Plato scale

The Plato scale measures dissolved extract by mass, using sucrose solutions as its reference. A wort reading of 15 °P therefore indicates roughly 15 grams of dissolved extract per 100 grams of wort, although wort extract is a mixture of sugars, dextrins, proteins, minerals, and other compounds rather than pure sucrose.¹

Brewers measure original gravity or degrees Plato before fermentation to characterize the starting wort. The value helps predict potential alcohol, compare batches, calculate brewhouse performance, and describe beer styles. Because fermentation converts sugars into ethanol and carbon dioxide, finished beer is usually lower in extract and requires correction for alcohol when its apparent or real extract is calculated.

Plato readings are commonly obtained from density, because dissolved material makes wort denser than water. Hydrometers and digital density meters determine specific gravity, after which a calibrated relationship converts the result to degrees Plato; refractometers instead measure refractive index and require special correction once alcohol is present.²

Plato and specific gravity are not interchangeable units, but they are closely related in ordinary wort. A commonly used approximation is °P ≈ 259 − (259 ÷ specific gravity), with gravity expressed relative to water; brewing software and laboratory instruments generally use more precise polynomial conversions. Temperature matters because density changes with temperature, so hydrometers are read at their calibration temperature or corrected accordingly.

Plato provides a compact way to track extract through the brewing process. Brewers compare pre-boil and post-boil readings to assess evaporation and kettle concentration, then compare original extract with fermented-beer measurements to estimate attenuation and alcohol content.³

In commercial brewing, extract readings also support process control: they can reveal dilution errors, incomplete lautering, excessive evaporation, or inconsistent malt performance. The scale is especially useful when reporting wort strength independently of the beer's eventual alcohol level. A high-Plato wort does not guarantee a high-alcohol beer, since yeast strain, fermentation conditions, and the proportion of fermentable to unfermentable extract determine how much sugar is converted.

The Plato scale is a sucrose-based convention rather than a direct chemical analysis of every substance in wort. Its readings are therefore operational estimates of extract, and colored, highly concentrated, carbonated, or alcohol-containing samples can require filtration, degassing, dilution, or instrument-specific correction.

Plato is closely related to the Brix scale, which also originated as a sucrose-based mass-percentage scale; for dilute, sugar-dominated liquids the values are nearly the same, but wort composition makes the equivalence approximate. The scale also appears outside finished-beer labeling: breweries use it for concentrated wort, high-gravity brewing, and fermentation models, while laboratories may report real extract, apparent extract, and alcohol by mass alongside the original-Plato value.

The scale honors the German chemist Fritz Plato, who refined nineteenth-century methods for relating liquid density to dissolved sucrose concentration. Brewing practice adopted the system because density tables offered a practical, reproducible estimate of wort extract before electronic analyzers became widespread.⁴

The degree sign and letter P are normally written together as °P, while “Plato” may refer either to the scale itself or to a numerical reading. In technical work, the measurement should be accompanied by the sample state and method—for example, original extract in °P or apparent extract in °P—because the same number can mean different things before and after fermentation.

Wort

The aqueous extract of malt and other brewing materials before fermentation.

Original extract

The concentration of dissolved extract in wort before fermentation, commonly reported in °P or specific gravity.

Specific gravity

The density of a liquid relative to the density of water at a specified reference temperature.

Real extract

The nonalcoholic dissolved and suspended residue remaining in fermented beer, expressed on a mass basis.

Apparent extract

An extract value inferred from beer density without fully correcting for alcohol's effect on density.

Brix scale

A sucrose-referenced scale expressing dissolved solids as an approximate percentage by mass.