Gay-Lussac's Law — the pressure law — describes a gas trapped in a rigid container: at constant volume the pressure is directly proportional to the absolute temperature, so P₁/T₁ = P₂/T₂. Heat the gas and the molecules strike the walls harder and more often, driving the pressure up in proportion to the kelvin temperature; cool it and the pressure falls. It is why sealed cans and pressure vessels become dangerous when heated, and why tyre pressure rises on a hot day.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: Gay-Lussac's Law as the constant-volume case of PV = nRT.
The Gay-Lussac's Law equations
The ratio of pressure to absolute temperature is constant for the gas while volume and amount stay fixed. To find a missing quantity, set the two ratios equal and cross-multiply. The pressure unit cancels — atm, kPa, bar or psi all work — but the temperatures must be in kelvin, since pressure is proportional to absolute temperature, measured from absolute zero.
Worked example — heating a sealed tank
Scenario: A rigid tank holds gas at 1 atm and 300 K (27 °C). It is heated to 600 K. What is the new pressure?
Doubling the absolute temperature doubles the pressure to 2 atm — the ratio P/T holds at 0.00333 atm/K throughout. Heat the tank to only 450 K and the pressure reaches 1.5 atm (a factor of 1.5, matching 450/300). This is exactly the hazard behind aerosol-can warnings: a sealed container's pressure climbs in lockstep with absolute temperature, and a strong enough heat source can push it past the bursting point.
Frequently Asked Questions
At constant volume, gas pressure is directly proportional to absolute temperature: P₁/T₁ = P₂/T₂. Double the kelvin, double the pressure.
P₂ = P₁·T₂/T₁. e.g. 1 atm × 600 K ÷ 300 K = 2 atm. New temp is T₂ = T₁·P₂/P₁.
It uses absolute temperature from absolute zero. Add 273.15 to °C; Celsius gives wrong, even negative, pressures.
A sealed can is constant volume — heating doubles the pressure as kelvin doubles, and it can burst.
Charles holds pressure constant (V vs T); Gay-Lussac holds volume constant (P vs T). Both are proportional to kelvin.