What is Specific Gravity?
Specific gravity (SG), also called relative density, is a dimensionless number that compares the density of a substance to the density of a reference — usually pure water at 4°C, which has a density of 1000 kg/m³. Because it is a ratio, specific gravity has no units. A value greater than 1 means the substance is denser than water and will sink; a value less than 1 means it is lighter and will float.
How to Use This Calculator
Enter the density of your substance in kilograms per cubic metre. The reference water density defaults to 1000 kg/m³, but you can change it if you need a different reference temperature or fluid. The calculator divides the substance density by the reference density and instantly returns the specific gravity along with a sink/float verdict.
The Formula Explained
The equation is simply $$\text{SG} = \frac{\rho_{\text{substance}}}{\rho_{\text{water}}}$$ Since both densities are in the same units, the units cancel out, leaving a pure ratio. Engineers, brewers, geologists and chemists use this value to identify materials, check fluid concentrations, and predict buoyancy.
Worked Example
Seawater has a density of about 1025 kg/m³. Dividing 1025 by 1000 gives a specific gravity of 1.025. $$\text{SG} = \frac{1025}{1000} = 1.025$$ Since this is greater than 1, an object that floats in fresh water will float slightly higher in seawater because seawater is denser.
FAQ
Is specific gravity the same as density? No. Density has units (e.g. kg/m³), while specific gravity is a dimensionless ratio of two densities.
Why use 1000 kg/m³ for water? That is the density of pure water at 4°C, its point of maximum density, making it the standard reference.
What does \(\text{SG} = 1\) mean? The substance has exactly the same density as water and will neither sink nor float — it stays suspended.
