Belgian Brewing Degrees are an obsolete calculation of the gravity of beer stipulated by Belgian law for the payment of taxes. It was developed as a practical way to analyze both regular and acidic (lambic and other sour style) beers, as the presence of acid led to errors in other methods.
Belgian brewing degrees were calculated after performing three analyses on the beer. First, the residual extract (n) of the beer was measured by distilling off the alcohol and analyzing the weight of the residual material left in the liquid. Using the standardized Doemens–Plato conversion table, this was converted to degrees Plato.
The original gravity obtained was converted to specific gravity by means of a Doemens–Plato conversion table. The factor 0.0013 was then subtracted. Finally, Belgian brewing degrees were obtained from the second decimal place of the resulting number. For example, if the final number calculated was 1.0641, the official Belgian brewing degrees would be 6.41. Tax and beer classifications were then based on this number.
Until the early 1990s, many Belgian breweries still used the Belgian gravity scale, but it has largely fallen out of favor with the Plato scale’s increasing prevalence. There are a number of Belgian beers, however, that are still named after their original gravities expressed in Belgian degrees, for example, Rochefort 6, 8, and 10.