Specific Gravity: Understanding Density and Buoyancy

by Yuvi K - December 21, 2023

What is Specific Gravity?

Specific Gravity (SG) is the ratio of the density of a substance when compared to the density of water. It is an important quantity when studying the effects of buoyancy, as well as the comparison of the densities of different substances. It is usually expressed in terms of “per cent” or “parts per thousand” (ppt). The density of water is 1.000, so any material with a Specific Gravity greater than 1.000 will sink in water and any material with a Specific Gravity less than 1.000 will float in water.

Why is Specific Gravity Important?

Specific Gravity is an important property of a material because it gives insights into the material’s behavior and potential uses. A material with a low Specific Gravity may be easier to transport and lift, making it useful in certain applications. Conversely, a material with a high Specific Gravity may be ideal for certain industrial processes that require a material with a heavy mass and high density.

Additionally, specific gravity is also important when studying the effect of buoyancy. A high Specific Gravity will make an object heavy and sink in the water, while a low Specific Gravity will make it lighter and float in the water. This is important for boats, which rely on the positive buoyant force to keep them afloat.

How to Calculate Specific Gravity

Specific gravity is calculated by dividing the density of the material by the density of water. This will give you a number that tells you how much heavier or lighter your material is than the same volume of water. Here is the formula used to calculate Specific Gravity:

SG = Density of Material/Density of Water

The Density of Water

The density of water is the amount of mass per unit volume of water. It is often expressed in g/cm3, kg/m3 or lb/ft3. The density of water varies slightly depending on the temperature, but it is usually considered to have a density of 1.00 g/cm3, 1000 kg/m3 or 62.43 lb/ft3.

Determining the Density of a Material

In order to calculate the Specific Gravity of a material, you must first determine its density. This can be done in several ways. The most common way is to use a scale or weigh boat to measure the mass of a given volume of the material. Once you have the mass, you can then calculate the density using the following formula:

Density = Mass/Volume

Examples of Specific Gravity of Common Substances

The following table contains the Specific Gravity of several common substances. As you can see, some substances such as Oak and Aluminum have a high Specific Gravity, meaning they are heavier than water and will sink in it. Others like Styrofoam have a low Specific Gravity, meaning they are lighter than water and will float in it.

Conclusion

Specific Gravity is an important property of a material because it can help us understand how it will behave in certain situations. It is calculated by dividing the density of the material by the density of water, and the density of water is usually considered to be 1.00 g/cm3, 1000 kg/m3 or 62.43 lb/ft3. Knowing the Specific Gravity of a material can help us determine where it will sink or float, as well as its potential uses.