Science, Tech, Math › Science Relative Density Definition Share Flipboard Email Print Dorling Kindersley: Dave King / 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 March 18, 2019 Relative density (RD) is the ratio of the density of a substance to the density of water. It is also known as specific gravity (SG). Because it is a ratio, relative density or specific gravity is a unitless value. If its value is less than 1, then the substance is less dense than water and would float. If relative density is exactly 1, the density is the same as water. If RD is greater than 1, the density is greater than that of water and the substance would sink. Examples The relative density of pure water at 4 C is 1.The relative density of balsa wood is 0.2. Balsa is lighter than water and floats on it.The relative density of iron is 7.87. Iron is heavier than water and sinks. Calculation When determining relative density, the temperature and pressure of the sample and reference should be specified. Usually the pressure is 1 am or 101.325 Pa. The basic formula for RD or SG is: RD = ρsubstance / ρreference If a difference reference is not identified, it may be assumed to be water at 4 °C. Instruments used to measure relative density include hydrometers and pycnometers. In addition, digital density meters may be used, based on a variety of principles.