The Numbers in Beer Style Descriptions

What do the numbers in the beer style descriptions mean? Here's a short explanation of each.

OG

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).

FG

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

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).

IBUs

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.