Combined Gas Law Calculator
The combined gas law relates the pressure, volume, and temperature of a fixed amount of gas between two states. It combines Boyle's law (pressure and volume are inversely proportional at constant temperature), Charles's law (volume is proportional to temperature at constant pressure), and Gay-Lussac's law (pressure is proportional to temperature at constant volume) into the single relationship P1V1/T1 = P2V2/T2. This calculator solves for any one of the six variables when the other five are known. Temperature must be entered in Kelvin. Pressure and volume can be any consistent units.
Combined gas law formula
P1 V1 / T1 = P2 V2 / T2
Rearrange for any unknown. For example, to find P2: P2 = P1 V1 T2 / (T1 V2). All temperatures must be in Kelvin. Pressure and volume units must be consistent between state 1 and state 2.
Special cases
- Boyle's Law: T1 = T2 (isothermal). Simplifies to P1 V1 = P2 V2.
- Charles's Law: P1 = P2 (isobaric). Simplifies to V1/T1 = V2/T2.
- Gay-Lussac's Law: V1 = V2 (isochoric). Simplifies to P1/T1 = P2/T2.
Frequently asked questions
What is the combined gas law?
The combined gas law merges Boyle's law (P and V), Charles's law (V and T), and Gay-Lussac's law (P and T) into one equation: P1V1/T1 = P2V2/T2. It applies to a fixed amount of ideal gas.
What units should I use?
Pressure can be in any consistent unit (atm, kPa, psi) as long as both P1 and P2 use the same unit. Temperature must be in Kelvin (K = degrees C + 273.15). Volume can be any consistent unit (L, mL, m^3).
What if I want to find the unknown pressure P2?
Select 'Solve for P2' and enter P1, V1, T1, V2, and T2. The calculator rearranges the equation to P2 = P1 V1 T2 / (T1 V2).
Can I keep one variable constant?
Yes. If volume is constant, enter V1 = V2. If temperature is constant, enter T1 = T2. If pressure is constant, enter P1 = P2. The equation reduces to the individual gas laws in each case.
Does this apply to real gases?
The combined gas law assumes ideal gas behavior. For real gases at high pressures or low temperatures, use the Van der Waals equation or a NIST compressibility factor instead.
Official sources
- NIST Chemistry WebBook: NIST WebBook Homepage.
- OpenStax University Chemistry: 9.3 Stoichiometry of Gaseous Substances.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.