70 kPa) it is ≈ 14.7 kPa. At the summit of Everest (P ≈ 33.7 kPa) it falls to ≈ 7.1 kPa — about a third of sea-level value.
It works well for ideal gases and most real mixtures at moderate pressures (< ~10 atm) and temperatures above the dew point. It breaks down for gases with strong intermolecular interactions (high-pressure steam, polar gases) or when components approach condensation. For those, use fugacity-based equations of state.
Also known as
TG we-Calculate Editorial Team. (2026). Partial Pressure Calculator — Dalton's Law [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/partial-pressure-calculator
TG we-Calculate Editorial Team. "Partial Pressure Calculator — Dalton's Law." TG we-Calculate. 2026. https://we-calculate.com/calculator/partial-pressure-calculator.
TG we-Calculate Editorial Team, "Partial Pressure Calculator — Dalton's Law," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/partial-pressure-calculator
@misc{wecalculate_partial_pressure_calculator, title = {Partial Pressure Calculator — Dalton's Law}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/partial-pressure-calculator}}, year = {2026}, note = {TG we-Calculate} }
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