Priming Sugar Depends on Temperature, Not Just Volume
Priming sugar is the last calculation of a batch and the one with the most obvious failure modes: too little and the beer is flat, too much and the bottles become a hazard.
The beer is already carbonated
The thing most priming charts obscure is that fermented beer already holds dissolved CO2, and how much depends on temperature. Cold beer holds more; warm beer has let more escape.
So the sugar you need is not "enough for 2.4 volumes" but "enough to make up the difference between what is already in solution and 2.4". The priming calculator works out the residual first, then the shortfall.
Temperature changes the answer by 77%
Take 19 litres at a target of 2.4 volumes with corn sugar, and vary only the beer's temperature:
- 4 °C → 69.7 g (residual 1.48 volumes)
- 10 °C → 91.3 g (1.20)
- 18 °C → 112.9 g (0.92)
- 21 °C → 118.8 g (0.84)
- 24 °C → 123.5 g (0.77)
Cold-conditioned beer needs barely half the sugar of warm beer. Use a generic chart that ignores temperature and you will over-prime a cold batch substantially.
Which temperature to enter
The one that matters is the highest temperature the beer reached after fermentation finished, not the temperature on bottling day. CO2 that came out of solution during a warm spell does not go back in when the beer cools.
A beer fermented at 20 °C and then cold-crashed to 3 °C should be primed for 20, not for 3. Getting this backwards is the commonest cause of over-carbonation.
Corn sugar and table sugar are not interchangeable by weight
Table sugar is sucrose and slightly more efficient per gram than dextrose, so it takes about 91% of the mass for the same result: where the calculator asks for 112.9 g of corn sugar at 18 °C, it asks for 102.7 g of table sugar.
Substituting one for the other gram-for-gram over-carbonates by about 10%. That is survivable once, and it compounds if it becomes habit.
Target the style, and respect the bottle
British ales sit around 1.5–2.0 volumes, American ales and lagers around 2.2–2.6, wheat beers and Belgians 3.0–4.5. Those higher figures are the dangerous ones.
Standard beer bottles are not rated for Belgian carbonation levels. Anything above about 3 volumes belongs in heavy Belgian-style bottles or champagne bottles, capped or corked appropriately. A bottle failing under pressure sends glass a long way.
Bottle bombs, and how to avoid them
Almost every burst bottle comes from one of three things: bottling before fermentation finished, priming with the wrong temperature, or an infection eating sugars the yeast could not.
The defence for the first is two stable hydrometer readings days apart. For the second, the rule above. For the third, sanitation — and treating a batch that keeps gaining carbonation as suspect rather than as merely lively.
Mix it in, gently
Dissolve the sugar in a little boiled water, cool it, and add it to the bottling vessel before the beer rather than dosing individual bottles. Racking on top of it mixes it evenly without splashing.
Uneven mixing is why one bottle in a batch gushes and another is flat, and it is entirely avoidable. Stir gently if you must — oxygen picked up at bottling is the other thing that spoils a good beer.