For a fixed amount of gas, pressure times volume divided by absolute temperature stays constant. From an initial state and the final pressure and temperature, this finds the final volume. Temperatures are entered in degrees Celsius and converted to kelvin for the calculation.
For a fixed amount of gas, pressure times volume divided by absolute temperature does not change. This is the combined gas law, gathering Boyle's law and Charles's law into one statement. From an initial state and the final pressure and temperature, it gives the final volume.
Here is pressure, volume and absolute temperature, with subscript 1 for the initial state and 2 for the final one. Solved for the volume it reads:
Two separate effects are simply multiplied together: raising the pressure squeezes the gas, and raising the temperature expands it.
Ratios cannot be taken in degrees Celsius. Going from 0 °C to 10 °C does not multiply the volume without limit; it is a change from 273.15 K to 283.15 K, a factor of 1.037. The Celsius scale has no true zero, so ratios on it mean nothing. This calculator accepts Celsius and converts to kelvin internally.
A gas at a pressure of 101.3, a volume of 2 and a temperature of 27 °C, brought to a pressure of 202.6 and a temperature of 127 °C, ends with a volume of about 1.333.
The absolute temperatures are 300.15 K and 400.15 K. Doubling the pressure halves the volume, and the temperature rises by a factor of 1.333, so 2 × 0.5 × 1.333 = 1.333.
The units of pressure and volume carry straight through to the answer. Kilopascals or atmospheres will both do, provided the same unit is used before and after.
The amount of gas must not change. If gas leaks from the vessel or is added to it, this relation no longer holds.