What is the Difference Between Density and Specific Gravity?
Density (ρ) — Absolute Measurement
Density is a dimensional physical quantity defined as mass divided by volume. It requires physical units such as kg/L, g/cm³, or kg/m³. For example, standard diesel fuel has a density of 0.832 kg/L. Review our master guide on the density formula V=m÷ρ.
Specific Gravity (SG) — Relative Dimensionless Ratio
Specific gravity (also called relative density) is a pure number without units. It compares the density of a substance directly against the density of pure reference water (1.000 kg/L at 4°C): SG = 0.832 kg/L ÷ 1.000 kg/L = 0.832. Check out diesel mass to volume conversions.
Specialized Hydrometer Density Scales by Industry
How major global industries express relative density and concentration.
| Scale Name | Units | Conversion Formula | Primary Industry |
|---|---|---|---|
| Density (ρ) | kg/L or kg/m³ | ρ = m / V | Physics, Chemical Engineering, General Trade |
| Specific Gravity (SG / RD) | Dimensionless (ratio) | SG = ρ_substance / ρ_water | Hydraulics, Battery Testing, Mining Slurries |
| API Gravity (°API) | Degrees API | °API = (141.5 / SG @ 60°F) - 131.5 | Global Petroleum, Crude Oil & Refining |
| Brix Scale (°Bx) | Degrees Brix (% sucrose wt) | 1° Bx = 1g sucrose / 100g aqueous | Wine, Brewing, Sugar Refining, Fruit Juices |
| Baumé Scale (°Bé Heavy) | Degrees Baumé | °Bé = 145 - (145 / SG) | Industrial Mineral Acids, Heavy Syrups, Electroplating |
| Plato Scale (°P) | Degrees Plato | °P ≈ °Bx ≈ (SG - 1) × 1000 / 4 | Commercial Brewing & Distilling |
1. API Gravity (°API) in Petroleum Logistics
The American Petroleum Institute (°API) scale is universally used in crude oil pricing. Water has an API gravity of 10.0° API. Light crude oils (e.g. Brent, WTI) exceed 38° API (SG < 0.835), whereas heavy bitumen drops below 20° API (SG > 0.934).
2. Brix Scale (°Bx) in Food, Wine & Agriculture
Named after Adolf Brix, 1° Brix represents 1 gram of dissolved sucrose sugar per 100 grams of aqueous solution. Used with optical refractometers to determine grape ripeness, honey quality (>80° Bx), and maple syrup density (66° Bx).
3. Baumé Scale (°Bé) in Chemical Manufacturing
Widely used in industrial acid production. For example, commercial 93.2% sulfuric acid is traditionally sold as 66° Baumé (SG = 1.835), providing a quick hydrometer quality check without chemical titration.
Frequently Asked Questions
Metrology questions on specific gravity and relative density.