The combined gas law stitches together the three simple gas laws into a single equation for a fixed amount of gas: P₁V₁/T₁ = P₂V₂/T₂. It says the group PV/T is the same before and after any change, so you can move a gas between two completely different states of pressure, volume and temperature and still tie them together. Hold one variable fixed and it collapses back to Boyle's (constant T), Charles's (constant P) or Gay-Lussac's (constant V) law — this is the master version that handles all three changing at once.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the combined gas law derived from PV = nRT.
The combined gas law equations
Because the amount of gas and the gas constant are fixed, the quantity PV/T is conserved between the two states. To solve any one variable, write both PV/T groups, set them equal, and rearrange. Pressure and volume units cancel in their pairs, so you only need consistency within each pair — but temperature, appearing alone in the denominator, must be the absolute kelvin value.
Worked example — two changes at once
Scenario: A gas occupies 2 L at 1 atm and 300 K. It is compressed and heated to 2 atm and 600 K. What is its new volume?
The doubling of pressure (which would halve the volume) is exactly cancelled by the doubling of absolute temperature (which would double it), so the volume lands back at 2 L. Heat the gas to 600 K without compressing and it expands to 4 L; compress to 2 atm without heating and it shrinks to 1 L. The combined law captures both effects in one step, and the constant PV/T = 0.00667 confirms the answer.
Frequently Asked Questions
P₁V₁/T₁ = P₂V₂/T₂ for a fixed amount of gas. The group PV/T stays constant between two states.
Rearrange for the unknown, e.g. V₂ = P₁V₁T₂/(T₁P₂). 1×2×600/(300×2) = 2 L.
It uses absolute temperature. Add 273.15 to °C. Celsius gives wrong or negative answers.
Combined compares two states of a fixed gas; ideal (PV=nRT) gives absolute values and handles changing moles.
The amount of gas (moles) and R. Pressure, volume and temperature may all change.