Science, Tech, Math › Science Calorie Definition in Chemistry Chemistry Glossary Definition of Calorie Share Flipboard Email Print Food calories are actually kilocalories. Because this is confusing, food energy may be reported in kilojoule units. Image Source / 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 June 07, 2018 A calorie is a unit of energy, but whether or not the "c" in the word is capitalized matters. Here's what you need to know: Calorie Definition A calorie is a unit of thermal energy equal to 4.184 joules or the amount of energy required to raise the temperature of 1 gram of liquid water 1°C at standard pressure. Sometimes a calorie (written with a lowercase "c") is called a small calories or a gram calorie. The symbol for the calorie is cal. When the word Calorie is written with an uppercase "C," it refers to the large calorie, food calorie, or kilogram calorie. The Calorie is 100 calories or the amount of thermal energy needed to heat one kilogram of water one degree Celsius. Calorie History Nicolas Clément, a French chemist and physicist, first defined the calorie as a unit of heat or thermal energy in 1824. The word "calorie" comes from the Latin word calor, which means "heat." The small calorie was defined in English and French dictionaries around 1841 to 1867. Wilbur Olin Atwater introduced the large calorie in 1887. Calorie Versus Joule The calorie is based on joules, grams, and degrees Celsius, so in a way it's a metric unit, but the official unit of energy in the International System of Units (SI) is simply the joule. In the modern era, it's more common to express thermal energy in terms of joules per kelvin per gram or kilogram. These values relate to the specific heat capacity of water. While the small calorie is still used sometimes in chemistry and the large calorie is used for food, joules (J) and kilojoules (kJ) are the preferred units.