Priming Sugar Calculator

How it's worked out

Residual CO₂. Beer leaving fermentation already holds dissolved CO₂, set by the warmest temperature it reached at atmospheric pressure. The tool uses the standard fit vols = 3.0378 − 0.050062·T + 0.00026555·T² (T in °F) — about 0.86 volumes at 70 °F, rising to 1.7 near freezing. You only sugar for the gap between this and your target.

Altitude. That residual figure assumes the beer settled under its own CO₂ at sea-level pressure, about one atmosphere. Where the air is thinner the blanket sits at lower pressure, and by Henry's law the beer holds proportionally less — roughly 82% of sea level in Denver, which trims about 0.15 volumes at 68 °F and asks for ~10% more priming sugar. Below a couple thousand feet it's negligible; in a mountain town it's worth dialing in. The same local pressure feeds the bottle-bomb check, since gauge pressure is measured against the thinner air outside.

Sugar for the gap. One volume of CO₂ is 1.964 g/L. The CO₂ to add is (target − residual) × 1.964 × litres, and the sugar is that divided by the source's yield. Corn sugar sold to homebrewers is dextrose monohydrate at 0.444 g CO₂/g — meaningfully less than table sugar's 0.514, because roughly a tenth of its weight is water of crystallisation. Mixing those two up is a common way to over- or under-shoot.

Headspace — and why carbonation can fall. A sealed package is a closed system: the CO₂ shares itself between the beer and the gas space until the dissolved level and the headspace pressure are in balance. Filling that space takes real CO₂ — for a 5 gal keg with half a gallon of headspace at 2.5 volumes, about 10 g of CO₂ ends up in the gas, which is why a keg can want ~15–20% more sugar than the same beer in bottles. The same physics runs in reverse: rack already-carbonated beer into a vessel with a lot of headspace and no top pressure, and CO₂ leaves solution to fill that space — the beer settles below where it started. The tool solves the full balance, so it accounts for both the extra sugar a big headspace needs and the drop an unprimed, heavily-headspaced beer will see.

Pressure & the bottle-bomb check. At the warmest temperature the sealed package will sit at, the tool re-solves the balance and reports the gauge pressure, adding the trapped air's share. Glass crown-cap bottles are flagged as carbonation climbs past about 3.5 volumes — that's heavy / champagne-bottle territory. PET flexes before it fails; can seams are rated lower; kegs and casks have a relief valve or vent and sit well inside their limits.

Gyle & speise. Priming with unfermented wort instead of sugar: the gyle's fermentable extract is °Plato × SG × 10 × fermentability grams per litre, yielding about 0.49 g CO₂ per gram of that extract. The tool returns the volume of gyle to add and what it works out to as a share of the batch — handy, traditional, and free of any off-flavour risk from refined sugar, at the cost of a small gravity bump.

Spund & top up. If the beer already carbonated part-way under a spunding valve, it never sat at atmospheric pressure, so the warmest-temperature residual no longer describes it. Switch the starting carbonation to already under pressure and the tool reads the dissolved CO₂ straight from the gauge — vols = k(T)·(gauge + atmospheric) − 0.0033, the same solubility fit the forced-carbonation tool uses — then primes only for the gap up to target. Read the gauge at a known beer temperature and set the conditioning temperature to match.

Natural carbonation by gravity. The oldest method of all: seal the vessel while the beer is still short of terminal and let the last of the fermentation carbonate it, no refined sugar involved. Each apparent gravity point of fermentation yields a fixed amount of CO₂ — derived from first principles as 0.989 g/L per point (about 0.50 volumes), from the alcohol each point makes and the carbon dioxide that comes with it. The tool turns the CO₂ gap (headspace and all) into the gravity to seal at, and reports a spunding-valve pressure to set as a safety backstop, so a misjudged terminal gravity vents off rather than building toward a bomb. It needs an accurate finishing gravity — a forced-ferment test is the honest way to get one — and a vessel rated for the pressure.

One thing no calculator can supply: live yeast. Priming only works if enough viable yeast is still in suspension to eat the sugar. Long-aged, heavily-fined, filtered or cold-crashed beers may need a fresh pitch at packaging. And note that nitro/mixed-gas pours are a different game — don't bottle-prime a beer to a nitro pour's apparent softness.