What Are Volumes of CO2?
Carbonation in beer is measured in "volumes" — the volume of CO2 gas relative to the liquid volume. For example, 2.4 volumes means the beer contains 2.4× its liquid volume in dissolved CO2 gas. Most American ales are carbonated at 2.2–2.6 volumes, which gives a lively but not aggressive carbonation. Highly carbonated Belgian or wheat beers can reach 3.0–3.5 volumes.
Why Beer Temperature Matters
CO2 is more soluble in cold liquid than warm liquid. After fermentation, some CO2 remains dissolved in your beer — more if it was fermented cold, less if it was warm. The priming sugar calculation must account for this "residual CO2" so you don't accidentally over-carbonate. A beer fermented at 68°F has less residual CO2 than one conditioning at 38°F.