Science, Tech, Math › Science The Formula For Gay-Lussac’s Law Share Flipboard Email Print buzbuzzer/Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Todd Helmenstine Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. our editorial process Todd Helmenstine Updated December 30, 2018 Gay-Lussac’s Law is a special case of the ideal gas law. This law only applies to ideal gases held at a constant volume allowing only the pressure and temperature to change. Gay-Lussac’s Law is expressed as:Pi/Ti = Pf/TfwherePi = initial pressureTi = initial absolute temperaturePf = final pressureTf = final absolute temperature It is extremely important to remember the temperatures are absolute temperatures measured in Kelvin, NOT °C or °F.