Dalton's Partial Pressure Calculator
Dalton's law of partial pressures states that the total pressure exerted by a mixture of non-reacting ideal gases is equal to the sum of the partial pressures of each individual gas. This principle is widely used in respiratory physiology (partial pressures of oxygen and carbon dioxide in air), chemical engineering (gas separation), and meteorology (water vapor pressure). Enter the partial pressures of up to five gases in any consistent pressure unit to calculate the total pressure. Blank fields are treated as zero.
Dalton's law formula
P_total = P(1) + P(2) + P(3) + ... + P(n)
Each partial pressure P(i) is the pressure that component i would exert if it alone occupied the entire volume at the same temperature. For ideal gases, mole fraction x(i) = P(i) / P_total.
Common applications
- Atmospheric air at sea level: P_O2 = 0.2095 atm, P_N2 = 0.7809 atm, P_Ar = 0.0093 atm, P_CO2 = 0.0004 atm. Total = 1.0001 atm.
- Respiratory physiology: alveolar oxygen partial pressure drives oxygen diffusion into blood. Normal P_O2 in alveoli is about 100 mmHg.
- Gas cylinders: partial pressures of mixed gases (such as nitrox for diving) are calculated to verify safe oxygen fractions.
Frequently asked questions
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 individual component: P_total = P1 + P2 + P3 + ... Each gas behaves independently as if it alone occupied the container.
What is a partial pressure?
The partial pressure of a gas in a mixture is the pressure it would exert if it alone occupied the same volume at the same temperature. It equals the mole fraction of that gas times the total pressure.
What are the units for partial pressure?
Any pressure unit may be used (atm, kPa, mmHg, psi) as long as all partial pressures use the same unit. The total pressure will be in the same unit.
How does this relate to mole fractions?
The partial pressure of gas i equals its mole fraction (n_i / n_total) multiplied by the total pressure. If you know the mole fraction and total pressure, you can find each partial pressure directly.
Is Dalton's law exact for real gases?
Dalton's law is exact for ideal gases. For real gases at moderate pressures and temperatures, it is a good approximation. At very high pressures, intermolecular forces between unlike molecules cause deviations.
Official sources
- NIST Chemistry WebBook: NIST WebBook Homepage.
- OpenStax University Chemistry: Gas Mixtures and Partial Pressures.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.