Degrees Brix (symbol °Bx) is a measurement of the fraction of sugar per hundred parts aqueous solution, by mass. It is measured with a saccharimeter, an instrument that measures specific gravity of the liquid, or more easily with a refractometer. For example, a 25°Bx solution is 25% sugar, or 1 part sugar to 3 parts water.

The Balling scale was developed by German chemist Karl Balling.^{[citation needed]} It refers to the concentration of dissolved solids (mostly sucrose) as the percentage sucrose at 17.5 °C. The Brix scale was originally derived when Adolf Ferdinand Wenceslaus Brix recalculated Balling's scale to a reference temperature of 15.5 °C. The Brix scale was subsequently recalculated again and now uses a reference temperature of 20 °C. Brix can be approximated as 261.3 × (1 − 1/g), where g is the specific gravity of the solution at 20 °C. The Plato scale which measures in Plato degrees is also a refinement of the Balling scale. It uses a reference temperature of 17.5 °C and a slightly different modulus, with the approximation 260 × (1 − 1/g), where g is the specific gravity of the solution at 17.5 °C.

The three scales are often used interchangeably since the differences are minor.

• Brix is primarily used in fruit juice, wine making, starch and the sugar industry.
• Plato is primarily used in brewing.
• Balling appears on older saccharimeters and is still used in the South African wine industry and in some breweries.

A Brix-measuring instrument for use in the vineyard

Brix is used in the food industry for measuring the approximate amount of sugars in fruits, vegetables, juices, wine, soft drinks and in the starch and sugar manufacturing industry. Different countries use the scales in different industries; in the UK brewing is measured with specific gravity X 1000, European brewers use Plato degrees, and US industries use a mix of specific gravity, Brix, degrees Baumé and Plato degrees. For fruit juices, one degree Brix is about 1-2% sugar by weight. This usually correlates well with perceived sweetness.

Since Brix is related to the concentration of dissolved solids (mostly sucrose) in a fluid it is therefore related to the specific gravity of the liquid. Because the specific gravity of sucrose solutions is well known, it can also be measured by refractometers. Modern Brix meters are digital refractometers that calculate the Brix value based on refractive index. These meters are typically portable, splashproof and very simple to use, so that they can be operated easily directly on location with little training. More and more often Brix is measured to determine ideal harvesting times of fruit and vegetables so that products arrive at the consumers in a perfect state or are ideal for subsequent processing steps such as vinification.

When a refractometer is used, it is correct to report the result as "refractometric dried substance" (RDS). One might speak of a liquid as being 20 °Bx RDS. This is a measure of percent weight of total dried solids and, although not technically the same as Brix degrees determined through a specific gravity method, renders an accurate measurement of sucrose content since the majority of dried solids are in fact sucrose. When an infrared Brix sensor is used, it measures the vibrational frequency of the sugar molecules, giving a Brix degrees measurement. This will not be the same measurement as Brix degrees using a density measurement because it will specifically measure dissolved sugar concentration instead of all dissolved solids. The index of refraction and density of mixtures such as ethanol and water are calculated using the Gladstone-Dale relation.

• Boulton, Roger; Vernon Singleton, Linda Bisson, Ralph Kunkee (1996). Principles and Practices of Winemaking. Chapman & Hall.  ISBN 0-412-06411-1
• Rene Martinez VitalSensors Technologies LLC. "VS1000B Series In-Line Brix Sensors for the Beverage Industry". http://www.vitalsensorstech.com/VS-1000B%20Brix%20Sensor%20Data%20Sheet.pdf. — Martinez describes the theory and practice of measuring brix on-line in beverages.