Density and specific gravity 16-02

We often describe a fluid by its “lightness or heaviness”. In solids we think of materials like cork or balsa wood as being very “light” and lead and gold as being “heavy”. In fluids we know that gasoline is lighter than water because it floats on water. The fact is that nothing is heavy or light by its self. It is only heavy or light compared to something else.

To calculate this “heaviness” or density we divide the mass of the material in pounds by its volume in cubic feet, or the mass in grams by its volume in cubic centimeters. This exercise generates a number that we can use to compare the densities of various fluids

The densities of ordinary substances vary from the least dense, hydrogen gas, with a density of 0.000089 grams per cubic centimeter to the element osmium which, at 22 grams per cubic centimeter, is only slightly more dense than gold or platinum. Density is such a characteristic property of each substance that it may even be used to identify the substance.

I once visited a gold mine in South Africa, and at the end of the tour we were shown a cubic foot block of pure gold (12″ x 12″ x 12″). The guide told us that any one of us could take the block home if we could carry it out the door. All of us were willing to give it a try, but quickly gave up when we were told it weighed 1200 pounds.

Since the gram is defined as the mass of a cubic centimeter of water, water must by definition have a density of 1 gram per cubic centimeter. It actually varies a little with changes in temperature, but its density is exactly 1 somewhere around 39 degrees Fahrenheit or 4 degrees Celsius

Specific gravity is also a measurement of heaviness. It is the term most commonly used in English speaking countries. To determine a specific gravity number you compare the density of a substance to that of an equal volume of fresh water. It is expressed without units. In the metric system, specific gravity is the same as in the English system. If something is 7.85 times as heavy as an equal volume of water (such as iron is) its specific gravity is 7.85. Its density is 7.85 grams per cubic centimeter, or 7.85 kilograms per liter, or 7.85 metric tons per cubic meter.

Gases are commonly compared to dry air, which has a density of 1.29 g per liter under so-called standard conditions (0° C and 1 atmosphere pressure). For example, liquid mercury has a density of 13.6 kg per liter. Its specific gravity is 13.6.

The gas carbon dioxide, which has a density of 1.976 g per liter under standard conditions, has a specific gravity of 1.53.

Because it is the ratio of two quantities that have the same dimensions (mass per unit volume). Specific gravity has no dimension.

In the pump business we assume the pump to be pumping cold fresh water so specific gravity is a useful term, but of the two methods of describing “heaviness”, density makes the most sense. When the US finally converts to the metric system, the term “specific gravity” should disappear.



  • On February 18, 2018