What is a Ideal Gas Calculator?
This calculator applies PV = nRT and the Boyle's and Charles's laws to compute the pressure, volume, temperature, or moles of a gas.
How it's calculated
PV = nRT (R = 8.314 J/mol·K)
Boyle's Law: P₁V₁ = P₂V₂ (constant temperature)
Charles's Law: V₁/T₁ = V₂/T₂ (constant pressure)
Temperature must always be entered in Kelvin (K).
Variable definitions
| Variable | Meaning | Unit |
|---|---|---|
| P | Pressure | Pa |
| V | Volume | m³ |
| n | Moles | mol |
| R | Gas constant | 8.314 J/mol·K |
| T | Absolute temperature | K |
Assumptions
This assumes the gas is ideal (negligible molecular volume, no intermolecular forces). Error stays small at normal temperature and pressure, but grows significantly at high pressure or low temperature, where real-gas corrections like the van der Waals equation become necessary.
Worked example
What volume does 2 moles of gas occupy at 25°C (298K), 1 atm (101,325 Pa)?
- V = nRT÷P = (2×8.314×298)÷101,325
- V ≈ 0.0489 m³ (48.9 L)
Why using Celsius gives wrong answers
Going from 20°C to 40°C looks like a doubling, but it is actually only a 7% increase — 293K to 313K. Gas laws describe the kinetic energy of molecules, and that energy scale starts at true zero (absolute zero), which only Kelvin represents correctly. That is why the proportional relationships in gas laws only hold when using Kelvin.
Good to know
- At standard conditions (0°C, 1 atm), one mole of any gas occupies 22.4 liters — regardless of what gas it is, because gas molecules themselves take up almost none of that volume.
- That is possible because a gas is mostly empty space — the molecules themselves account for roughly 0.1% of the total volume.
- Ideal gases do not actually exist — the model assumes zero molecular volume and no intermolecular forces, but real gases behave close enough to this at normal temperature and pressure to make the model useful.
- At high pressure or low temperature, real gases deviate significantly, which is why the van der Waals equation exists as a correction.
- Winter tire pressure loss is Charles's Law in action — at constant volume, lower temperature means lower pressure. A 10°C drop typically costs about 0.1 bar.
Frequently asked questions
Q. Can I enter temperature in Celsius?
No. It must be converted to Kelvin (°C + 273.15) first.
Q. How many liters is 1 mole of gas?
22.4 liters at standard conditions (0°C, 1 atm). At 25°C, 1 atm, it is about 24.5 liters.
Q. When does the ideal gas law break down?
At high pressure or low temperature, error increases significantly, since molecular volume and intermolecular forces can no longer be ignored.