Charles's law says that heat a gas at constant pressure and it expands — proportionally, and predictably. Formalized by Jacques Charles in the 1780s from observations that all gases expand by roughly the same fraction per degree of heating, it is one of the three simple gas laws (alongside Boyle's and Gay-Lussac's) that combine into the ideal gas law. This article covers where the law comes from, why Kelvin is non-negotiable, and where the direct-proportion model breaks down.
The V/T = constant relationship
Charles's law states that for a fixed amount of gas at constant pressure, volume is directly proportional to absolute temperature: V/T = constant, or equivalently V₁/T₁ = V₂/T₂ when comparing two states. Physically, temperature is a measure of the average kinetic energy of gas molecules — heat the gas and its molecules move faster and collide with the container walls harder and more often. At constant pressure, the only way to keep the outward force on the walls balanced is for the container (or the gas boundary, like a piston or balloon skin) to expand, giving the molecules more room and lowering the collision frequency back to the original pressure. Cool the gas and the reverse happens: the volume shrinks. This is why a helium balloon left outside on a cold day visibly deflates even though no gas has leaked out — the volume has simply followed the temperature down.
Why the temperature must be in kelvin
Charles's law is a statement about direct proportion from a true zero, and only the Kelvin scale has a true zero — the point at which molecular motion (and, in the idealized limit, gas volume) theoretically stops. Celsius and Fahrenheit are both offset scales with arbitrary zero points, so a proportional relationship that holds in kelvin does not hold in either of them. Converting is one line: K = °C + 273.15, or K = (°F − 32) × 5/9 + 273.15. The calculator above does this conversion automatically and flags it whenever you enter a Celsius or Fahrenheit value, but doing it by hand is the single step most students skip — and skipping it produces volumes that are wrong by a wide margin, sometimes even negative.
How Charles's law fits with Boyle's and the combined gas law
Charles's law is one of three simple gas laws, each of which holds one variable fixed. Boyle's law (PV = constant) holds temperature fixed and relates pressure and volume. Charles's law (V/T = constant) holds pressure fixed and relates volume and temperature. Gay-Lussac's law (P/T = constant) holds volume fixed and relates pressure and temperature. When pressure is not actually constant — for example, a gas cylinder heated in a fixed-volume tank, or a weather balloon rising through changing atmospheric pressure — none of the three simple laws applies cleanly on its own, and you need the combined gas law, P₁V₁/T₁ = P₂V₂/T₂, or the full ideal gas law, PV = nRT, which folds all three simple laws together with Avogadro's law.
Where the simple model breaks down
Charles's law describes an ideal gas — molecules with negligible size and no attraction to each other. Real gases deviate from this, especially at low temperatures approaching condensation and at high pressures where the molecules' own volume stops being negligible. It also assumes the amount of gas and the pressure genuinely stay constant: a rigid sealed container cannot expand, so heating gas inside one raises pressure instead of volume (that is Gay-Lussac's law, not Charles's), and a real balloon's rubber tension adds a small pressure contribution of its own that a strict constant-pressure model ignores. For everyday temperature ranges and moderate pressures — the range most classroom and workshop problems live in — Charles's law's direct proportion is accurate enough to use without correction.