Partial Pressure Calculator
Gas partial pressure from share and total.
What it does
Air is a team of gases sharing one pressure. Dalton says each contributes its fraction of the total: oxygen near twenty one percent of 101.3 kilopascals gives 21.27 with 80.03 left for the rest. Nitrogen at seventy eight percent of 760 millimeters of mercury gives 592.8.
The tool above multiplies share by total in kilopascals, millimeters of mercury, or atmospheres. Units ride through untouched, so lab and dive math both work. Repeating per gas builds the full mixture while the remainders cross check the sum.
Ideal behavior underlies the line. High pressures bend real gases off Dalton, temperature moves totals, and humidity adds its own vapor share. Diving and altitude work layer those corrections on top.
This page models ideal shares only. It does not handle solubility, reactions, or altitude totals. For breathing mixes, confirm with tables and training.
How to use
- Enter the gas percent share.
- Enter the mixture total pressure.
- Pick kPa, mmHg, or atm.
- Read partial pressure first.
- Read remainder to check the sum.
- Repeat per gas for full mixes.
Formula
- fraction
- Gas share in percent, 0 to 100.
- total
- Mixture total in kPa, mmHg, or atm.
- rest
- Total minus this gas.
Ideal gas mixtures with one shared unit. At zero fraction the partial is zero; at 100% it equals total. Non ideal behavior at high pressure sits outside this line.
Worked example
Example: oxygen 21% at 101.3 kPa
Defaults: 21% at 101.3 kPa (oxygen in air). Unit choices are kPa, mmHg, atm.
- Inputs: 21% oxygen at 101.3 kPa total.
- Partial: 0.21 times 101.3 is 21.27 kPa.
- Rest: 101.3 minus 21.27 is 80.03 kPa rest.
- Nitrogen 78% of 760 mmHg gives 592.8 mmHg.
- Dalton sums to total. Tool shows the same.
Reference table
| Share and total | Partial | Rest |
|---|---|---|
| 21% of 101.3 kPa | 21.27 kPa | 80.03 kPa rest |
| 78% of 760 mmHg | 592.8 mmHg | 167.2 mmHg rest |
| 4% of 50 kPa | 2 kPa | 48 kPa rest |
| 100% of 2 atm | 2 atm | 0 atm rest |
| 50% of 200 kPa | 100 kPa | 100 kPa rest |
Limits
- Ideal mixes. High pressure deviates.
- One shared unit per run.
- No solubility or reactions.
- No altitude totals.
Common mistakes
- Mixing percent with fractions off by 100x.
- Mixing kPa with mmHg totals.
- Reading partial as total.
- Forgetting humidity vapor share.
- Applying at extreme pressures.
Try it yourself
Quick checks. Answers sit tucked under each question.
Nitrogen 78% of 760 mmHg?
Partial is 0.78 times 760 is 592.8 mmHg with 167.2 mmHg rest.
4% of 50 kPa?
Partial is 0.04 times 50 is 2 kPa with 48 kPa rest.
Questions
How is partial pressure figured?
Fraction times total. Here 21.27 kPa.
What is the remainder?
Parts sum to the whole by Dalton. Here 80.03 kPa rest.
Which units work?
Pick kPa, mmHg, or atm. Math matches units throughout.
Does Dalton always hold?
Yes for ideal mixes. Real gases deviate at high pressure.
Is air exactly 21% oxygen?
No. Temperature and humidity shift real air from dry standard.
How do I get percent back?
Divide partial by total times 100 to invert.