How this calculator works
When beer ferments, carbon dioxide dissolves in it until the pressure and temperature reach equilibrium. Colder fermentation leaves more CO2 in the beer, so less priming sugar is needed at bottling. Priming calculators estimate that residual CO2 from the fermentation temperature, subtract it from the style target, and convert the missing gas to sugar using fermentation chemistry. Sucrose produces four molecules of CO2 per molecule, and dextrose monohydrate produces two, so dextrose needs more weight for the same gas. The result assumes you bottle the whole batch and that the beer is not already carbonated.
Formula
residual CO2 (volumes) = 3.0378 - 0.050062 x T + 0.00026555 x T^2, with T in degrees Fahrenheit. CO2 to add = target - residual. CO2 mass = volumes to add x 1.96 g per litre per volume x litres. sugar mass = CO2 mass / sugar yield, where the yield is 0.514 g CO2 per g sucrose, 0.444 for dextrose, 0.366 for dry malt extract and 0.40 for honey.
Worked example
A 19 L batch fermented at 20 °C (68 °F) has a residual of 3.0378 - 3.4042 + 1.2275 = 0.861 volumes. For an American pale ale at 2.4 volumes, you need to add 1.539 volumes. That is 1.539 x 1.96 x 19 = 57.3 g of CO2, which needs 57.3 / 0.514 = 111.5 g of sucrose, or 5.87 g per litre. Corn sugar would need 129.0 g.
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Frequently asked questions
When should I add priming sugar?
Dissolve the sugar in a small amount of boiling water, cool it, and add it to the bottling bucket before racking the beer on top. Mix gently without splashing, which would add oxygen. Adding dry sugar directly to bottles gives uneven carbonation and can cause gushers.
How carbonated should different styles be?
English bitters are lightly carbonated at about 1.8 volumes, stouts about 2.0, American ales 2.3 to 2.5, lagers around 2.6 and wheat beers or Belgian ales up to 3.0. Very high carbonation needs stronger bottles and more careful conditioning.
Why use the fermentation temperature rather than the bottling temperature?
The residual CO2 was set by the highest temperature the beer reached after fermentation, when dissolved gas escaped. If the beer warmed during a diacetyl rest or conditioning, that higher temperature gives the correct residual figure. Using a lower bottling temperature would overestimate the CO2 already present.
How can I avoid over-carbonation?
Measure the batch volume accurately, use the highest temperature the beer reached, and do not add extra sugar for flavour. Store bottles in a warm room for about two weeks, then chill one and test it before opening the rest. If a batch is already overcarbonated, vent and re-cap carefully rather than opening warm bottles.