Charles's law formula
At constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature: V1/T1 = V2/T2. Solving for the new volume gives V2 = V1·T2 ÷ T1. Temperatures must be in kelvin, not Celsius, because the ratio only works on an absolute scale. To also change pressure, switch to the combined gas law calculator.
The ratio V/T is therefore constant for the sample, and the calculator shows it in the badge.
How to use this calculator
Enter the initial volume V1 and both temperatures in kelvin. To convert from Celsius, add 273.15. The calculator divides V1 by T1, multiplies by T2 and returns V2 in the same volume unit. It guards against T1 = 0 K, which would make the ratio undefined.
The single biggest error in Charles's law problems is leaving temperatures in Celsius. Heating a gas from 27 °C to 327 °C looks like a twelvefold rise, but in kelvin it is 300 K to 600 K — only a doubling. Convert first, then compare. Fix the volume instead and the pressure rises with temperature, which is Gay-Lussac’s law.
Worked example
A 2 L balloon at 300 K is heated to 600 K at constant pressure. V2 = (2 × 600) ÷ 300 = 4 L. Doubling the absolute temperature doubles the volume.
Read the result as a proportion, not an addition: the gas gained 2 L because the absolute temperature ratio was 2. The relationship also breaks down near condensation, where the gas turns to liquid. To bring the amount of gas into the picture as well, use the ideal gas law calculator.
Frequently asked questions
- What is Charles's law?
- Charles's law states that at constant pressure the volume of a fixed amount of gas is directly proportional to its absolute temperature.
- Why must temperature be in kelvin?
- The direct proportion only holds on an absolute scale. Using Celsius gives wrong ratios, so convert by adding 273.15 to Celsius first.
- What stays constant in Charles's law?
- Pressure and the amount of gas stay constant. Only volume and temperature change together.
- What if T1 is zero?
- Absolute zero would make the formula divide by zero, so the calculator requires T1 greater than 0 K.
- Does the volume unit matter?
- No — V2 comes back in whatever unit you used for V1, because the formula multiplies V1 by a plain temperature ratio. Litres in, litres out; cubic metres in, cubic metres out. Only the temperatures have a mandatory unit, and that unit is kelvin.