Combined Gas Law Calculator
Leave one of the six pressure, volume, and temperature values blank and it solves the missing one.
Leave exactly one field blank and the calculator finds it from P₁V₁/T₁ = P₂V₂/T₂. Temperatures must be in kelvin, since the law only balances on an absolute scale. Pressure and volume can be any consistent units.
Pressure 1
1
Volume 1
2
Temperature 1
300K
Pressure 2
2
Volume 2
1
Temperature 2
300K
How it works
The combined gas law rolls Boyle's, Charles's, and Gay-Lussac's laws into a single relationship: P₁V₁/T₁ = P₂V₂/T₂. It describes a fixed amount of gas moving from one set of conditions to another. Squeeze it, heat it, or both — as long as no gas escapes, that ratio stays constant.
In practice you know five of the six values and want the sixth. A gas at 1 atm, 2 L, and 300 K that you warm and compress will settle at some new pressure or volume, and the law pins down exactly what. Leave the unknown box empty and the calculator rearranges the equation for you.
Temperature is the one that trips people up: it has to be in kelvin, not Celsius. The law relies on an absolute temperature scale, so 0 has to mean genuinely no thermal energy. If your temperatures are in Celsius, add 273.15 first, otherwise the ratios won't balance.
Frequently asked questions
Why must temperature be in kelvin?
The law divides by temperature, and that only makes physical sense on an absolute scale where zero means zero energy. Celsius has an arbitrary zero, so using it would give wrong ratios. Convert with K = °C + 273.15.
What units should pressure and volume use?
Any consistent units work, because they appear on both sides of the equation and cancel out. Just keep P₁ and P₂ in the same unit, and V₁ and V₂ in the same unit — atm with atm, liters with liters.
When does the combined gas law not apply?
It assumes a fixed amount of an ideal gas. If gas is added or removed, or the gas is near condensing into a liquid, the relationship breaks down. For those cases you'd reach for the ideal gas law with moles included.