Charles's Law Calculator (V₁/T₁ = V₂/T₂)
By Santaji GadepatilinUpdated October 27, 2026
At constant pressure, a gas expands in proportion to its absolute temperature. Charles's law explains why hot-air balloons rise and why tyres lose pressure in cold weather.
How to use the Charles’s law calculator
Enter any three of the initial and final volume and temperature. The fourth is calculated. Pressure and amount of gas stay constant.
Charles’s law formula
V₁ ÷ T₁ = V₂ ÷ T₂ (T in kelvin)
Worked example
A 2 L balloon is warmed from 20 °C (293.15 K) to 80 °C (353.15 K):
V₂ = 2 × 353.15 ÷ 293.15 = 2.41 L
Volume–temperature table
Starting at 2 L and 25 °C, at constant pressure:
| Temperature | Volume |
|---|---|
| -50 °C | 1.497 L |
| -20 °C | 1.698 L |
| 0 °C | 1.832 L |
| 25 °C | 2.000 L |
| 50 °C | 2.168 L |
| 100 °C | 2.503 L |
| 200 °C | 3.174 L |
Absolute zero
Extend the volume–temperature line backwards and it reaches zero volume at −273.15 °C (0 K). This was one of the first clues to the existence of absolute zero.
Everyday examples
- Hot-air balloons: heating the air inside expands it, lowering its density so the balloon rises.
- Balloons in the cold: a party balloon shrinks when taken outside on a cold day.
- Tyres: on a hot day tyre pressure rises (pressure and temperature together — see Gay-Lussac’s law).
- Baking: gas bubbles in dough expand in the oven, helping bread rise.
Gay-Lussac’s law
At constant volume, pressure is proportional to absolute temperature: P₁ ÷ T₁ = P₂ ÷ T₂. A sealed can at 1 atm and 20 °C reaches about 1.27 atm at 100 °C.
A common mistake
Using °C directly would suggest the volume quadruples (80 ÷ 20) — completely wrong. Always use absolute temperature.
Frequently asked questions
What is Charles's law?
At constant pressure, a gas's volume is proportional to its absolute temperature — V₁/T₁ = V₂/T₂.
Why must temperatures be in kelvin?
The law is based on absolute zero. The calculator converts °C and °F to kelvin for you.
Where is it seen in daily life?
Hot-air balloons rising, balloons shrinking in a freezer, and tyre pressure warnings in cold weather.
What is Charles’s law?
At constant pressure, the volume of a gas is directly proportional to its absolute temperature: V₁ ÷ T₁ = V₂ ÷ T₂.
Why must temperature be in kelvin?
Because volume is proportional to absolute temperature. Using °C gives wrong results, and fails completely below 0 °C.