Beer already contains CO₂
Fermentation saturates the beer with carbon dioxide, and how much stays dissolved depends on temperature. Warm beer holds less. That residual amount is your starting point, and you only need to add the difference.
Use the warmest temperature the beer reached after fermentation finished, not the fermentation set point and not the temperature on bottling day. Once the CO₂ has come out of solution it does not go back in on its own.
- 0 °C / 32 °F → 1.71 volumes residual
- 10 °C / 50 °F → 1.20 volumes
- 20 °C / 68 °F → 0.86 volumes
- 21 °C / 70 °F → 0.84 volumes
Where the sugar rates come from
One volume of CO₂ means one litre of gas dissolved per litre of beer, which is 1.96 grams. Fermentation converts sugar to CO₂ at a ratio fixed by the chemistry:
Corn sugar sold for brewing is dextrose monohydrate, which carries a water molecule and is about 91% glucose by weight. Dry malt extract is only partly fermentable, around 75%. Dividing 1.96 by each yield gives the rate per litre per volume:
- Corn sugar (dextrose monohydrate) — 4.41 g/L per volume
- Table sugar (sucrose) — 3.81 g/L per volume
- Dry malt extract — 5.35 g/L per volume
These sit at the top of the range you will see elsewhere. Kai Troester's derivation at Braukaiser arrives at the same 4.4 g/L/vol for corn sugar by the same route. Calculators that skip the monohydrate correction land around 4.0 g/L/vol; Brewer's Friend and Brew Your Own worked examples fall between 4.2 and 4.35. Anything in 4.0–4.4 is defensible — the difference is about 10%, well inside the spread of real-world bottling.
One number worth keeping separate in your head: 1.96 g is the mass of CO₂ in one volume, not the mass of sugar needed to make it. The sugar figure is always larger, because only part of each gram of sugar becomes CO₂.
Worked example
A 19 litre batch, target 2.4 volumes, beer that sat at 20 °C after fermentation:
129 grams is close to the three-quarter-cup rule of thumb that has been passed around homebrewing for decades, which is a good sign that the arithmetic is sound.
Typical targets
- British ale, cask-conditioned — 1.5 to 2.0 volumes
- American ale, IPA, porter — 2.2 to 2.6 volumes
- Lager, pilsner — 2.4 to 2.8 volumes
- Belgian ale, saison — 2.8 to 3.5 volumes
- German wheat beer — 3.3 to 4.5 volumes
Above about 3 volumes, use bottles rated for it. Standard crown-capped beer bottles are not designed for Belgian pressures, and the failure mode is glass.