Science, Tech, Math › Science The Formula for the Combined Gas Law Share Flipboard Email Print Yagi Studio / 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 Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated August 15, 2019 The combined gas law ties together Boyle's law, Charles' law, and Gay-Lussac's law. Basically, it states that as long as the amount of gas doesn't change, the ratio between the pressure-volume and temperature of a system is a constant. There is no "discoverer" of the law as it simply puts together concepts from other cases of the ideal gas law. The Combined Gas Law Formula The combined gas law examines the behavior of a constant amount of gas when pressure, volume and/or temperature is allowed to change. The simplest mathematical formula for the combined gas law is: k = PV/T In words, the product of pressure multiplied by volume and divided by temperature is a constant. However, the law is usually used to compare before/after conditions. The combined gas law is expressed as: PiVi/Ti = PfVf/Tf where: Pi = initial pressureVi = initial volumeTi = initial absolute temperaturePf = final pressureVf = final volumeTf = final absolute temperature It is extremely important to remember that the temperatures are absolute temperatures measured in Kelvin, NOT °C or °F. It is also important to keep your units constant. Do not use pounds per square inch for pressures initially to find Pascals in the final solution. Uses of the Combined Gas Law The combined gas law has practical applications in situations where pressure, volume, or temperature can change. It is used in engineering, thermodynamics, fluid mechanics, and meteorology. For example, it can be used to predict cloud formation and the behavior of refrigerants in air conditioners and refrigerators.