Ideal Gas Law Calculator

The ideal gas law (PV=nRT) expresses the relationship between pressure, volume, moles, and temperature of a gas — the foundation of chemistry and physics problems involving gases.

Enter the moles (mol), volume (L), and temperature (K) to calculate the pressure (atm) using the gas constant R=0.0821 L·atm/mol·K.

For example, if 1 mol of gas occupies 22.4 L at standard conditions (0°C, 273.15 K), the pressure is about 1 atm.

Ideal Gas Law Calculator Calculation Method

P: pressure (atm), n: moles (mol), R: gas constant (0.0821), T: temperature (K), V: volume (L)

The pressure (atm) is 0 .

About This Calculator

The ideal gas law relates pressure, volume, temperature and amount for a gas: PV = nRT. It is an excellent approximation at ordinary temperatures and pressures and the starting point for most gas calculations.

Enter the moles, volume and temperature to get the pressure (or rearrange for any missing variable), with temperature in kelvin.

The ideal gas law merges Boyle, Charles and Avogadro into a single equation of state. One mole of an ideal gas occupies 22.41 L at 0 °C and 1 atm, and 24.47 L at 25 °C — the molar volume used in most modern laboratory work. Real gases depart from the prediction near condensation, at high pressure or at low temperature.

Key Formula:P × V = n × R × T (R = 8.314 J/(mol·K))

Frequently Asked Questions

What is the ideal gas constant R?

R is 8.314 J/(mol·K), or 0.08206 L·atm/(mol·K) if you work in litres and atmospheres. Choose the value whose units match the rest of your problem.

Why must temperature be in kelvin?

Kelvin starts at absolute zero, so 0 K means zero molecular motion. Celsius values would give negative results and nonsense ratios — room temperature is 293.15 K, not 20.

When does the ideal gas law break down?

At high pressure or low temperature, when molecules are close together and intermolecular forces matter. Real gases are then modelled with the van der Waals equation instead.

Which value of R should I use?

Match R to the units of pressure and volume: 0.08206 L·atm/(mol·K) with litres and atmospheres, 8.314 J/(mol·K) with pascals and cubic metres, or 62.36 L·mmHg/(mol·K) with millimetres of mercury.

Why does doubling the Celsius temperature not double the volume?

The law uses absolute temperature in kelvin. Going from 300 K to 600 K doubles the volume at constant pressure, but going from 20 °C to 40 °C is only 293 K to 313 K and raises the volume by about 7 %.

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Sources & last update

2026-10-19