How to Calculate Partial Pressure with Dalton’s Law and the Ideal Gas Law
By Tools Star Hub Editorial Team · Published
Quick answer: the partial pressure of a gas is its mole fraction times the total pressure, P = X × P(total). Open the Partial Pressure Calculator, pick one of four modes (mole fraction × total, gas mixture in moles, total from partials, or the ideal gas law P = nRT/V), enter your values and press Calculate. The result is shown with its working steps and, in most modes, in atm, kPa, Pa, bar and mmHg.
In a mixture of gases that do not react, each gas pushes on the container walls as if it were alone. Dalton’s law says the total pressure is the sum of those separate pressures. Chemistry homework, diving gas planning and breathing gas questions all come back to this idea, and the arithmetic is easy to slip on when units change or a percentage is used where a fraction is needed.
Which calculation modes are there?
The calculator has four modes. Choose the one that matches the numbers you already have.
| Mode | You enter | You get |
|---|---|---|
| Mole fraction × total | Mole fraction X (0 to 1) and total pressure | Partial pressure of that gas |
| Gas mixture (moles) | Moles of two or more gases and total pressure | Each gas’s mole fraction and partial pressure |
| Total from partials | Two or more partial pressures | Total pressure |
| Ideal gas P = nRT/V | Moles, temperature and volume | Pressure of the gas |
How does the mole fraction mode work?
Enter the mole fraction X and the total pressure, and the tool multiplies them. X must be between 0 and 1 and the total must be above 0.
A common mistake is typing a percentage, such as 21 for 21% oxygen. Any X above 1 is rejected with a message telling you to divide a percentage by 100 first, so 21% becomes 0.21.
How do I work out partial pressures for a whole mixture?
Gas mixture mode starts with two rows, N₂ and O₂, and you can add more gases. Enter the moles of each gas and the total pressure. The tool adds the moles, divides each gas’s moles by that sum to get its mole fraction X, and multiplies each X by the total pressure.
The result lists every gas with its X and its partial pressure, so you can check that the fractions add up to 1 and the partials add up to the total. At least two gases are needed, because a single gas would simply have the whole pressure.
How do I find the total pressure from partial pressures?
Total from partials mode is Dalton’s law in its plainest form. Enter at least two partial pressures in the same unit and the tool adds them up. This is the mode to use when a question gives you the pressure of each gas and asks for the total.
How does the ideal gas mode calculate pressure?
Ideal gas mode uses P = nRT/V with R = 8.314462618 J/(mol·K). Enter the amount in moles, the temperature and the volume.
Temperature can be in kelvin or degrees Celsius. Celsius is converted by adding 273.15, and the result must be above 0 K. Volume can be in litres, which are divided by 1,000 to give cubic metres, or entered directly in m³. With n in moles, T in kelvin and V in m³, the formula gives pressure in pascals, which the tool then converts to the other units.
This mode is useful for finding the partial pressure of one gas from its own amount, because in an ideal mixture each gas behaves as if it filled the container alone.
Which pressure units are supported?
Five units are available: atm, kPa, Pa, bar and mmHg. The tool uses 1 atm = 101,325 Pa and 1 mmHg = 101,325 ÷ 760 Pa. Apart from mixture mode, the answer is shown converted into all five units so you can copy whichever one your question asks for.
| Unit | Value in pascals |
|---|---|
| 1 atm | 101,325 Pa |
| 1 kPa | 1,000 Pa |
| 1 bar | 100,000 Pa |
| 1 mmHg | 101,325 ÷ 760 Pa (about 133.32 Pa) |
How are inputs read and results shown?
Commas are removed from what you type, so 101,325 is read as 101325. Negative values are rejected. Results are shown to six significant figures, and very large or very small numbers (1e9 or more, or below 1e-4) switch to exponential notation.
Every result comes with the steps used to reach it. Calculate runs the sum, Copy result copies it, and Clear empties the form.
Which mode should I pick for a textbook problem?
Read the question for the quantities it gives. If it states a percentage or fraction of a gas and a total pressure, use mole fraction × total, remembering to turn the percentage into a fraction. If it lists the moles or amounts of several gases in one container with a total pressure, use gas mixture mode, which saves you working out every fraction by hand.
If it gives the pressure of each gas and asks what the container reads, use total from partials. If it gives an amount in moles, a temperature and a volume, use the ideal gas mode, then compare the result with other gases in the same container if the question goes on to ask about a mixture.
Many multi-part questions use two modes in a row. For example, you might find one gas’s pressure with P = nRT/V and then add it to the others in total from partials.
How can I check that an answer makes sense?
A partial pressure can never be larger than the total pressure, and in a mixture the partial pressures should add back up to the total. In gas mixture mode the mole fractions should also add up to 1, give or take rounding.
For the ideal gas mode, a useful reference is that one mole at 0 °C in about 22.4 litres sits close to 1 atm. If your answer is thousands of times larger or smaller, check whether a volume in litres was entered as m³ or a Celsius temperature as kelvin.
How do I use the calculator step by step?
- Choose a mode. Pick mole fraction × total, gas mixture, total from partials or the ideal gas law.
- Pick units. Select atm, kPa, Pa, bar or mmHg for pressure, and K or °C and L or m³ in ideal gas mode.
- Enter your values. Use fractions between 0 and 1, not percentages, and positive numbers only.
- Press Calculate. Read the answer and the steps underneath it.
- Copy or clear. Use Copy result to paste the answer elsewhere, or Clear to start again.
Example
Oxygen in air at sea level
X = 0.21 and total = 1 atm gives 0.21 atm, which the tool also shows as about 21.28 kPa and about 159.6 mmHg.
A nitrogen and oxygen mixture
3 mol N₂ and 1 mol O₂ at 2 atm total: X is 0.75 and 0.25, so the partial pressures are 1.5 atm and 0.5 atm.
Adding partials
0.78 atm + 0.21 atm + 0.01 atm gives a total pressure of 1 atm.
One mole at 0 °C in 22.4 L
T becomes 273.15 K and V becomes 0.0224 m³. P = 1 × 8.314462618 × 273.15 ÷ 0.0224, about 101,388 Pa, which is just over 1 atm.
What are the limits of these results?
Ideal behaviour is assumed
Dalton’s law and P = nRT/V describe ideal gases. Real gases at high pressure or low temperature can differ.
Keep units consistent
In total from partials mode, enter every value in the same unit, because the tool adds the numbers as typed.
Six significant figures
Round the displayed answer to the precision your question or data supports.
FAQ
- What happens if I type 21 for 21% oxygen?
- Mole fraction must be between 0 and 1. Divide a percentage by 100 first, so 21% becomes 0.21.
- What value of R does the ideal gas mode use?
- It uses R = 8.314462618 J/(mol·K), with temperature in kelvin and volume in cubic metres.
- Can I enter the temperature in Celsius?
- Yes. Choose °C and the tool adds 273.15. The result has to be above 0 K.
- Can I enter volume in litres?
- Yes. Litres are divided by 1,000 to give cubic metres before the formula is applied.
- How many gases can the mixture mode handle?
- It needs at least two. It starts with N₂ and O₂ rows and you can add more.
- Does the mixture mode need mole fractions?
- No. Enter moles for each gas and the total pressure, and the tool works out each mole fraction for you.
- What does 1 mmHg equal in this tool?
- 101,325 ÷ 760 pascals, so 760 mmHg equals exactly 1 atm.
- Why is my answer shown like 2.5e-5?
- Values below 1e-4 or at least 1e9 are shown in exponential notation to keep them readable.
- Can I type numbers with thousands separators?
- Yes. Commas are stripped, so 101,325 is read as 101325.
- Can a partial pressure be negative?
- No. Negative inputs are rejected because amounts and pressures cannot be below zero.
- How do I convert a percent composition to mole fraction?
- Divide the percent by 100 (21% oxygen → 0.21) before multiplying by total pressure.
- When should I pick mixture mode over ideal-gas mode?
- Use mixture mode when you know total pressure and composition; use ideal-gas when you have n, V, and T.
- Can torr and mmHg be treated as equal here?
- For ordinary lab work they are treated as equal; keep labels consistent in the report.
- What if my mole fractions sum to 0.99?
- Round-off is common—renormalize or add the missing component before trusting the total.
- How precise should R be for class problems?
- Use the R value matching your units (for example L·atm) as stated in the tool or textbook.
- Does gauge pressure work in these formulas?
- Convert to absolute pressure first by adding atmospheric pressure.
- Can I solve for volume given partial pressure?
- Rearrange the ideal-gas relation with that gas’s moles, T, and its partial pressure.
- Why reject a mole fraction of 1.2?
- Fractions cannot exceed 1 for a single component in a simple mixture model.
- How do I report scientific notation answers?
- Keep significant figures from your inputs; scientific notation is fine for tiny pressures.
- Are dissolved oxygen problems in water different?
- Yes—use Henry’s law for solubility, not Dalton’s gas-mixture shortcut alone.
Work through your gas problem with the free Partial Pressure Calculator.
Convert other units with the Unit Converter.
Round answers with the Significant Figures Calculator.