Charles' Law Calculator
Charles' Law describes the direct proportionality between the volume of a gas and its absolute temperature at constant pressure. When you heat a gas, it expands; when you cool it, it contracts. This relationship was first observed by Jacques Charles in 1787 while experimenting with hydrogen-filled balloons. The law is a cornerstone of gas behavior and is applied in balloon flight, industrial gas processes, meteorology, and teaching chemistry. Enter three of the four values (V1, T1, V2, T2) to solve for the fourth. Temperatures can be entered in Celsius or Kelvin. The calculator automatically converts and displays results in both units.
Charles' Law formula
V1 / T1 = V2 / T2 (T must be in Kelvin)
V2 = V1 * T2 / T1
T2 = T1 * V2 / V1
T(K) = T(degC) + 273.15
Charles' Law examples
- At 0 degC (273 K), a gas occupies 1.00 L. At 100 degC (373 K), it occupies 1.37 L.
- A hot air balloon heated from 20 degC to 100 degC expands its air by about 27%.
- Weather balloons expand as they rise because temperature and pressure both drop, but pressure effect dominates.
- Liquid nitrogen (-196 degC / 77 K) causes a balloon to shrink to about 27% of its room-temperature volume.
Charles' Law: frequently asked questions
What is Charles' Law?
Charles' Law states that at constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature (in Kelvin): V1/T1 = V2/T2. Doubling the absolute temperature doubles the volume. Discovered by Jacques Charles in 1787 and later confirmed by Gay-Lussac.
Why must temperature be in Kelvin?
Charles' Law requires absolute temperature because volume is proportional to the actual kinetic energy of gas molecules. Zero Kelvin (absolute zero, -273.15 degC) is the theoretical temperature at which gas volume would be zero. Using Celsius would give incorrect results because 0 degC is not zero kinetic energy.
What happens when a gas is cooled at constant pressure?
When a gas cools at constant pressure, the molecules move more slowly and the gas contracts. Its volume decreases proportionally with the decrease in absolute temperature. Cooling from 300 K to 150 K (halving absolute temperature) would halve the volume at constant pressure.
What are common applications of Charles' Law?
Hot air balloons: heating air increases volume and decreases density, producing lift. Bread rising: yeast CO2 expands as the oven heats. Tire pressure warming: tires expand as temperature rises (this is more accurately Gay-Lussac's Law at constant volume). Weather balloon expansion at altitude.
How do I convert between Celsius and Kelvin?
K = degC + 273.15. Always convert to Kelvin before using Charles' Law. For example, 25 degC = 298.15 K. The calculator accepts inputs in either Celsius (converted to Kelvin internally) or Kelvin directly.
Official sources
- NIST: NIST WebBook - Thermophysical Properties of Fluid Systems.
- NOAA: NOAA Atmospheric Science Education.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.