🌡️ Charles's Law Calculator
V₁/T₁ = V₂/T₂ at constant pressure. Temperatures in Kelvin.
How to Use This Calculator
Select the variable you want to calculate, enter the three known values, and click Calculate. Charles's Law applies at constant pressure and constant amount of gas. Temperature must be in Kelvin for the formula to work correctly.
Choose your unknown: V₂ (final volume), T₂ (final temperature), V₁ (initial volume), or T₁ (initial temperature).
Convert all temperatures to Kelvin before typing them in. Add 273.15 to any Celsius value. 20°C = 293.15 K, 100°C = 373.15 K.
Enter the two volume values in any consistent unit (litres, mL, or m³) and the one known temperature in Kelvin.
Click Calculate. For example, a gas at 5 L and 273 K heated to 546 K gives V₂ = 5 × 546 / 273 = 10 L, exactly double the original.
Charles's Law Formula
Charles's Law says that at constant pressure, volume is directly proportional to absolute temperature. Doubling the Kelvin temperature doubles the volume. Halving the Kelvin temperature halves the volume. This happens because hotter gas molecules move faster and collide with the container walls more forcefully, pushing the walls outward if pressure is kept constant. V₁ and T₁ are the starting conditions, V₂ and T₂ are the final conditions.
Worked Examples
Where This Calculation Comes Up
Charles's Law appears in any problem involving a gas that is heated or cooled at constant pressure. In the lab, you might collect a gas at room temperature and need to correct the volume to 0°C for comparison with a theoretical prediction at STP. You do that by applying V₂ = V₁ × T₂ / T₁ with T₂ = 273.15 K. This kind of temperature correction is standard in gas collection experiments.
Hot air balloons are the most visual demonstration of Charles's Law. Heating the air inside the balloon from about 20°C (293 K) to 120°C (393 K) expands the volume by a factor of 393/293, roughly 1.34 times. Since the expanded hot air is less dense than the cool air outside, the balloon rises. Weather balloons carrying instruments up to the stratosphere expand continuously as they ascend and surrounding pressure drops, and the combined gas law applies there. But for understanding the temperature-volume relationship alone, Charles's Law is the starting point.
Frequently Asked Questions
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?
The law requires absolute temperature. At 0 K, theoretical volume is zero. Celsius values don't have a true zero.
What happens to gas volume when heated?
Volume increases proportionally with absolute temperature. Doubling T (in K) doubles V.
What is absolute zero?
0 K = −273.15°C. At absolute zero, gas theoretically occupies no volume (ideal gas extrapolation).
How is Charles's Law used?
Used in hot air balloons, cooking, weather balloons, and calculating gas volumes at different temperatures.