Formula & Calculator
Specific Gravity
Compares the density of a substance to a reference fluid (usually water at 4°C), a dimensionless ratio used throughout process and daily-life calculations.
Interpretation
Specific gravity: SG = ρ_substance / ρ_reference (water at 4°C). Dimensionless density ratio. Example: Mercury SG=13.6.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| SG | Specific gravity | |
| rho_substance | Density of the substance | kg/m3 |
| rho_reference | Density of reference fluid (water = 1000 kg/m3) | kg/m3 |
What it means
Specific gravity (SG) is a dimensionless quantity defined as the ratio of the density of a substance to the density of a reference material, usually water at 4°C (1000 kg/m³). It is widely used in industry because it is easy to measure with hydrometers and pycnometers. SG is used to characterise fluids, solids, and slurries. In petroleum engineering, API gravity is derived from SG. In hydrometry, SG of soil particles is used for compaction and consolidation calculations. In chemical engineering, SG is used in conversion between mass and volume, and in sizing equipment like separators and storage tanks. Since SG is independent of units, it is convenient for reporting and comparing materials. It is also used in quality control to check product purity or composition. Understanding specific gravity is essential for process design and material handling.
Worked example
Specific Gravity – Two Examples
Real‑World| Parameter | Value |
|---|---|
| ρ_substance | 850 kg/m³ |
| ρ_reference | 1000 kg/m³ |
| Parameter | Value |
|---|---|
| ρ_substance | 13,600 |
| ρ_reference | 1000 |
Common mistakes
- Reference substance: Usually water at 4°C (density 1000 kg/m³) – but sometimes air or other reference.
- Units: Both densities in the same units – SG is dimensionless.
- Temperature dependence: Density changes with temperature; use the same reference temperature for both.
- Specific gravity vs. density: SG is not density; it is a ratio.
- Specific gravity of gases: Reference is air; SG_air = 1.
Applications
Specific gravity (SG) is the ratio of the density of a substance to the density of a reference (usually water at 4°C). It is a dimensionless quantity used to characterise materials in engineering, chemistry, and geology. Engineers use SG to calculate mass from volume, to determine buoyancy, and to select materials for flotation or sedimentation. In the oil and gas industry, SG is used to classify crude oils and to design separators. In mineral processing, it is essential for gravity separation techniques. SG is also used in the design of hydraulic and pneumatic systems. By knowing SG, professionals can convert between mass and volume, assess the purity of substances, and design equipment such as hydrometers and density meters.
- Characterisation of fluids and solids in chemical and petroleum engineering
- Design of separators, decanters, and hydrocyclones
- Buoyancy calculations for vessels, pontoons, and marine structures
- Quality control in production and refining processes
- Conversion between mass and volume for inventory management
Frequently Asked Questions
Specific gravity (SG) is the ratio of the density of a substance to the density of a reference fluid (usually water at 4°C): SG = ρ_substance / ρ_reference. It is dimensionless. For liquids, the reference is water; for gases, it is air.
- Comparing SG values at different reference temperatures – density varies with temperature.
- Using SG for gases without specifying the reference (usually air at STP).
- Assuming SG is the same as density – they are proportional but not equal.
Density = SG × ρ_reference. For liquids, if water is the reference, ρ = SG × 1000 kg/m³ (at 4°C).
- Water: 1.0
- Mercury: 13.6
- Ethanol: 0.79
- Crude oil: 0.85‑0.95
It is easy to measure with hydrometers and is used for quality control, concentration monitoring, and blending calculations.
Generally, as temperature increases, density decreases (for liquids), so SG decreases. For gases, SG is also temperature‑dependent but often referenced to STP.
API gravity is a scale used for petroleum liquids: API = (141.5/SG) – 131.5. Higher API means lighter oil. It is used in the oil industry.
Pump power depends on density (ρ), so SG is used to convert water‑based power calculations to other liquids.