In the United States, carbonation level is expressed in volumes of carbon dioxide. A volume of carbon dioxide is defined as the volume of gas that could be removed from a volume of beer at 68 °F (20 °C) at one atmosphere of pressure. For example, a liter of beer with 2.5 volumes would fill a 2.5-L bag with carbon dioxide if all the gas were removed at 68 °F (20 °C) and atmospheric pressure. This really is a weird unit of measure! Almost all other countries express carbon dioxide in grams per liter, a much more obvious expression of concentration.
Most beers in the United States contain between 2.5 and 2.6 volumes of CO2. Beers such as Bud, Miller and Coors fall into this carbonation range. These same beers served on draft have slightly lower levels of CO2 and fall about 0.05 to 0.1 volumes lower than their bottled brethren. Bottled lagers from Europe have a little less carbonation, about 0.1 volume less, than American lagers, but they seem dramatically less fizzy because most European lagers are all-malt, and that has a dramatic effect on beer body.
Traditional English ales served from casks have very low levels of CO2, usually somewhere around 1.8 volumes. Since beer at 55 °F (13 °C) contains about 1.3 volumes of carbon dioxide when it is sitting in an unpressurized carbon dioxide environment, English ales fall at the very low end of the CO2 scale when compared with other beers from around the world. Bottled ales tend to be higher in carbonation, but they still have less than most lagers. Typical values fall between 2.2 and 2.4 volumes of CO2.
The wheat and fruit beers of the world, such as Berliner weiss beers, Bavarian hefe-weizens, Belgian wit beers, and lambics, have very high levels of CO2 to give them a light and refreshing palate. These beers have CO2 levels ranging from three to four volumes. Like Champagne, these beers are often served in fluted glassware that presents them with a certain elegance.
Some generalizations can be drawn about carbonation and flavor. Beers that have complex palates usually have lower levels of CO2, so the beer’s true identity isn’t masked by carbonation. Beers with less complex palates that are meant to be served ice cold typically have more carbonation. Since carbonation stimulates the trigeminal nerve, the nerve that is also stimulated by spicy foods, some noted brewing experts have given these beers the nickname “pain beers.”
Pain beers often derive a significant portion of their flavor from the carbonation, and many taste downright nasty when allowed to lose their high level of carbonation.
When craft beers are dragged into the picture, things get a little jumbled. Most craft beers have complex palates, leading one to speculate that they also have low CO2 levels. However, many craft beers have CO2 levels equaling or exceeding American lagers. When consumed, the beers don’t seem to be overcarbonated because their full flavors are able to carry a higher level of CO2 without seeming unbalanced.
This just shows that rules of thumb are useful guides but cannot be rigorously applied to all scenarios. It is up to the brewer to discover the right level of carbonation.