Science, Tech, Math › Science What Is an Experimental Constant? Explanation and Examples of Constants Share Flipboard Email Print Constants do not change during an experiment. This experiment is being conducted at a constant temperature and pressure. B2M Productions, Getty Images Science Chemistry Scientific Method Basics Chemical Laws Molecules Periodic Table Projects & Experiments 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 08, 2018 A constant is a quantity that does not change. Although you can measure a constant, you either cannot alter it during an experiment or else you choose not to change it. Contrast this with an experimental variable, which is the part of an experiment that is affected by the experiment. There are two main types of constants you may encounter in experiments: true constants and control constants. Here is an explanation of these constants, with examples. Physical Constants Physical constants are quantities which you cannot change. They may be calculated or defined. Examples: Avogadro's number, pi, the speed of light, Planck's constant Control Constants Control constants or control variables are quantities a researcher holds steady during an experiment. Even though the value or state of a control constant may not change, it is important to record the constant so the experiment may be reproduced. Examples: temperature, day/night, duration of a test, pH