Combined gas law formula
The combined gas law unites the three simple gas laws into one relationship: P1V1/T1 = P2V2/T2. Solving for the new volume gives V2 = P1·V1·T2 ÷ (T1·P2). Temperatures must be in kelvin. When only pressure changes use the Boyle's law calculator; when only temperature changes use the Charles's law calculator.
How to use this calculator
Enter the initial state (P1, V1, T1) and the final pressure and temperature (P2, T2). Keep pressures in one consistent unit and temperatures in kelvin. The calculator solves for V2 and guards against T1 = 0 or P2 = 0, both of which would make the result undefined.
Worked example
A 2 L gas at 1 atm and 300 K is heated to 600 K while pressure stays at 1 atm. V2 = (1 × 2 × 600) ÷ (300 × 1) = 4 L. With pressure unchanged this reduces to Charles's law. Once the amount of gas changes too, the combined law is no longer enough and you need the ideal gas law calculator.
Frequently asked questions
- What is the combined gas law?
- It combines Boyle's, Charles's and Gay-Lussac's laws into one equation, P1V1/T1 = P2V2/T2, for a fixed amount of gas where pressure, volume and temperature can all change.
- What units do I use?
- Use consistent pressure and volume units, and always use kelvin for temperature. Add 273.15 to Celsius to convert.
- When should I use this instead of a simpler law?
- Use it whenever two or three of pressure, volume and temperature change at once. If only one variable changes, a simpler single gas law is enough.
- Why guard against T1 = 0 and P2 = 0?
- Both appear in the denominator. Dividing by zero is undefined, and neither absolute zero temperature nor zero pressure is physically realistic for a gas.
- When should I use the ideal gas law instead?
- Use PV = nRT when the amount of gas changes or when you need an absolute value rather than a before-and-after comparison. The combined gas law assumes a fixed number of moles, so it only relates two states of the same sealed sample.