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Partial Pressure Calculator

Calculate partial pressures with Dalton's law from mole fractions, gas mixtures or the ideal gas law, add partial pressures to a total, and convert between atm, kPa, Pa, bar and mmHg.

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What this tool does

This partial pressure calculator applies Dalton's law in four ways. It can turn a mole fraction and a total pressure into a partial pressure, split a total pressure across a mixture of gases from their amounts in moles, add separate partial pressures to get a total, or find the pressure of a single gas from the ideal gas law. Every answer is shown in atm, kPa, Pa, bar and mmHg, with the working written out step by step so you can check it against a textbook or a lab sheet.

How to use

  1. Choose a mode: mole fraction × total, gas mixture, total from partials, or ideal gas.
  2. Enter the values for that mode. Mole fractions are decimals from 0 to 1, amounts are in moles, and temperatures can be kelvin or degrees Celsius.
  3. Pick the pressure unit you are working in. The result is converted to the other four units automatically.
  4. Press Calculate, read the steps, and use Copy result to paste the answer and working into notes. Clear empties every field.

Examples

  • Oxygen in dry air at sea level

    Dry air is about 20.95% oxygen by volume, so its mole fraction is about 0.2095. At a total pressure of 1 atm, P(O₂) = 0.2095 × 1 atm = 0.2095 atm, which is 21.23 kPa or 159.2 mmHg.

  • A three-gas cylinder

    A cylinder at 200 kPa holds 3.0 mol of nitrogen, 1.5 mol of oxygen and 0.5 mol of argon, 5.0 mol in total. The mole fractions are 0.60, 0.30 and 0.10, so the partial pressures are 120 kPa, 60 kPa and 20 kPa. They add back to 200 kPa.

  • Gas collected over water

    Hydrogen is collected over water at 25 °C with a total pressure of 755.0 mmHg. Water's vapour pressure at 25 °C is about 23.8 mmHg, so the dry hydrogen pressure is 755.0 − 23.8 = 731.2 mmHg. Use the total-from-partials mode in reverse by checking that 731.2 + 23.8 returns 755.0 mmHg.

  • Ideal gas pressure in a flask

    0.500 mol of gas at 25 °C in a 10.0 L flask: T = 298.15 K and V = 0.0100 m³, so P = 0.500 × 8.314462618 × 298.15 ÷ 0.0100 = 123,948 Pa, about 124.0 kPa or 1.223 atm.

Main features

  • Dalton's law from a mole fraction, with a check that rejects fractions below 0 or above 1.
  • A mixture mode for up to 10 gases that computes each mole fraction and confirms the fractions add to 1.
  • Total pressure from up to 10 partial pressures.
  • Ideal gas pressure from n, T and V with kelvin or Celsius and litres or cubic metres.
  • Exact conversions between atm, kPa, Pa, bar and mmHg.

How it works

Dalton's law of partial pressures says that in a mixture of gases that do not react, each gas pushes on the container walls as if it were alone, and the total pressure is the sum of those separate pushes. Because pressure in an ideal mixture is proportional to the number of molecules, each gas's share of the total pressure equals its share of the moles.

Calculation method

Mole fraction mode uses Pᵢ = Xᵢ × P_total. The fraction must be at least 0 and no more than 1, and the total pressure must be greater than zero.

Mixture mode first adds the moles: n_total = n₁ + n₂ + … . Each mole fraction is Xᵢ = nᵢ ÷ n_total, and each partial pressure is Xᵢ × P_total. The tool also adds the fractions and shows the sum, which should be 1 apart from rounding.

Total mode uses P_total = P₁ + P₂ + … + Pₙ, with every value in the same unit.

Ideal gas mode uses P = nRT ÷ V with R = 8.314462618 J/(mol·K). Celsius is converted with T(K) = T(°C) + 273.15, litres are divided by 1,000 to get cubic metres, and the answer comes out in pascals before conversion.

Unit conversion goes through pascals: 1 atm = 101,325 Pa, 1 bar = 100,000 Pa, 1 kPa = 1,000 Pa and 1 mmHg = 101,325 ÷ 760 Pa, about 133.322 Pa. Results are rounded to six significant figures for display only.

Mole fraction validation

A mole fraction is a ratio of part to whole, so it cannot be negative and cannot exceed 1. If you type a percentage such as 21 instead of 0.21, the calculator stops and asks for a decimal rather than returning a partial pressure 21 times larger than the total. In mixture mode the fractions are computed from the moles, so they always add to 1 and you do not have to normalise them yourself.

Worked example: breathing gas at depth

A diver breathes air at 30 metres of seawater, where the absolute pressure is about 4.0 bar. With an oxygen fraction of 0.21, P(O₂) = 0.21 × 4.0 = 0.84 bar, and with a nitrogen fraction of 0.79, P(N₂) = 0.79 × 4.0 = 3.16 bar. The two add back to 4.0 bar. This is the same calculation dive planners use to check oxygen exposure, although real dive planning needs proper training, tables or a dive computer, not a web calculator.

Common mistakes to avoid

Mixing units is the most frequent error: adding a value in kPa to a value in mmHg gives a meaningless total. Another is using Celsius directly in P = nRT/V, which can even give a negative pressure. A third is forgetting that a gas collected over water is wet, so the measured total includes water vapour.

When Dalton's law works well

The law is very accurate for gases at ordinary pressures and temperatures, such as air, breathing mixtures, combustion gases and most classroom problems. It becomes less exact at high pressure or low temperature, where molecules take up a noticeable share of the volume and attract one another, and for gases that react, dissolve or condense.

Tips

  • Convert percentages to decimals before using mole fraction mode: 78% becomes 0.78.
  • For gas collected over water, subtract the water vapour pressure at the measured temperature from the total.
  • Keep every partial pressure in the same unit before adding them.
  • Use kelvin in your own notes as well, because P = nRT/V only works with absolute temperature.

Limitations

The calculator assumes ideal gases that do not react with each other. It does not apply van der Waals or other real-gas corrections, it does not look up vapour pressures, and it does not handle gas solubility in liquids (Henry's law). Results are shown to six significant figures, which can be more precision than your measurements support, so round the final answer to match your data.

See the disclaimer for what these tools do not cover.

FAQ

What is partial pressure?
It is the pressure one gas in a mixture would exert if it filled the whole container alone at the same temperature.
What is the formula for partial pressure?
Pᵢ = Xᵢ × P_total, where Xᵢ is the gas's mole fraction. You can also add every partial pressure to get the total: P_total = P₁ + P₂ + … .
How do I find a mole fraction?
Divide the moles of one gas by the total moles of all gases. The mixture mode does this for you and shows each fraction.
Why does the calculator reject a mole fraction of 21?
A mole fraction must be between 0 and 1. If you mean 21%, enter 0.21.
Which pressure units are supported?
atm, kPa, Pa, bar and mmHg. Torr is effectively the same as mmHg for everyday work, since 1 torr is exactly 101,325 ÷ 760 Pa.
Can I use Celsius in the ideal gas mode?
Yes. Choose °C and the tool adds 273.15 before calculating. Temperatures at or below absolute zero are rejected.
What value of R does the tool use?
8.314462618 J/(mol·K), the exact SI value defined since 2019, with volume in cubic metres and pressure in pascals.
Does Dalton's law work for real gases?
It is a close approximation at everyday pressures. At high pressure or near condensation, real-gas behaviour causes small differences that this tool does not model.
How many gases can I add?
Up to 10 in the mixture and total modes, which covers almost every homework, lab and diving problem.
How do I convert mmHg results to kPa?
Divide mmHg by 7.5006 for an approximate kPa value, or switch the tool’s unit before calculating.
Can I mix mole fractions that do not sum to 1?
They should sum to 1 for a complete mixture; fix composition first if the total drifts.
When is the ideal-gas mode appropriate?
Use it for dilute gases near ordinary temperatures and pressures; dense or polar gases need corrections.
How does water vapor affect dry-gas partial pressures?
Subtract vapor pressure from total pressure when you need dry-gas mole fractions.
Can I back-calculate mole fraction from a known partial pressure?
Divide the partial pressure by the total pressure when Dalton’s law applies.
Why did raising temperature change the ideal-gas result?
Ideal-gas mode uses PV=nRT relationships—temperature shifts computed pressures when amount and volume are fixed.
Do I enter gauge or absolute pressure?
Dalton’s law needs absolute total pressure; convert gauge readings by adding atmospheric pressure.
How many decimal places should lab reports keep?
Match your instrument precision; avoid false accuracy beyond the measured total pressure.
Is volume percent the same as mole percent for ideal gases?
Yes under ideal-gas assumptions at the same T and P; real gases can deviate.
Can dissolved gases in liquids use this tool?
No—Henry’s law and solubilities differ from gas-mixture partial pressures.
What if one component is a condensable vapor?
Watch saturation limits; condensation means the simple gas-mixture model no longer holds.

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