Relative Density Definition

What Is Relative Density?

Relative density compares the density of a material to that of pure water.
Relative density compares the density of a material to that of pure water. Dorling Kindersley: Dave King / Getty Images

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.

Relative Density 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.

Calculating Relative Density

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.

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Helmenstine, Anne Marie, Ph.D. "Relative Density Definition." ThoughtCo, Nov. 12, 2017, thoughtco.com/definition-of-relative-density-605608. Helmenstine, Anne Marie, Ph.D. (2017, November 12). Relative Density Definition. Retrieved from https://www.thoughtco.com/definition-of-relative-density-605608 Helmenstine, Anne Marie, Ph.D. "Relative Density Definition." ThoughtCo. https://www.thoughtco.com/definition-of-relative-density-605608 (accessed November 24, 2017).