Specific Gravity
Density relative to water.
Formula
Variables
How it works
Specific gravity compares how dense a substance is to water by taking a ratio of their densities. Because it is a ratio of the same kind of quantity, the units cancel, so specific gravity has no units. A value greater than 1 means the substance is denser than water and will tend to sink in it; a value less than 1 means it is less dense than water and may float.
When to use it
Use this when you want to quickly judge whether a material is heavier or lighter than water without worrying about units. It is common in fluid mechanics, materials science, and chemistry when identifying liquids, checking buoyancy, or comparing unknown samples.
Worked example, step by step
A liquid has density 850 kg/m³. Use SG = ρ / ρ_water SG = 850 / 1000 SG = 0.85 So the specific gravity of the liquid is 0.85, meaning it is less dense than water.
Common mistakes to avoid
- Forgetting that specific gravity is unitless and writing kg/m³ as the final answer
- Using the wrong reference density instead of water's density of 1000 kg/m³
- Reversing the ratio and calculating ρ_water / ρ instead of ρ / ρ_water
Did You Know?
Ice floats because its specific gravity is below 1, a rare property that lets fish survive under frozen lakes.
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