Specific Gravity Calculator: SG, Brix, and API Gravity

Specific gravity (SG) is the ratio of the density of a substance to the density of a reference substance, typically water at 4 degrees Celsius (where water is densest, at 1,000 kg/m^3 or 1 g/cm^3). Because it is dimensionless, specific gravity is the same regardless of the unit system used. Values above 1.0 indicate a substance denser than water (which sinks); values below 1.0 indicate a substance less dense than water (which floats). Specific gravity is widely used in brewing, winemaking, food production, petroleum refining, and chemical engineering because it provides a quick check on purity, concentration, or composition without needing to know the exact unit system.

Degrees Brix (symbol: Bx) is a measure of dissolved sugar content in a liquid, defined as grams of sucrose per 100 grams of solution. It is measured with a refractometer or hydrometer and closely related to specific gravity: a higher sugar concentration gives a denser solution and thus a higher SG. A simplified approximation is SG ≈ 1 + (Brix / 1,000) x 4, but more commonly: SG ≈ 1 + 0.004 x Brix for the 0-30 Brix range. For greater precision, the Plato scale or official ICUMSA tables should be used. Brix is used extensively in the food and beverage industry to measure juice, wine, syrup, and fruit sugar levels.

API gravity is a measure of how heavy or light a petroleum liquid is compared to water. It was developed by the American Petroleum Institute (API) and is defined as: API = 141.5 / SG minus 131.5, where SG is measured at 60 degrees Fahrenheit (15.6 degrees Celsius). API gravity above 31.1 is considered light crude oil; between 22.3 and 31.1 is medium crude; below 22.3 is heavy crude; and below 10 (denser than water) is extra-heavy crude. Light crudes with high API gravity are generally more valuable because they yield more gasoline and distillates per barrel.

At approximately 15-20 degrees Celsius, representative specific gravity values include: water at 1.000, whole milk at approximately 1.030, seawater at approximately 1.025, ethanol (pure) at approximately 0.789, petrol (gasoline) at approximately 0.720 to 0.775, diesel fuel at approximately 0.820 to 0.850, honey at approximately 1.38 to 1.45, and mercury at approximately 13.6. These values vary with temperature, so all measurements should note the temperature at which they were taken.

Temperature significantly affects specific gravity because most liquids expand when heated, lowering their density. For water, SG is 1.000 at 4 degrees Celsius (maximum density), and decreases at both lower and higher temperatures. In the petroleum industry, SG and API gravity are measured at a standard temperature of 60 degrees Fahrenheit (15.6 degrees Celsius) and corrected using standardised tables (ASTM D1298 or similar) if measured at another temperature. In brewing and winemaking, Brix measurements made with a refractometer need temperature correction. Density meters and hydrometers often include built-in or referenced correction tables.