Volumes of CO2, and the part everyone skips
Carbonation is measured in volumes: one volume means the beer holds a quantity of dissolved CO2 that would occupy the same space as the beer itself at standard conditions. Ordinary American pale ale sits around 2.4 volumes. British cask-style ales are much lower, near 1.5. German wheat beers run past 3.5, which is why they come in heavy bottles.
The step people skip is that finished beer is already carbonated. Fermentation produces far more CO2 than the beer can hold, most of it leaves through the airlock, but what stays dissolved depends on temperature — cold liquid holds more gas. The standard relation for residual CO2 in volumes as a function of the highest temperature the beer has reached in degrees F is
CO2residual = 3.0378 − 0.050062 T + 0.00026555 T²
At 68 degrees F that is 0.88 volumes. At 45 degrees F it is 1.32 volumes. That difference — nearly half a volume — is roughly a third of the priming sugar for a typical beer. Prime a cold-conditioned beer using a figure worked out for a warm one and you overshoot badly, which is the mechanism behind a large share of the bottle failures people report.
Note that the temperature that matters is the highest the beer reached after fermentation ended, not the current one. Once CO2 has come out of solution on a warm day it does not go back in when the beer cools again in a sealed but unpressurised fermenter.
The sugar arithmetic
One volume of CO2 corresponds to about 1.96 grams of CO2 per litre of beer. Dextrose monohydrate — corn sugar — is roughly 91% glucose by weight, and yeast converts glucose to CO2 at about 0.49 grams of CO2 per gram of sugar. That works out to about 4.39 grams of corn sugar per litre per volume of CO2.
Run the standard case: 5 US gallons is 18.93 litres, and going from 0.83 residual volumes at 70 degrees F to a 2.4 volume target is a gap of 1.57 volumes. That is 4.39 x 18.93 x 1.57, or about 130 grams — 4.6 ounces. That matches the figure published in the common priming charts for exactly those conditions.
Other sugars scale by how much CO2 they yield per gram:
| Sugar | Weight relative to corn sugar | Notes |
|---|---|---|
| Corn sugar (dextrose monohydrate) | 1.00 | The reference. Roughly 9% of the weight is water of crystallisation. |
| Table sugar (sucrose) | 0.87 | Fully fermentable and anhydrous, so you need less by weight |
| Dry malt extract | 1.47 | Only partly fermentable, and the fermentable fraction varies by product |
The DME figure is the least reliable of the three, because extract fermentability differs between products and the unfermentable portion contributes nothing. Treat it as an approximation and aim slightly low if you use it. Honey, maple syrup and other syrups vary so much in water and sugar content that a single factor for them is not defensible.
Bottle bombs
An over-pressurised glass bottle fails suddenly and throws fragments hard enough to cut. This is not a theoretical hazard and it is worth being blunt about the three ways it happens.
The first is bottling too early. If gravity is still falling, the yeast will keep working after the cap goes on, and the sugar it eats is added to whatever you primed with. Take a gravity reading, wait two or three days, take another, and only bottle when they match. The second is a measurement error — sugar measured by volume rather than weight, or a batch primed for a larger volume than actually went in the bucket. The third is an infection, where a wild yeast or bacterium slowly ferments sugars the brewing yeast left alone, over weeks or months. That one gives no warning at bottling time and is why long-stored bottles of a batch that seemed fine can still fail.
Store conditioning bottles in a closed crate or a plastic tub, at floor level, away from where people sit. If one goes, the box keeps the glass in the box.
Kegging and the rest of the batch
If you keg rather than bottle, priming sugar is optional — you can force carbonate with a regulator and a pressure chart, which removes the timing risk entirely and lets you change your mind about carbonation level afterwards. Priming a keg works and uses the same numbers as above, just without the bottle failure hazard, since a keg is built for far more pressure than it will ever see.
Before you get here, the gravity readings that tell you fermentation actually stopped are the same ones that give the alcohol and attenuation figures. If the batch volume in the bottling bucket came out short of what you planned, the brewing water and boil-off calculator is where that error started.
Questions people ask
Why does the temperature I enter matter so much?
Because it decides how much CO2 the beer is already holding, and that gets subtracted from your target. A beer that sat at 45 degrees F holds about 1.32 volumes; the same beer that reached 75 degrees F holds about 0.75. For a 2.4 volume target that is the difference between needing 1.08 volumes of priming and needing 1.65 — roughly a 50% difference in sugar. Use the highest temperature the beer has reached since fermentation finished, not the temperature it is at now, because CO2 that gassed off on a warm day does not come back.
Can I use table sugar instead of corn sugar?
Yes, and you need about 13% less of it by weight because sucrose is anhydrous and fully fermentable while corn sugar carries water of crystallisation. Where 130 grams of corn sugar is called for, use about 113 grams of table sugar. The old claim that table sugar produces a cidery flavour comes from an era of much larger sugar additions in extract brewing; at priming quantities, which are a fraction of a percent of the beer, no detectable difference has held up. What does matter is that you weigh it rather than measuring by the cup.
How do I prime individual bottles instead of the whole batch?
Divide the total sugar weight by the number of bottles and weigh that amount into each one, or use commercially made priming tablets or drops with a stated CO2 contribution. The per-bottle figure is shown above. Batch priming into a bottling bucket is generally more consistent because one weighing error affects everything equally rather than concentrating in one bottle, but it needs gentle, thorough mixing without splashing — stir slowly with the racking cane, do not whisk oxygen into finished beer. Per-bottle priming avoids the mixing problem and creates a weighing problem instead.
The beer has been in the bottle three weeks and it is still flat. What now?
Check the temperature first. Yeast will not carbonate a bottle stored below about 60 degrees F in any reasonable time; move the box somewhere in the high sixties and give it another two weeks. If it is warm and still flat, the likely causes are too few viable yeast cells left in suspension after a long cold conditioning, a sugar addition that did not mix evenly through the bucket, or caps that are not sealing. Invert a bottle and watch for a slow bead of bubbles at the cap to check the seal. Adding more sugar to already-capped bottles is not a fix and is how bottles end up over-primed.
Is there a limit on what I can brew at home?
There are rules, at both federal and state level, and they differ from one state to another and change over time. They cover things like how much may be produced, who may produce it, and what may be done with it afterwards. This page does not state any of them as fact because doing so would be wrong somewhere. Look up current federal guidance and your own state's rules before scaling up. Separately, distilling spirits at home is a distinct federal matter with its own licensing regime and is not covered anywhere on this site.