About this tool
The Priming Sugar Calculator is a free, in-browser homebrewing tool that works out how much fermentable sugar to add at bottling to carbonate your beer to a target level. When you bottle, the yeast ferments the priming sugar in the sealed bottle and the CO2 it produces dissolves into the beer. Enter your batch volume, the beer's temperature at bottling, the target carbonation in volumes of CO2, and the sugar type, and it returns the sugar weight in grams and ounces. Everything is computed locally in your browser.
The key subtlety is residual CO2: warm beer still holds dissolved CO2 from fermentation, and the warmer it is, the less it retains. The tool estimates residual CO2 (volumes) from the highest recent beer temperature with CO2_res = 3.0378 − 0.050062 × T + 0.00026555 × T² (T in °F). The CO2 you must add is the target minus this residual, the required CO2 mass is CO2_to_add × 1.96 g/L × volume in liters, and the sugar weight is that mass divided by the sugar's CO2 yield: corn sugar/dextrose 0.46, table sugar (sucrose) 0.51, anhydrous dextrose 0.50, honey 0.46, and dry malt extract about 0.33 (DME is only partly fermentable, so you need more of it).
As a worked example, carbonating 19 L of beer that finished at 20 °C (68 °F) to 2.4 volumes needs about 125 g of corn sugar. Typical style targets are British ales 1.5–2.0 volumes, American ales 2.2–2.7, and German wheat beers 3.3–4.5. Dissolve the sugar in a little boiled water, gently mix it into the whole batch before filling bottles for even carbonation, and never exceed your bottles' pressure rating — over-priming or bottling before fermentation is complete can produce dangerous bottle bombs.