Charles's Law describes how a fixed amount of gas behaves when you change its temperature at constant pressure: the volume is directly proportional to the absolute temperature, so V₁/T₁ = V₂/T₂. Heat the gas and it expands; cool it and it contracts — in exact proportion to the temperature measured in kelvin. The one rule that trips people up is that temperature must be absolute: a gas at 600 K really does occupy twice the volume it has at 300 K, but 600 °C and 300 °C are not in a 2:1 ratio at all.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: Charles's Law as the isobaric case of PV = nRT.
The Charles's Law equations
The ratio of volume to absolute temperature is a constant for the gas while pressure and amount stay fixed. To find any one quantity, set the two ratios equal and cross-multiply. Volume units cancel, so litres or millilitres both work, but the temperatures must be in kelvin — the proportionality is to absolute temperature, measured from absolute zero, not from the freezing point of water.
Worked example — heating a gas
Scenario: A 2-litre sample of gas at 300 K (27 °C) is heated to 600 K at constant pressure. What is the new volume?
Doubling the absolute temperature doubles the volume to 4 litres — the ratio V/T stays at 0.00667 L/K throughout. Heat the same gas to only 450 K and it would expand to 3 litres (a factor of 1.5, matching 450/300). Note that if you had wrongly used Celsius — 27 °C to 327 °C — you would get a completely different and incorrect answer, which is why the kelvin scale is essential here.
Frequently Asked Questions
At constant pressure, gas volume is directly proportional to absolute temperature: V₁/T₁ = V₂/T₂. Double the kelvin, double the volume.
V₂ = V₁·T₂/T₁. e.g. 2 L × 600 K ÷ 300 K = 4 L. New temp is T₂ = T₁·V₂/V₁.
The law uses absolute temperature from absolute zero. Add 273.15 to °C. Celsius gives wrong, even negative, volumes.
It expands in proportion to kelvin. 300 K → 330 K (+10%) grows volume by 10%. Basis of hot-air balloons.
Yes — it's the isobaric case. If P changes too, use P₁V₁/T₁ = P₂V₂/T₂ or PV = nRT.