Dalton's law says the total pressure of a gas mixture equals the sum of each gas's partial pressure, and each partial pressure equals its mole fraction times the total. Dry air at 101.325 kPa is 20.95% oxygen by moles, so oxygen's partial pressure is 0.2095 × 101.325 = 21.23 kPa.
Chemistry students, respiratory therapists, divers planning gas blends and lab staff collecting gas over water all lean on Dalton's law of partial pressures. This calculator works in two directions. Add up to four partial pressures to get the total pressure, each gas's mole fraction and a donut showing how the pressure divides. Or enter the total pressure with the moles of one gas and of the whole mixture to get that gas's partial pressure. Every answer stays in the unit you pick, so mixing kPa with mmHg is up to you to avoid. The law assumes ideal gases that do not react; at high pressures or with strongly interacting gases such as ammonia and hydrogen chloride, real partial pressures drift from these values.
Total pressure adds partial pressures; One partial pressure uses the mole fraction.
kPa, atm, mmHg or bar. All pressures you enter must use it.
Fill gases A and B, plus C and D if present, or the total pressure and the two mole amounts.
The working lists each mole fraction; the donut chart can be downloaded as PNG, SVG or CSV.
Result Pressure (in the chosen unit): 21.2276
Mole fraction: 0.2095
Share of total pressure: 20.95%
Detail: Other gases: 80.0974 kPa
Dry air is 20.95% oxygen by moles, so at one standard atmosphere oxygen contributes about 21.2 kPa.
Result Pressure (in the chosen unit): 1
Mole fraction: 0.5
Share of total pressure: 50%
Detail: Largest share: Gas A
Partial pressures of 0.5, 0.3 and 0.2 atm add up to exactly one atmosphere.
P_total = P_A + P_B + P_C + …; P_i = x_i × P_total, with x_i = n_i ÷ n_total
P_totalP_ix_in_i, n_totalJohn Dalton observed in 1801 that each gas in a non-reacting mixture exerts the pressure it would exert alone in the same volume and temperature. Because every ideal gas obeys PV = nRT with the same V and T, pressure is proportional to moles, so a gas's share of the pressure equals its share of the moles. In sum mode the tool adds the entered pressures and divides each by the total; in fraction mode it multiplies the mole fraction by the total pressure.
In an ideal gas, pressure depends on the number of particles hitting the walls, not their mass. A mole of helium and a mole of xenon at the same volume and temperature exert the same pressure, so mole fraction, not mass fraction, sets each gas's share.
The trapped gas is saturated with water vapour. Subtract the vapour pressure of water at the bath temperature from the measured total. At 25 °C that is about 3.17 kPa (23.8 mmHg), so 101.3 kPa total leaves about 98.1 kPa for the dry gas.
Oxygen stays about 20.95% of dry air at altitude, but the total pressure falls. At roughly 3,000 m the pressure is near 70 kPa, so oxygen's partial pressure is about 0.2095 × 70 ≈ 14.7 kPa, which is why breathing feels harder.
For ideal gases at the same temperature and pressure, volume percent equals mole percent, so 78% nitrogen by volume means a mole fraction of 0.78. This does not hold for liquids or for mass percentages.
Divers track oxygen partial pressure because oxygen becomes toxic above about 1.4 to 1.6 atm. Air at 40 m depth is about 5 atm total, so oxygen sits near 0.21 × 5 = 1.05 atm, while enriched mixes reach the limit at shallower depths.
It shows the mole fraction of the gas with the largest partial pressure, named in the Detail line. The working lists the mole fraction of every gas you entered.
Using this tool in an assignment, article or report? Copy a citation, or a link for your website.
Tools Directory Editorial Team. (2026, October 27). Dalton’s Law Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/daltons-law-calculator/
Tools Directory Editorial Team. “Dalton’s Law Calculator.” Tools Directory, 27 Oct. 2026, https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/daltons-law-calculator/. Accessed 27 Oct. 2026.
Tools Directory Editorial Team. “Dalton’s Law Calculator.” Tools Directory. Last modified October 27, 2026. Accessed October 27, 2026. https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/daltons-law-calculator/.
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