Table of Contents
What Is Specific Gravity?
Specific gravity (SG) is the dimensionless ratio of a substance's density to the density of a reference substance. For solids and liquids, the reference is almost always water at 4°C (maximum density, 999.97 kg/m³) or at a specified temperature. For gases, the reference is usually dry air at standard conditions. Because it is a ratio of two densities, specific gravity has no units.
A specific gravity greater than 1.0 means the substance is denser than water and will sink in it. A value less than 1.0 means it is less dense and will float. This simple test is widely used in geology, brewing, petroleum engineering, and quality control to identify and characterize materials without complex laboratory equipment.
Formula
Where ρ is density in consistent units. The API gravity scale used in the petroleum industry is related by: API = (141.5 / SG) - 131.5. The Baumé scale for liquids heavier than water is: Bé = 145 - (145 / SG).
Common Specific Gravities
| Material | Specific Gravity |
|---|---|
| Gold | 19.32 |
| Iron | 7.87 |
| Aluminum | 2.70 |
| Concrete | 2.30 |
| Seawater | 1.025 |
| Ice | 0.917 |
| Gasoline | 0.72 |
| Ethanol | 0.789 |
| Mercury | 13.56 |
Applications
- Brewing: Measuring sugar content via wort density to track fermentation.
- Petroleum: Classifying crude oil grades (light, medium, heavy) using API gravity.
- Geology: Identifying mineral samples based on density measurements.
- Medicine: Urinalysis uses urine specific gravity to assess hydration and kidney function.
- Battery testing: Checking electrolyte concentration in lead-acid batteries.
Frequently Asked Questions
Is specific gravity the same as density?
No. Density has units (kg/m³, g/cm³, etc.) while specific gravity is a dimensionless ratio. However, in CGS units where water density is 1.000 g/cm³, the numerical value of SG equals the density in g/cm³.
Why does temperature matter?
Density changes with temperature, so both the substance and reference temperatures must be specified. A common notation is SG 20/4, meaning the substance at 20°C compared to water at 4°C.
What is API gravity?
API gravity is an inverse scale used by the petroleum industry. Higher API means lighter oil. Crude oil with API > 31.1 is "light," 22.3-31.1 is "medium," and < 22.3 is "heavy." It is calculated as API = 141.5/SG - 131.5.