Priming Sugar for a Carbonation Level

Bottle conditioning is a small, deliberate, fully sealed fermentation. A measured dose of sugar goes into finished beer, the surviving yeast ferments it, and the CO₂ produced has nowhere to go but into solution. Getting the dose right requires three things the arithmetic makes explicit: how much CO₂ the target carbonation represents, how much the beer already holds, and how much CO₂ a gram of your particular sugar actually yields.

Carbonation is quoted in "volumes", meaning litres of CO₂ gas at 0 °C and one atmosphere dissolved per litre of beer. That definition is what fixes the 1.9635 in the equation — it is the density of CO₂ at those conditions, 44.009 g/mol over 22.414 L/mol, and it is a physical constant rather than a fitted one. British ales sit near 1.5 to 2.0 volumes, North American lagers and pale ales around 2.4 to 2.6, German wheat beers and Belgian saisons at 3.0 and above. The yield term $y$ is stoichiometry: corn sugar as dextrose monohydrate yields 44.4% of its weight as CO₂, anhydrous dextrose 48.9%, and table sugar 51.4%, which is the theoretical ceiling for any fermentable. Dried malt extract is lower still and variable, because only part of it is fermentable at all.

The residual term is the one most often skipped, and skipping it produces gushers. Beer that has just finished fermenting already holds dissolved CO₂ — how much depends on the WARMEST temperature it reached after fermentation ended, since that is the point at which the beer last equilibrated with the gas above it. A beer cold-crashed to 2 °C but which sat at 20 °C for a week beforehand carries the 20 °C figure, roughly 0.85 volumes, not the cold one. Using the cold-crash temperature instead credits the beer with CO₂ it does not have and over-primes it substantially. That is the classic mistake on this page, and it is the one with physical consequences.

Two safety points, stated plainly because glass is involved. Prime only beer that has genuinely finished fermenting — two or three identical hydrometer readings several days apart, not a guess based on the airlock going quiet. Residual unfermented extract does not care that you have already added priming sugar, and a beer bottled at 1.020 that was heading for 1.010 carries ten points of unplanned carbonation. And respect the pressure rating of what you are filling: ordinary crown-capped beer bottles are safe to roughly 3.5 volumes, and Belgian or champagne bottles with the appropriate closure are what the higher styles require. Distribute the priming sugar evenly by dissolving it, boiling it briefly, and gently mixing it into the whole batch — sugar dosed bottle by bottle is how one carton ends up containing both flat beer and a grenade.