Combined Gas Law Calculator
Enter a gas's starting pressure, volume, and temperature and its new pressure and temperature to find the final volume — solved straight from P₁V₁/T₁ = P₂V₂/T₂.
One law, three variables
The combined gas law links pressure, volume, and absolute temperature, so a single calculation handles changes in all three at once.
Temperatures in kelvin
Both temperatures must be absolute (kelvin). Add 273.15 to a Celsius reading before you enter it, and keep both pressures in the same unit.
What is the combined gas law?
Pressure, volume, and temperature in one relationship
This combined gas law calculator solves P₁V₁/T₁ = P₂V₂/T₂ — the rule that, for a fixed amount of gas, the pressure times the volume divided by the absolute temperature stays constant. It merges three classic gas laws (Boyle's, Charles's, and Gay-Lussac's) into one relationship, so you can predict how a gas behaves when its pressure, volume, and temperature all change together. Enter the starting state and the new pressure and temperature, and the calculator returns the final volume the gas occupies.
Enter the initial pressure, volume, and temperature plus the new pressure and temperature to get the final volume V₂ instantly.
Rearranging P₁V₁/T₁ = P₂V₂/T₂ to isolate the final volume gives a single multiply-and-divide step.
V₂ = (P₁ × V₁ × T₂) / (T₁ × P₂)Because the law is a ratio, the pressure and volume units cancel as long as you stay consistent: use the same pressure unit for both pressures, and the final volume comes back in whatever volume unit you used for V₁. Temperature is the exception — it must always be absolute, in kelvin.
Suppose a gas starts at 100 kPa, 2 L, and 300 K, and is then compressed to 150 kPa while warming to 350 K.
Multiply the top
100 × 2 × 350 = 70,000 — initial pressure times initial volume times the new temperature.
Multiply the bottom
300 × 150 = 45,000 — initial temperature times the new pressure.
Divide
70,000 ÷ 45,000 = 1.555556 L — the final volume V₂. Higher pressure shrinks the gas, but the higher temperature pushes back, so it lands near 1.56 L.
The relationship is exact for an ideal gas, but a few conditions matter in practice.
Fixed amount of gas, absolute temperature, ideal behaviour
The combined gas law only holds when the amount of gas stays fixed — no gas added, leaked, or used up in a reaction. Temperatures must be in kelvin, since the law uses absolute temperature; a Celsius value would give the wrong ratio. It also assumes ideal-gas behaviour, which is accurate at moderate pressures and temperatures but drifts at very high pressure or near a gas's condensation point, where real-gas corrections are needed.