Science & engineering · Gas laws
Gay-Lussac's law calculator.
Solve the missing pressure or temperature of a gas at constant volume.
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How does Gay-Lussac's Law work?
Gay-Lussac's law, also called Amontons's law: the pressure of a gas is directly proportional to its absolute temperature, so P1 / T1 = P2 / T2. It holds for a fixed amount of gas in a rigid container (constant volume). Temperatures must be on an absolute (kelvin) scale: add 273.15 to Celsius, which this tool does for you.
Worked example. A gas at 360 kPa and 297 K is heated to 323 K in a rigid container. P2 = 360 x 323 / 297 = 391.5 kPa. The OpenStax example (9.5) reports this as 390 kPa after rounding to two significant figures.
Pressure and volume must be in the same unit on both sides of the equation, and the answer comes back in that unit. Temperatures are converted to kelvin internally, as the source requires.
Frequently asked questions
What is Gay-Lussac's law?
At constant volume, the pressure of a fixed amount of gas is proportional to its kelvin temperature: P1/T1 = P2/T2. OpenStax calls it Amontons's law.
Why does a sealed can's pressure rise when heated?
Its volume is fixed, so by this law the pressure scales with absolute temperature. Never heat a sealed container.
Can I use gauge pressure?
The law needs absolute pressure. If you have gauge pressure, add atmospheric pressure (about 101.3 kPa at sea level) first.
Which temperature units work?
Celsius, Fahrenheit or kelvin; they are converted to kelvin before the calculation.
Sources
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Formulas and the worked example are from the source above. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Education and general engineering reference only. These laws assume an ideal gas at constant amount and the held-constant quantity named on this page; real gases at high pressure or low temperature deviate. Not a substitute for a safety calculation on pressure vessels or compressed gas equipment.