About the Charles's Law Calculator
This Charles's law calculator finds how the volume of a gas changes with temperature when pressure is held constant, or what temperature is needed to reach a target volume. Pick the unknown — V₁, T₁, V₂ or T₂ — and enter the other three. You can type temperatures in Celsius, Kelvin or Fahrenheit; the calculator converts them to Kelvin behind the scenes, which is the step most people forget.
It is useful for chemistry students, for explaining why a balloon shrinks in the freezer or a tyre looks flat on a cold morning, and for rough checks on hot-air balloons and gas thermometers. The chart shows the straight-line relationship between volume and absolute temperature.
The law assumes an ideal gas, a closed container that can expand freely (constant pressure) and a fixed amount of gas. Temperatures at or below absolute zero (0 K, −273.15 °C) are not physically meaningful and are rejected.
With the default inputs, the answer is 2.5031. Change any value above to recalculate instantly.
How to use the charles's law calculator
- 1Choose which value to solve for.
- 2Select your temperature unit (°C, K or °F) and volume unit.
- 3Enter the three known values.
- 4Read the answer; kelvin equivalents are shown for checking your working.
Formula and method
Charles's law states that at constant pressure, the volume of a fixed amount of ideal gas is directly proportional to its absolute temperature. Plotting volume against temperature gives a straight line that extrapolates to zero volume at absolute zero, −273.15 °C.
Because the proportionality only holds on an absolute scale, the calculator first converts every temperature to kelvin (K = °C + 273.15, or K = (°F − 32) × 5/9 + 273.15), solves the ratio, and converts a temperature answer back to your chosen unit. Volumes can be in any unit as long as both states match.
- V₁, V₂
- Initial and final volume
- T₁, T₂
- Initial and final absolute temperature (K)
Worked examples
Heating a gas from 25 °C to 100 °C
Convert to kelvin: 298.15 K and 373.15 K. Then V₂ = 2 × 373.15 ÷ 298.15 ≈ 2.503 L — a 25% rise, not the fourfold increase the Celsius numbers would suggest.
Balloon in the freezer
A 5 L balloon at 22 °C (295.15 K) cooled to −18 °C (255.15 K) shrinks to 5 × 255.15 ÷ 295.15 ≈ 4.32 L.
Temperature needed to expand a gas
To grow 400 mL at 300 K to 500 mL, T₂ = 300 × 500 ÷ 400 = 375 K (about 101.85 °C).
Frequently asked questions
What is Charles's law?+
Charles's law says that at constant pressure the volume of a gas is directly proportional to its absolute temperature: V₁/T₁ = V₂/T₂. Heating a gas makes it expand; cooling makes it contract.
Why must temperature be in kelvin for Charles's law?+
The law is a direct proportion, which only works on a scale that starts at true zero. On the Celsius scale 0 °C is not zero energy, so ratios of Celsius temperatures give wrong answers. Kelvin starts at absolute zero.
What is the difference between Charles’s law and Boyle’s law?+
Charles's law relates volume and temperature at constant pressure. Boyle's law relates pressure and volume at constant temperature. The combined gas law merges both.
What are everyday examples of Charles's law?+
Hot-air balloons rise because heated air expands and becomes less dense; a basketball feels softer outdoors in winter; a sealed bag of chips puffs up in a hot car.
What happens at absolute zero?+
Extrapolating Charles’s law, an ideal gas would reach zero volume at 0 K (−273.15 °C). Real gases liquefy long before that, which is why the calculator rejects temperatures at or below absolute zero.