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Partial Pressure Calculator

ChemistryGas Laws and Thermodynamics
Method
Partial pressure (kPa) —
In atmospheres (atm)
—
In mmHg (torr)
—
In bar
—
Mole fraction
—

Partial pressure is the pressure one gas in a mixture would exert alone. By Dalton's law it equals the gas's mole fraction times the total pressure: oxygen at 20.95 mol per 100 mol of dry air at 1 atm gives 0.2095 atm, or 21.23 kPa. For a single gas in a known volume, use p = nRT ÷ V instead.

About this tool

Chemistry students, respiratory therapists checking oxygen tension and divers planning gas blends all need the pressure one component of a mixture contributes. This calculator offers two routes. The mole fraction route takes the moles of your gas, the total moles and the total pressure in atm, kPa, mmHg, bar or psi, and applies Dalton's law. The ideal gas route takes moles, absolute temperature in kelvin and container volume in litres and applies p = nRT ÷ V. Every result is shown in kPa with atm, mmHg and bar conversions, plus the line-by-line working. Both routes assume ideal gas behaviour, which holds well near room conditions but drifts at high pressure or near condensation, and neither accounts for water vapour unless you include it in the mole counts.

How to use it

  1. Pick a method

    Choose Mole fraction when you know the mixture composition and total pressure, or Ideal gas law when you know temperature and volume.

  2. Enter the moles of your gas

    Give the amount of the component you care about, in moles.

  3. Fill in the remaining inputs

    For mole fraction, add total moles and total pressure and pick its unit inside the field. For the ideal gas law, add temperature in kelvin and volume in litres.

  4. Read the result

    The partial pressure appears in kPa, with atm, mmHg and bar beneath it and the working steps.

Examples

Oxygen in air at 1 atm

Method
Mole fraction
Moles of this gas
20.95
Total moles in the mixture
100
Total pressure
1
Pressure unit
atm

Result Partial pressure (kPa): 21.23
In atmospheres (atm): 0.2095
In mmHg (torr): 159.22
In bar: 0.21228
Mole fraction: 0.2095

  1. Mole fraction x = 20.95 ÷ 100 = 0.2095
  2. Total pressure = 1 atm = 101.325 kPa
  3. p = x × P_total = 0.2095 × 101.325 = 21.2276 kPa

Oxygen is 20.95 of every 100 moles of dry air, so at one standard atmosphere it contributes 0.2095 atm, about 21.2 kPa.

1 mol at 273.15 K in 22.414 L

Method
Ideal gas law
Moles of this gas
1
Temperature
273.15
Container volume
22.414

Result Partial pressure (kPa): 101.32
In atmospheres (atm): 1
In mmHg (torr): 760
In bar: 1.01325
Mole fraction: Not used

  1. p = n·R·T ÷ V = 1 × 8.314462618 × 273.15 ÷ 22.414 L = 101.3249 kPa

The classic molar volume at 0 °C gives back one atmosphere, a quick check on the ideal gas route.

2 of 5 mol at 760 mmHg

Method
Mole fraction
Moles of this gas
2
Total moles in the mixture
5
Total pressure
760
Pressure unit
mmHg (torr)

Result Partial pressure (kPa): 40.53
In atmospheres (atm): 0.4
In mmHg (torr): 304
In bar: 0.4053
Mole fraction: 0.4

  1. Mole fraction x = 2 ÷ 5 = 0.4
  2. Total pressure = 760 mmhg = 101.325 kPa
  3. p = x × P_total = 0.4 × 101.325 = 40.53 kPa

A mole fraction of 0.4 takes 40 % of the total: 304 mmHg, or 40.53 kPa.

How it is calculated

p_i = x_i × P_total, with x_i = n_i ÷ n_total; or p_i = n_i·R·T ÷ V

p_i
partial pressure of gas i
x_i
mole fraction of gas i
n_i, n_total
moles of gas i and of the whole mixture
R
gas constant, 8.314462618 J/(mol·K)
T, V
absolute temperature in K and volume in L

Dalton's law says ideal gases in a mixture act independently, so each one's share of the pressure equals its share of the molecules. The total pressure is first converted to kPa (1 atm = 101.325 kPa = 760 mmHg, 1 bar = 100 kPa). In the ideal gas route, R × T ÷ V with V in litres gives kPa directly, because one joule per litre is one kilopascal.

When not to use it

  • Gases at many bar or close to liquefying need a real-gas equation of state.
  • Dissolved gas tension in liquids follows Henry's law, not this calculation.
  • Humid mixtures need water vapour counted, or its vapour pressure subtracted first.

Common mistakes

  • Entering temperature in degrees Celsius instead of kelvin.
  • Using mass in grams where moles are required.
  • Using percent composition by mass as if it were mole fraction.
  • Forgetting that the total pressure must include every gas, including water vapour.

Frequently asked questions

What is Dalton's law of partial pressures?

It states that the total pressure of a mixture of non-reacting ideal gases equals the sum of the pressures each gas would exert alone in the same volume and temperature. It follows that each gas's partial pressure is its mole fraction times the total.

Why is volume percent the same as mole fraction for gases?

For ideal gases, equal volumes at the same temperature and pressure hold equal numbers of moles (Avogadro's law). So 20.95 % oxygen by volume in air means a mole fraction of 0.2095.

What is the partial pressure of oxygen at sea level?

Dry air at 1 atm holds oxygen at about 0.2095 atm, which is 21.23 kPa or roughly 159 mmHg. Inside the lungs it is lower because water vapour and carbon dioxide take part of the total.

Why must temperature be in kelvin?

The ideal gas law is proportional to absolute temperature. Celsius has an arbitrary zero, so 25 °C must be entered as 298.15 K; otherwise the pressure would come out about twelve times too small.

How do I get moles from grams?

Divide the mass by the molar mass. For example, 16 g of O2 at 32.00 g/mol is 0.5 mol. Then enter that value as the moles of your gas.

Can the partial pressure exceed the total pressure?

No. A mole fraction is at most 1, so a single gas's partial pressure is never larger than the total. The calculator flags moles of a gas that exceed the total moles.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 27). Partial Pressure Calculator. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/partial-pressure-calculator/

MLA

Tools Directory Editorial Team. “Partial Pressure Calculator.” Tools Directory, 27 Oct. 2026, https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/partial-pressure-calculator/. Accessed 27 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Partial Pressure Calculator.” Tools Directory. Last modified October 27, 2026. Accessed October 27, 2026. https://tools.milunir.com/chemistry/gas-laws-and-thermodynamics/partial-pressure-calculator/.

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