Pick a gas and enter the amount, volume and temperature to calculate the real gas pressure with the van der Waals equation, compared with an ideal gas.
Last updated: November 8, 2026
A gas law corrected for real gases
The van der Waals equation adapts the ideal gas law to real gases with two corrections: attraction between molecules (a) and the volume the molecules take up (b): • (P + a n² ÷ V²) × (V − n b) = n R T • P = n R T ÷ (V − n b) − a n² ÷ V² • R = 0.08314 L·bar/(mol·K) Example: 1 mol of CO₂ (a = 3.640, b = 0.04267) in a 1 L container at 300 K exerts about 22.4 bar. The ideal gas law gives 24.9 bar; with Z = 0.90 the real pressure is about 10% lower. The constants differ slightly between sources. At very high pressure and near the critical point the equation is also only approximate.
Common questions about the van der Waals equation
a stands for the attraction between molecules, b for the volume of the molecules themselves. A large a pulls the pressure below the ideal value.
Other tools you may find useful
In moles.
Container volume, litres.
Kelvin: °C + 273.15.
Attraction between molecules.
Volume of the molecules.