Ideal Gas Law Calculator

PV = nRT relates the four properties of a gas, with R the universal gas constant at 8.3145 joules per mole per kelvin. Every gas law people learn separately is a special case of it: hold temperature fixed and you have Boyle's law, hold pressure fixed and you have Charles's. There is no need to remember them as distinct rules.

The mistake that dominates all others is entering Celsius. The equation is a proportionality and kelvin has an absolute zero, so a temperature of 20°C entered as 20 rather than 293.15 gives an answer wrong by roughly a factor of fifteen — and, worse, a plausible-looking one. If a result seems absurd, the temperature is the first thing to check.

The units have to be consistent too. In SI that means pascals, cubic metres, moles and kelvin. Litres and atmospheres work with a different value of R, and mixing the two conventions silently produces nonsense — so everything here is SI, with the familiar units offered as conversions rather than inputs.

It is called the ideal gas law because real gases only approximate it. The model assumes molecules have no volume and do not attract one another, which holds well at ordinary pressures and temperatures and breaks down near condensation — at high pressure or low temperature, expect real behaviour to diverge.