How much CO2 does fermentation actually produce?

Roughly 0.46 grams of CO2 per gram of sugar fermented, alongside an equal mass of ethanol. For a typical 20 litre batch dropping from 1.050 to 1.010, that is on the order of 200 to 230 grams of CO2.

What is residual dissolved CO2?

Even after fermentation ends, the beer holds some CO2 in solution. The amount depends on temperature: colder liquid holds more. This residual level is expressed in volumes and is the starting point before you prime or force-carbonate.

Why does temperature matter for dissolved CO2?

CO2 solubility falls as temperature rises. A beer fermented warm at 20C retains roughly 0.85 volumes of CO2, while one cold-conditioned near 2C retains close to 1.7 volumes. This residual is subtracted from your target carbonation.

Can I use this to size a closed pressure fermenter?

It gives the total CO2 generated, which is the gas a sealed vessel must either hold under pressure or vent through a spunding valve. Combine the total mass with your vessel headspace to estimate pressure build, though exact pressure also depends on temperature and headspace volume.

Does the same formula apply to wine and cider?

Yes. The 0.46 grams CO2 per gram sugar relationship comes from the fermentation stoichiometry and holds for any sugar fermented by yeast, so wine, cider, and mead all follow it.

CO2 Production During Fermentation

When yeast ferments sugar it produces almost equal masses of ethanol and carbon dioxide. This tool estimates how much CO2 your batch generates as it drops from original gravity to final gravity, how much stays dissolved at your fermentation temperature, and how much is left over for natural carbonation.

How it works

The chemistry is fixed by the fermentation equation. One molecule of glucose yields two molecules of ethanol and two of CO2:

C6H12O6 -> 2 C2H5OH + 2 CO2

By mass that works out to about 0.46 g of CO2 for every gram of sugar fermented (2 x 44 / 180). The tool estimates fermentable sugar from the gravity drop:

sugar mass (g) = (OG points - FG points) x 2.6 x volume (L)

where gravity points are the SG decimals times 1000 (so 1.050 is 50 points). Total CO2 produced is then sugar mass x 0.46.

Residual dissolved CO2

Not all of that CO2 escapes. The liquid retains a temperature-dependent residual level, approximated by:

residual volumes = 3.0378 - 0.050062 x T(F) + 0.00026555 x T(F)^2

with temperature in Fahrenheit. Multiplying residual volumes by 1.96 g/L and the batch volume gives the dissolved CO2 mass. The net figure (total minus dissolved) is the gas vented through the airlock or held by a sealed fermenter.

Tips and notes

Use the residual dissolved CO2 as the baseline when priming: subtract it from your target carbonation volumes so you do not over-prime and create gushers or bottle bombs.
Warm fermentations retain less CO2, so they need more priming sugar to reach the same finished carbonation.
The sugar-from-gravity step is an estimate; real wort attenuation varies with yeast and recipe. Every calculation runs locally in your browser.