Last updated: July 3, 2026
Partial Pressure Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The partial pressure calculator applies Dalton's law: Pᵢ = xᵢP_total, where xᵢ = nᵢ/n_total. Enter component moles, total moles, and total pressure to get the component partial pressure in the same pressure unit plus its mole fraction.
Partial pressure equals the component mole fraction times the total pressure. The mole fraction is component moles divided by total moles.
Key Takeaways
- Partial pressure is a component gas's pressure contribution in a mixture.
- Dalton's law gives Pᵢ = xᵢ × P_total for ideal gas mixtures.
- Mole fraction is xᵢ = nᵢ/n_total and has no unit.
- The partial pressure uses the same unit as the total pressure you enter.
- For a complete ideal mixture, all partial pressures add to P_total.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
P_i = x_i × P_total, where x_i = n_i / n_total
Where:
- P_i=Partial pressure of gas component i(same as total pressure)
- x_i=Mole fraction of component i(dimensionless)
- P_total=Total pressure of the gas mixture(atm, torr, bar, kPa, or chosen unit)
- n_i=Moles of component i(mol)
- n_total=Total moles of all gas components(mol)
Worked Examples
Oxygen-like component in an atm mixture
A gas has 2 mol out of 10 mol total at 5 atm total pressure.
- 1Find the mole fraction: xᵢ = nᵢ ÷ n_total = 2 ÷ 10 = 0.2.
- 2Apply Dalton's law: Pᵢ = xᵢ × P_total.
- 3Pᵢ = 0.2 × 5 atm = 1 atm.
Gas component at atmospheric pressure in torr
A component contributes 3 mol in a 4 mol mixture at 760 torr total pressure.
- 1Calculate mole fraction: xᵢ = 3 ÷ 4 = 0.75.
- 2Multiply by total pressure: Pᵢ = 0.75 × 760 torr.
- 3The partial pressure is 570 torr.
Trace component in a low-pressure mixture
A minor component has 1 mol out of 8 mol total at 2 atm total pressure.
- 1Find xᵢ = 1 ÷ 8 = 0.125.
- 2Use Pᵢ = xᵢ × P_total.
- 3Pᵢ = 0.125 × 2 atm = 0.25 atm.
Introduction
Partial pressure is the pressure a gas component would exert if it alone occupied the mixture volume at the same temperature. This calculator uses Dalton's law, Pᵢ = xᵢP_total, where the mole fraction xᵢ is nᵢ/n_total. It pairs well with the mole calculator for amount-of-substance work and the molarity calculator when gas calculations connect to solution chemistry. The concept is defined in physical chemistry texts and summarized by IUPAC's pressure terminology and LibreTexts gas-mixture notes.
Dalton's law of partial pressures
For an ideal gas mixture, total pressure is the sum of all component partial pressures: P_total = P₁ + P₂ + P₃ + …. Because ideal gases share the same volume and temperature, each component's pressure contribution is proportional to its mole fraction. That gives the working equation Pᵢ = xᵢ × P_total.
xᵢ is dimensionless and ranges from 0 to 1.
All partial pressures must use the same pressure unit as P_total.
The sum of all mole fractions is 1 for a complete mixture.
The sum of all partial pressures equals total pressure for ideal mixtures.
How mole fraction controls pressure
Mole fraction counts molecules, not mass. A light gas and a heavy gas with the same number of moles have the same mole fraction and therefore the same ideal partial pressure at a fixed total pressure. This makes partial pressure especially useful for air, respiratory gases, vapor mixtures, and reaction atmospheres.
If nᵢ is greater than n_total, the mixture description is inconsistent; re-check the component list before using the result.
How to calculate partial pressure
Use the calculator or follow the manual workflow below. The most important step is confirming that n_total includes every gas component in the mixture, including inert diluents and carrier gases.
Measure or calculate nᵢ for the gas component of interest.
Add all gas-component moles to get n_total.
Compute xᵢ = nᵢ/n_total.
Multiply by total pressure: Pᵢ = xᵢP_total.
Repeat for each component if you need the full pressure distribution.
Pressure units and conversions
Dalton's law is unit-neutral: if P_total is entered in atm, the partial pressure is in atm; if it is entered in torr, the answer is in torr. Convert only when comparing with a reference in another unit. Common reference conversions are 1 atm = 760 torr = 101.325 kPa = 1.01325 bar.
| Total pressure | Mole fraction | Partial pressure |
|---|---|---|
| 1 atm | 0.21 | 0.21 atm |
| 760 torr | 0.75 | 570 torr |
| 101.325 kPa | 0.50 | 50.6625 kPa |
| 2 bar | 0.125 | 0.25 bar |
Where partial pressure is used
Partial pressures appear in gas-phase equilibrium constants, Henry's-law solubility, anesthesia and diving calculations, combustion, environmental monitoring, and controlled-atmosphere synthesis. For stoichiometric preparation of a gas component, use the grams to moles calculator or compare amount units with the concentration calculator.
When a gas is collected over water, subtract water-vapor pressure from the measured total before assigning dry-gas partial pressures.
Assumptions and limitations
The equation assumes ideal-gas behavior and a homogeneous mixture at a common temperature. Real gases at high pressure, low temperature, or strong intermolecular attraction can deviate from simple mole-fraction scaling. In those cases, fugacity or activity-coefficient corrections may be needed instead of raw partial pressure.
Best for low-to-moderate pressure ideal gas mixtures.
Not a substitute for vapor-liquid equilibrium calculations near condensation.
Use dry-gas corrections when water vapor contributes to total pressure.
Keep significant figures aligned with measured moles and pressure.
Quick Reference Card
Partial Pressure — Quick Reference
Quick reference • Partial Pressure Calculator
Pᵢ = (nᵢ/n_total) × P_totalValid range: Use 0 ≤ nᵢ ≤ n_total and P_total ≥ 0; best for ideal gas mixtures.
Common Values
⚠ Watch Out
- •Do not mix pressure units within the same calculation.
- •n_total must include all gas components, not just reactive gases.
- •nᵢ cannot be greater than n_total for a physical mixture.
- •Correct for water vapor when working with wet collected gases.
- •Real gases at high pressure may need fugacity corrections.
Pro Tips
- →Compute all mole fractions first; they should sum to 1.
- →Use total pressure in the unit you want for the answer.
- →For quick air estimates, multiply total pressure by 0.21 for oxygen and 0.78 for nitrogen.
- →Keep more digits during intermediate mole-fraction calculations, then round the final pressure.
- →When total pressure is 760 torr, mole fraction times 760 directly gives torr.
FAQs
What is partial pressure?
Partial pressure is the pressure contribution of one gas component in a mixture. It is the pressure that component would exert if it alone filled the same volume at the same temperature.
What formula does this calculator use?
It uses Dalton's law in mole-fraction form: Pᵢ = xᵢ × P_total, with xᵢ = nᵢ/n_total.
Can I use atm, torr, kPa, or bar?
Yes. Enter total pressure in any pressure unit; the partial pressure comes out in that same unit because mole fraction is dimensionless.
Why do I need total moles?
Total moles determine the mole fraction. A gas with 2 mol in a 10 mol mixture has xᵢ = 0.2, so it contributes 20% of the total ideal pressure.
What if component moles exceed total moles?
That is physically inconsistent because one component cannot contain more moles than the whole mixture. Recalculate n_total by summing every component.
Does Dalton's law work for real gases?
It is an excellent approximation for many low-pressure gas mixtures, but high-pressure or strongly nonideal systems may require fugacity-based corrections.