Science & engineering · Gas laws
Combined gas law calculator.
Solve any one missing pressure, volume or temperature of a fixed amount of gas.
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Result
Leave exactly one box empty.
How does Combined Gas Law work?
The combined gas law merges Boyle's, Charles's and Gay-Lussac's laws: P1 x V1 / T1 = P2 x V2 / T2. It holds for a fixed amount of gas, with temperature on the kelvin scale. Temperatures must be on an absolute (kelvin) scale: add 273.15 to Celsius, which this tool does for you.
Worked example. 13.2 L of gas at 153 atm and 300 K is released to 3.13 atm at 310 K. V2 = 13.2 x 153 x 310 / (300 x 3.13) = 667 L (OpenStax Example 9.10).
Pressure and volume must be in the same unit on both sides of the equation, and the answer comes back in that unit. Temperatures are converted to kelvin internally, as the source requires.
Frequently asked questions
What is the combined gas law?
P1V1/T1 = P2V2/T2 for a fixed amount of gas. It reduces to Boyle's law when T is constant, Charles's law when P is constant and Gay-Lussac's law when V is constant.
When should I use the combined gas law instead of the ideal gas law?
Use it when you know an initial state and want a final state and the amount of gas does not change. The ideal gas law PV = nRT is for a single state and needs the moles.
Do pressure units matter?
Use one pressure unit for both states and one volume unit for both states. Only temperature needs to be absolute, which the tool handles.
Which box do I leave empty?
Exactly one of the six. The calculator solves for it and reports temperature in the unit you chose.
Sources
Checked
Formulas and the worked example are from the source above. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Education and general engineering reference only. These laws assume an ideal gas at constant amount and the held-constant quantity named on this page; real gases at high pressure or low temperature deviate. Not a substitute for a safety calculation on pressure vessels or compressed gas equipment.