Charles's Law Calculator
Determine the final volume of a gas after a temperature change using Charles's Law. Applies whenever pressure is constant and a gas is heated or cooled in a flexible container.
About this calculator
Charles's Law describes the direct relationship between the volume and absolute temperature of a gas held at constant pressure. It is expressed as V₁/T₁ = V₂/T₂, where V₁ and T₁ are the initial volume and temperature, and V₂ and T₂ are the final values. Rearranging for final volume gives: V₂ = (V₁ × T₂) / T₁. Temperature must always be in Kelvin for this law to work correctly, since Kelvin is an absolute scale with no negative values — Charles's Law breaks down if Celsius is used. The law predicts that gas volume increases proportionally with absolute temperature: doubling the Kelvin temperature doubles the volume. It explains why a balloon shrinks in cold air and expands when heated.
How to use
A balloon contains 3 L of air at an initial temperature of 300 K. It is moved into a warm room where the temperature is 360 K. Using the formula: V₂ = (V₁ × T₂) / T₁ = (3 × 360) / 300 = 1080 / 300 = 3.6 L. Enter 3 in 'Initial volume', 300 in 'Initial temperature', and 360 in 'Final temperature'. The calculator returns 3.6 L — the balloon expands by 0.6 L as expected when temperature increases at constant pressure.
Frequently asked questions
Why must temperature be in Kelvin when using Charles's Law?
Charles's Law is based on the absolute temperature scale because it describes a proportional relationship between volume and temperature that only holds when zero temperature means zero molecular motion. The Kelvin scale starts at absolute zero (−273.15°C), where gas volume theoretically reaches zero. If you used Celsius, the zero point would be arbitrary (the freezing point of water), and the proportional relationship would break down mathematically. Always convert Celsius to Kelvin by adding 273.15 before plugging values into Charles's Law.
What is the difference between Charles's Law and Boyle's Law?
Both are gas laws derived from the ideal gas law, but they hold different variables constant. Boyle's Law holds temperature constant and describes the inverse relationship between pressure and volume (P₁V₁ = P₂V₂). Charles's Law holds pressure constant and describes the direct relationship between volume and temperature (V₁/T₁ = V₂/T₂). In practice, Boyle's Law applies to compression processes (like a syringe or pump), while Charles's Law applies to heating or cooling processes where the gas can freely expand or contract, such as a balloon or a gas piston.
How does Charles's Law explain everyday phenomena like balloons deflating in cold weather?
When a balloon is taken outside on a cold day, the air inside cools and the gas molecules move more slowly, exerting less pressure on the balloon walls. Because the balloon is flexible and the outside pressure stays roughly constant, the balloon simply shrinks to maintain pressure equilibrium — its volume decreases in proportion to the drop in absolute temperature, exactly as Charles's Law predicts. The same principle explains why car tire pressure drops in winter (though there the volume is fixed, so it interacts with Boyle's Law too) and why hot air balloons rise when the air inside is heated to reduce density.