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.

Worked example

For a 20-litre batch dropping from OG 1.050 to FG 1.010, that is 40 gravity points. Fermentable sugar mass is 40 × 2.6 × 20 = 2,080 grams. Total CO2 produced is roughly 2,080 × 0.46 = 957 grams. Fermented at 20°C (68°F), the residual dissolved CO2 is approximately 0.86 volumes. Across 20 litres at 1.96 g/L per volume, that is about 33.7 grams dissolved, leaving roughly 923 grams of CO2 vented through the airlock.

Understanding carbonation volumes

Brewers measure dissolved CO2 in “volumes” — one volume equals one litre of CO2 gas (at standard pressure and temperature) per litre of liquid. Common carbonation targets are:

The residual dissolved CO2 figure this tool calculates is the baseline from which natural or forced carbonation begins. For bottle conditioning, subtract this residual from your target and calculate the priming sugar needed to generate the remaining CO2 in a sealed bottle. For force carbonation, set your regulator to the pressure needed to reach the target at the serving temperature.

Temperature and its critical role

Temperature is the key variable controlling how much CO2 remains dissolved. At lower fermentation or conditioning temperatures, beer holds significantly more CO2 in solution:

• At 2°C (36°F): approximately 1.7 volumes of residual CO2
• At 10°C (50°F): approximately 1.2 volumes
• At 18°C (64°F): approximately 0.9 volumes
• At 24°C (75°F): approximately 0.7 volumes

This matters most for bottle conditioning. A beer fermented at warm temperatures and bottled without cold crashing retains very little residual CO2, so it needs more priming sugar to reach the target carbonation. A beer cold-conditioned at near-freezing temperatures already holds substantial dissolved CO2 and needs much less priming — or none at all if the residual alone reaches the target.

Getting this calculation wrong in the direction of too much priming sugar results in over-carbonated beer at best and bottle explosions at worst.

Closed pressure fermenters

If you ferment in a sealed vessel (a keg, a conical with a spunding valve, or a pressure-capable fermenter), the CO2 produced during fermentation builds pressure rather than escaping through an airlock. The total CO2 mass this tool calculates represents the gas your vessel must safely contain — or vent through a spunding valve set to your target pressure. Combine the total mass with your vessel’s headspace volume to estimate the equilibrium pressure using Henry’s law, though this requires knowing your fermentation temperature and the gas constant for CO2. Most spunding valve users set a target pressure by style rather than calculating from first principles, then let the valve vent excess.