An empirically derived hydrometer scale developed in 1843 by German scientist Karl Balling, and improved by Fritz Plato to measure density of beer wort in terms of percentage of extract by weight. The scale expresses the density as the percentage of sucrose by weight, so a wort measured at 12° Plato (abbreviated 12°P) has the same density as a water−sucrose solution containing 12% sucrose by weight.
Degrees Plato are more popular in central European brewing, and occasionally feature in beer names -- some Slovak or Czech breweries feature 10° and 12° versions of their beers, for instance. Many Belgian breweries also do this, examples include Rochefort and Maredsous.
The relationship between degrees Plato and specific gravity is not linear, but a good approximation is that 1° Plato is worth 4 "brewers points" (the thousandths' part of the SG measurement), so 12°P corresponds to an original gravity of 1.048.
These equations will allow the homebrewer to convert between °P and specific gravity. Note that these are approximations.
Derive Specific Gravity from Measured °P
Derive °P from Measured Specific Gravity