The density of a substance measures how much mass it has in a given volume. The formula for density is mass divided by volume:

Specific gravity is a ratio of the density of a substance to the density of a reference substance, usually water. Since the density of water is one gram per cubic centimeter, you calculate specific gravity by dividing the density of a substance by one gram per cubic centimeter. Since a number divided by one is itself, the specific gravity of a substance is the density absent the units of measure.

#### TL;DR (Too Long; Didn't Read)

To find the specific gravity of a substance, divide its density by that of water.

## Find the Density

Determine the density of a substance. This may be done by dividing the mass by the volume of the substance or more directly through the use of instruments such as a hydrometer. For example, you measure the volume of a balloon as 2 liters and its weight (minus the weight of the rubber balloon) as 276 grams. This works out to 138 grams per liter or .138 grams per cc.

## Divide by Density of Water

Divide the density of the substance by the density of water. Water has a density of one gram per cubic centimeter (1 g per cm^{3}). Following the example, dividing .138 grams per cc by 1 gram per cc gives the unitless number, .138.

## Quotient is Density

The quotient is the specific gravity of the substance. In the example, .138 is the specific gravity of helium.