What priming sugar is actually doing in the bottle
Bottle conditioning is a second, tiny fermentation carried out in a sealed container. You add a measured dose of fermentable sugar to finished beer, cap it, and the yeast still in suspension converts that sugar to ethanol and carbon dioxide. The ethanol is negligible — a normal priming dose adds about a quarter of a percent ABV. The carbon dioxide has nowhere to go, so it dissolves into the beer until the gas in the headspace and the gas in the liquid reach equilibrium. That equilibrium is what you taste as carbonation.
Brewers measure carbonation in volumes: the volume of gas, at 0 °C and one atmosphere, that would be released from a given volume of beer. A beer at 2.5 volumes holds two and a half litres of CO2 gas for every litre of liquid. Converted to mass, one volume is about 1.96 g of CO2 per litre of beer, which is the constant that turns a carbonation target into a weight of gas you have to manufacture.
The step almost every quick chart omits is that the beer is not starting from zero. Carbon dioxide is a fermentation product, and beer that has just finished fermenting is already saturated with it at whatever temperature it sat at. A beer resting at 40 °F holds about 1.46 volumes before you add anything; the same beer warmed to 75 °F holds only about 0.78 volumes, because gas solubility falls as temperature rises. Prime a cold-crashed beer using a figure calculated for a warm one and you over-carbonate by more than half a volume, which is the difference between a nice ale and a bottle that foams out of the neck.
The formula explained, term by term
The calculation has three moving parts, and each answers a separate question.
How much gas do you still need? That is the target minus the residual: Ct − Cr. The residual comes from a quadratic fit to CO2 solubility data, Cr = 3.0378 − 0.050062 T + 0.00026555 T² with T in Fahrenheit. Over the range this fit is used for, roughly 32 to 80 °F, it falls steadily as temperature rises. Use the warmest temperature the beer has reached since fermentation ended, because once dissolved gas has escaped at a high temperature it does not come back when the beer cools again in a sealed vessel that was open at the time.
How much CO2 is that by weight? Multiply the shortfall in volumes by 1.96 g/L and by the litres you are bottling.
How much sugar produces that weight of CO2? This is pure stoichiometry. One molecule of glucose (180.16 g/mol) ferments to two molecules of ethanol and two of CO2 (88.02 g/mol), so glucose yields 88.02 ÷ 180.16 = 0.4886 g of CO2 per gram. Sucrose (342.30 g/mol) is a disaccharide that takes up a water molecule when it inverts, then yields four CO2: 176.04 ÷ 342.30 = 0.5143 g per gram. That is why table sugar is about 5% lighter on the scale than corn sugar for identical carbonation — a fact worth knowing when a recipe specifies one and you have the other.
Dry malt extract and honey are mixtures, not pure sugars, so their yields are working figures rather than exact chemistry. This calculator treats DME as about 75% fermentable extract and honey as about 80% fermentable sugar by weight, which put them at roughly 1.33× and 1.25× the corn-sugar weight. Both are approximations that vary by brand and batch.
Worked example: 5 US gallons at 70 °F to 2.4 volumes
You have a pale ale sitting at 70 °F in the fermenter, five US gallons going into the bottling bucket, and you want the 2.4 volumes typical of the style. You are using corn sugar.
- Convert the volume. 5 US gal × 3.78541 = 18.927 L.
- Residual CO2. 3.0378 − (0.050062 × 70) + (0.00026555 × 70²) = 3.0378 − 3.50434 + 1.30120 = 0.8347 volumes.
- Gas still needed. 2.4 − 0.8347 = 1.5653 volumes.
- Mass of CO2. 1.96 × 18.927 × 1.5653 = 58.07 g.
- Sugar weight. 58.07 ÷ 0.4886 = 118.9 g, which is 4.19 oz.
- Per bottle. 18.927 L ÷ 0.35488 L per 12 oz bottle = 53.3 bottles, so 118.9 ÷ 53.3 = 2.23 g per bottle.
Now change one thing. Suppose you cold-crashed at 38 °F before bottling but the beer had previously sat at 70 °F for a week. The residual is still 0.83 volumes, not the 1.5 volumes the cold temperature suggests, because the gas left the beer while it was warm. Enter the warmest temperature, which is why this field is labelled the way it is.
Choosing a target and reading the result
Pick the target from the style, then check it against your bottles. Most British and American ales sit between 1.9 and 2.5 volumes; German wheat beers and Belgian ales run 3.0 to 4.0; cask-conditioned real ale sits near 1.0 to 1.5. Below 2.0 volumes the beer reads soft and the head collapses quickly; above about 3.0 the carbonation starts to scrub aroma and produce a prickly, acidic bite that suits a saison and ruins a mild.
Standard American crown-capped bottles are comfortable to about 3.0 volumes and are being pushed hard beyond 3.2. If you want a Belgian tripel at 3.6 volumes, use bottles built for it — heavy Belgian glass or champagne bottles with a cage — and store them where a failure will not throw glass. The pressure inside a bottle roughly doubles between 2.0 and 4.0 volumes at the same temperature.
The per-bottle figure is for people dosing individual bottles with a scale or with carbonation drops rather than batch priming. Dosing bottles individually removes the risk of a badly mixed bucket, but introduces the risk of an inaccurate scale — 2.2 g is at the edge of what a 0.1 g kitchen scale reads honestly.
If your bottles are consistently under-carbonated by a similar amount every batch, suspect two things before you change the math. First, corn sugar is sold as dextrose monohydrate and carries about 9% water that every published chart ignores. Second, a beer that spent a long time cold or was heavily fined may have too few yeast cells left in suspension to finish the job in a reasonable time; give the bottles three weeks at 68–72 °F before you judge them. Serving temperature also fools people: cold beer releases its gas reluctantly and tastes flatter than the same bottle at cellar temperature.
Corn sugar in grams for 5 US gallons (18.93 L)
| Beer temperature | Residual CO₂ (vol) | To 2.0 vol | To 2.4 vol | To 2.8 vol |
|---|---|---|---|---|
| 60 °F / 15.6 °C | 0.99 | 76.7 | 107.1 | 137.5 |
| 65 °F / 18.3 °C | 0.91 | 83.1 | 113.5 | 143.8 |
| 70 °F / 21.1 °C | 0.83 | 88.5 | 118.9 | 149.2 |
| 75 °F / 23.9 °C | 0.78 | 92.9 | 123.2 | 153.6 |
For table sugar multiply these by 0.950; for dry malt extract multiply by 1.330; for honey multiply by 1.250. Typical style targets: British and American ales 1.9–2.5 volumes, lagers 2.4–2.7, German wheat beers 3.0–4.0, Belgian ales 2.5–4.0, cask ale 1.0–1.5.
Bottle bombs come from unfinished beer, not from priming arithmetic
The dose on this page assumes fermentation is complete. If you bottle a beer that is still dropping gravity, the residual fermentable sugar carbonates the bottle on top of your priming dose, and there is no upper bound on where that ends. Ten gravity points of unfermented extract can add several volumes of CO2 — far past the burst pressure of any beer bottle. Confirm with two identical hydrometer readings three days apart using the ABV calculator before you prime, and be especially careful with beers containing fruit, honey or a diastatic strain, which can restart weeks later.
Ways a priming dose goes wrong
- Using the cold-crash temperature. The residual figure depends on the warmest temperature since fermentation ended, not the temperature on bottling day. Getting this backwards over-primes by around half a volume.
- Priming the batch size instead of the bottled volume. A five-gallon batch usually yields four and a half gallons into bottles once trub and samples are gone. Priming for five over-carbonates by about 10%.
- Swapping sugars gram for gram. Corn sugar, table sugar and DME are not interchangeable by weight. DME needs about a third more.
- Pouring the syrup in and not mixing. Stratified priming solution gives you a case where the last bottles gush and the first are flat. Rack the beer onto the cooled syrup rather than stirring the bucket hard, which oxidises the beer.
- Judging carbonation at two weeks. Three weeks at 68–72 °F is the normal minimum, and cold-conditioned or heavily fined beers can take longer because there is less yeast left to do the work.
- Priming a keg with a bottling dose. Kegs hold headspace pressure differently and are usually force-carbonated instead — use the keg carbonation PSI calculator for that.
Priming, force carbonation and spunding
There are three ways to get gas into beer, and the choice changes the arithmetic entirely. Priming, described here, manufactures the CO2 in the package from a measured sugar dose; it needs live yeast, three weeks of warmth, and produces a small yeast deposit that many drinkers regard as a feature. Force carbonation dissolves gas from a cylinder into a keg under pressure, is reversible, takes days rather than weeks, and needs no yeast at all. Spunding seals the fermenter near the end of fermentation and captures the beer's own CO2 under a pressure-relief valve, giving fully natural carbonation with no priming sugar and no yeast deposit — but it requires a pressure-rated fermenter and gravity readings accurate enough to know when to seal.
Commercial bottle-conditioned beers usually combine methods: the beer is filtered or centrifuged, then re-seeded with a measured dose of fresh yeast alongside the priming sugar so that the conditioning is predictable rather than dependent on whatever survived. If you are bottling a beer that has been in a cold vessel for two months, doing the same thing with a small dose of rehydrated dry yeast is worth more than any adjustment to the sugar weight.
Whichever route you take, the target in volumes is the same number, and the styles that suit each level do not change. What changes is only how you get there, and how long it takes.