Combined Gas Law Calculator
Find a gas's new volume from changing P and T.
Results update as you type.
How it works
The combined gas law brings together the three simple gas laws into a single equation that relates pressure, volume, and temperature for a fixed amount of gas, and this calculator uses it to find a gas's final volume when both pressure and temperature change. You enter the initial pressure, volume, and temperature along with the new pressure and temperature, and it returns the resulting volume. The law states that pressure times volume divided by absolute temperature is constant, so as long as no gas is added or removed, this combination holds steady even as the individual quantities shift. It is the practical tool for real situations where more than one condition changes at once, such as a weather balloon rising into thinner, colder air, a scuba tank warming in the sun, or an engine cylinder compressing and heating a fuel mixture. As always with gas laws, temperatures must be in kelvin, the absolute scale measured from absolute zero, or the calculation will be wrong. Pressure and volume can be in any consistent units because the law depends on ratios. Chemistry and physics students use the combined gas law as a bridge to the full ideal gas law. Enter the five known values to find the final volume.
P₁V₁ / T₁ = P₂V₂ / T₂
The combined gas law merges Boyle's, Charles's, and Gay-Lussac's laws into one relationship, stating that the quantity pressure times volume divided by absolute temperature stays constant for a fixed amount of gas. Rearranging to solve for the final volume gives the initial pressure times volume times the final temperature, all divided by the initial temperature times the final pressure. Temperatures must be in kelvin for the relationship to hold correctly.
Source:Gas laws (combined gas law)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter initial pressure (p₁), initial volume (v₁), initial temperature (t₁), final pressure (p₂) and final temperature (t₂).
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Final volume (V₂) updates instantly as you type — no button to press.
Frequently asked questions
What is the combined gas law?
The combined gas law relates the pressure, volume, and temperature of a fixed amount of gas, stating that pressure times volume divided by absolute temperature remains constant. It merges Boyle's law, Charles's law, and Gay-Lussac's law into one equation, making it useful whenever two or more of these properties change at the same time rather than one in isolation.
When should I use it instead of the simpler gas laws?
Use the combined gas law when more than one condition changes at once. If only pressure and volume change at constant temperature, Boyle's law suffices; if only volume and temperature change at constant pressure, Charles's law works. But when pressure, volume, and temperature all vary together, the combined gas law is the correct single equation to handle the situation.
Why must temperature be in kelvin?
Gas laws require an absolute temperature scale that starts at absolute zero, which the kelvin scale provides. Celsius and Fahrenheit place zero at arbitrary points, so plugging them into the equation would produce meaningless results. Convert Celsius to kelvin by adding 273.15 before using the combined gas law, and make sure both temperatures use the same scale.
What units should pressure and volume use?
Pressure and volume can be in any units you like, such as atmospheres, kilopascals, litres, or cubic metres, as long as you use the same unit for both pressure values and the same unit for both volume values. Because the law is built on ratios, the units cancel out, so only temperature has the strict requirement of being in kelvin.
How does it lead to the ideal gas law?
The combined gas law says pressure times volume divided by temperature is constant for a fixed amount of gas. If you also account for the amount of gas in moles, that constant becomes the universal gas constant, giving the ideal gas law, pressure times volume equals the number of moles times the gas constant times temperature. The combined gas law is essentially the ideal gas law with the amount held fixed.