Quick answer
The partial pressure of a gas is P_i = x_i × P_total, where x_i = n_i ÷ n_total (Dalton's law). Oxygen at 0.21 mole fraction in air at 101.325 kPa has a partial pressure of 21.28 kPa.
Enter the total pressure, moles of the gas and total moles. The calculator returns the partial pressure and mole fraction.
Related tools: Pressure Calculator: Pressure from force and area, with unit conversions. Mole Calculator: Convert between mass, moles and number of particles. Percent Yield Calculator: Percent yield of a reaction.
How to use the Partial Pressure Calculator
- Enter total pressure (above 0).
- Enter moles of the gas (above 0).
- Enter total moles of all gases (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Total pressure | A number | > 0 |
| Moles of the gas | A number | > 0 |
| Total moles of all gases | A number | > 0 |
Formula and method
Formula
P_i = x_i × P_total, x_i = n_i ÷ n_total
IEEE-754 double precision; shown to the decimals indicated.
Output has the same pressure unit as the total pressure.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Total pressure = 101.325; Moles of the gas = 0.21; Total moles of all gases = 1.
| Result | Value |
|---|---|
| Partial pressure | 21.27825 |
| Mole fraction | 0.21 |
Example 2
Inputs: Total pressure = 760; Moles of the gas = 2; Total moles of all gases = 5.
| Result | Value |
|---|---|
| Partial pressure | 304 |
| Mole fraction | 0.4 |
Example 3
Inputs: Total pressure = 2.5; Moles of the gas = 3; Total moles of all gases = 12.
| Result | Value |
|---|---|
| Partial pressure | 0.625 |
| Mole fraction | 0.25 |
The first line is the headline result (partial pressure). Below it the partial Pressure Calculator lists mole fraction, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Worked example
0.21 × 101.325 = 21.278 kPa. Nitrogen at 0.78 has 79.0 kPa. Partial pressures add up to the total pressure.
Applications
Divers watch oxygen partial pressure to avoid toxicity (about 1.4 bar maximum), and altitude lowers it. Gas solubility follows Henry's law with the partial pressure. The moles of the gas can't exceed total moles.
How the result changes with total pressure
Holding every other input at the first worked example, this table changes only total pressure. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Total pressure | Partial pressure | Mole fraction |
|---|---|---|
| 50.66 | 10.6386 | 0.21 |
| 101.3 | 21.273 | 0.21 |
| 202.7 | 42.567 | 0.21 |
| 405.3 | 85.113 | 0.21 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Moles of the gas: entering "6" shows "The gas cannot have more moles than the total." beside that field and no result.
- Total pressure: entering "0" shows "Total pressure must be greater than 0." beside that field and no result.
Choosing between the Partial Pressure Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Pressure Calculator](/science/pressure-calculator/) | force and area | Pressure |
| [Mole Calculator](/science/mole/) | start from and molar mass | Amount |
| [Percent Yield Calculator](/science/percent-yield/) | actual yield and theoretical yield | Percent yield |
| [Electron Configuration Calculator](/science/electron-configuration/) | element and charge | Electron configuration of H |
| [Dilution Calculator](/science/dilution-ratio/) | solve for | Final volume V₂ |
Background: chemistry calculations
Most basic chemistry calculations chain three relations: moles n = mass ÷ molar mass, molarity M = n ÷ volume in litres, and dilution C₁V₁ = C₂V₂. Keep units consistent: grams with g/mol, litres with mol/L.
Molar masses come from standard atomic weights, which differ slightly between tables and with the isotopic make-up of a real sample.
Assumptions and limitations
- Moles of the gas cannot exceed total moles
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for moles of the gas and total pressure to confirm this.
Frequently asked questions
What is Dalton's law?
Total pressure is the sum of partial pressures.
Why does altitude reduce oxygen?
The total pressure falls.
Does it need mole fractions?
Yes. The partial pressure is the mole fraction (moles of the gas ÷ total moles) times the total pressure.
What does the Partial Pressure Calculator calculate?
Partial pressure of a gas in a mixture (Dalton's law). It returns Partial pressure and Mole fraction.
What formula does the Partial Pressure Calculator use?
P_i = x_i × P_total, x_i = n_i ÷ n_total. Output has the same pressure unit as the total pressure.
What do I need to enter in the partial Pressure Calculator?
You enter Total pressure, Moles of the gas and Total moles of all gases.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Moles of the gas cannot exceed total moles.
What is an example of a partial Pressure result?
With total pressure 101.325, moles of the gas 0.21 and total moles of all gases 1, the partial pressure is 21.27825.
Where does the method come from?
The method follows: OpenStax, Chemistry 2e (Rice University): gases and Dalton's law of partial pressures.
Related calculators
- Pressure Calculator — Pressure from force and area, with unit conversions.
- Mole Calculator — Convert between mass, moles and number of particles.
- Percent Yield Calculator — Percent yield of a reaction.
- Electron Configuration Calculator — Ground-state electron configuration of an element or monatomic ion, with noble-gas shorthand and unpaired electrons.
- Molarity Calculator — Concentration of a solution from solute mass, molar mass and volume.
Sources
- OpenStax, Chemistry 2e (Rice University): gases and Dalton's law of partial pressures
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.