Priming Sugar Calculator

In short Every homebrewing book quotes about four grams of corn sugar per litre per volume of CO2 as a number to memorise. It is not a number to memorise, it is stoichiometry, and doing it properly explains three things the rule of thumb cannot. One volume of CO2 means one litre of gas per litre of beer, which at 0°C and one atmosphere weighs 44.009 ÷ 22.414 = 1.9635 grams. Glucose ferments to two molecules of CO2 per molecule of sugar, so every gram of glucose yields 0.4886 grams of CO2. Divide and you get 4.0188 grams per litre per volume. That is where the four comes from. FIRST CONSEQUENCE: TABLE SUGAR IS NOT THE SAME WEIGHT. Sucrose is a larger molecule yielding four CO2 rather than two, and it works out at 3.8179 grams against glucose’s 4.0188 — you need 95% as much by weight. That is exactly why the old advice offers three quarters of a cup of corn sugar or two thirds of a cup of table sugar. It is not a preference, it is molar mass. SECOND: THE CORN SUGAR IN THE BAG IS NOT DRY. Homebrew shops sell dextrose monohydrate, which carries a water of crystallisation that ferments into nothing at all. That water is 9.1% of the weight, so you need 110% of what anhydrous dextrose would need. A recipe quoting four grams per litre without saying which one is quietly a tenth out. THIRD: THE BEER ALREADY HAS CO2 IN IT. Dissolved carbon dioxide left from fermentation depends on the HIGHEST temperature the beer has reached since it finished, not the temperature now — about 0.86 volumes at 20°C and 1.20 at 10°C. Ignore it and you overshoot by most of a volume, which is the commonest cause of gushers. ⚠️ A SAFETY NOTE THAT IS NOT DECORATIVE. Over-carbonating bursts glass, and a bottle that fails fractures rather than leaking. Standard beer bottles take about 3 volumes, swing-tops about 2.5, heavy Belgian bottles about 4.5 and champagne bottles about 6. Those are rules of thumb, not ratings. Leave margin, and never bottle a beer that has not finished fermenting — residual sugar carbonates on top of whatever you added.