The Numbers in Beer Style Descriptions
What do the numbers in the beer style descriptions mean? Here's a short
explanation of each.
- Stands for "Original Gravity." This is the specific gravity of the
unfermented wort. The primary contribution to specific gravity is sugar,
some of which is fermented into alcohol, and some of which remains in the
finished beer to give sweetness and body. A specific gravity of 1.040
corresponds to a 10% (by weight) sugar solution, and will produce, on average,
about 4% alcohol (measured by volume).
- Stands for "Final Gravity." The AHA added FG numbers to the style
guidelines in 1995, and the charts have not yet been updated. The final
gravity is the specific gravity of the fermented beer. It will always be
less than the original gravity because during fermentation heavy sugars are
converted to lighter carbon dioxide and alcohol. The gravity is reduced both
by the reduced sugar content, and because alcohol is lighter than water.
Thus the sugar content of the finished beer cannot be calculated directly
from the final gravity, without taking the alcohol content into account.
See the useful formulæ page for
details on this computation.
- Alcohol content is measured in percent. Just to keep you on your toes,
the percentage may be either by volume or weight. Since alcohol is lighter
than water, the %volume (frequently abbreviated v/v) number is larger than
the %weight (w/w) number by a factor of 1.25. The numbers in this table are
%volume, which is the standard for wines and liquors in the US, and for
all alcoholic beverages in most parts of the world. For some reason, beer
in the US is commonly measured in %weight (e.g., 3.2% beer has at most 3.2%
by weight of alcohol, or 4% by volume).
- The International Bitterness Unit measures the bitterness of the
beer from the hops. There are other sources, but the hops are
usually the primary source of bitterness. One IBU corresponds to 1mg of
isomerized alpha acid per liter of beer. The threshhold of taste is about
12IBUs; below that level there is no perceptible bitterness (for most people,
most of the time). Generally, at higher beer OG values, it requires higher
levels of IBUs to give the same perceived bitterness, as balanced by the
residual sweetness of the finished beer.
- Color (SRM)
- Beer color in the US is measured by the "Standard Reference Method" (SRM),
or in "degrees Lovibond". The numbers are about the same between the two
scales, and they tend to be used interchangably. Higher values of the
color measurement correspond to darker beers. The scale is not linear: 10SRM
is not twice as dark as 5SRM, for example. The Europeans (and most of the rest
of the world) use a different scale: EBC. A very rough conversion from
SRM to EBC is to multiply by 3. This only works at all well for SRM values
less than 4 or 5.
Back to the Beer Page.
Spencer W. Thomas