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 = ρ_{substance} / ρ_{reference}

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.