Charles’s Law Calculator
Enter a starting volume and two Kelvin temperatures to find the final volume of a gas at constant pressure — and see how volume tracks absolute temperature.
Volume from temperature
Enter the initial volume and the two temperatures and the calculator returns the final volume V₂ using V₁/T₁ = V₂/T₂.
Use Kelvin
Temperatures must be absolute — convert °C to kelvin by adding 273.15 before you start, or the ratio will be wrong.
What is Charles's law?
Volume grows with absolute temperature
This Charles's law calculator turns three measurements — a starting volume and two absolute temperatures — into the final volume of a gas. Charles's law states that, at constant pressure, the volume of a fixed amount of gas is directly proportional to its temperature on the absolute (Kelvin) scale: heat the gas and it expands, cool it and it contracts, by exactly the same ratio as the temperatures. Discovered by Jacques Charles and later quantified by Joseph Gay-Lussac, it is one of the simple gas laws that combine into the ideal gas law, and it explains everyday effects from a hot-air balloon rising to a sealed bag puffing up in the sun.
Enter the initial volume and the initial and final temperatures in kelvin to get the final volume instantly.
Because volume divided by absolute temperature stays constant, V₁/T₁ = V₂/T₂, you rearrange to solve for the final volume directly.
V₂ = (V₁ × T₂) / T₁Take a worked example: 1 litre of gas at 273.15 K (0 °C) is heated to 373.15 K (100 °C) at constant pressure. Multiply the initial volume by the final temperature, 1 × 373.15 = 373.15, then divide by the initial temperature, 373.15 / 273.15 ≈ 1.366099 L. The gas occupies about 37 % more space simply because its absolute temperature rose by the same proportion. Notice that only the ratio of the two temperatures matters, so the final volume comes back in whatever unit you used for the initial volume.
The formula is exact for an ideal gas, but a couple of practical points are worth keeping in mind.
Absolute temperature, constant pressure, ideal gas
Both temperatures must be in kelvin — using Celsius gives a meaningless ratio, and a temperature of zero or below is rejected because absolute temperature cannot be negative. Charles's law also assumes the pressure and the amount of gas stay fixed, so it does not apply when a container is sealed and rigid or when gas escapes. Real gases follow it closely at everyday conditions but deviate near condensation or at very high pressure.