The gas is the number people never look at
A five and a half gallon batch going from 1.052 to 1.012 ferments about 1.76 kilograms of sugar. Stoichiometry says sugar becomes ethanol and carbon dioxide in fixed proportion — 180.16 grams of glucose gives 92.14 of ethanol and 88.02 of CO2 — so that batch produces about 859 grams of gas. Converted to a volume at 68 degrees F and ordinary pressure, that is about 469 litres, or 124 gallons.
Put that against the vessel. A six and a half gallon fermenter holding five and a half gallons has one gallon of headspace. The fermentation is going to push roughly 124 times that volume of gas out through whatever is at the top, over about three days. That is the fact that reframes the whole subject: headspace is not storage. It is a passage, and the reason the pressure inside a fermenter stays ordinary is entirely that the gas keeps leaving.
Which is why a blocked airlock is a serious condition rather than an inconvenience. There is no version of the arithmetic in which one gallon of empty space absorbs 124 gallons of gas. If the exit closes while the fermentation is running, the pressure goes up until something gives, and both glass and plastic give suddenly.
Krausen is a property of your vessel, not a constant
The foam rise on top of a fermentation is commonly a substantial fraction of the batch volume and it is one of the least predictable numbers in brewing. It moves with gravity, with yeast, with temperature, with how much headspace there is in the first place, and with the shape of the vessel — a narrow carboy pushes krausen up further than a wide bucket holding the same beer, because the same volume of foam has less area to spread over.
So the calculator takes it as an input and says plainly that it supplies no figure. What it does with your figure is useful: it tells you the largest batch that fits inside your vessel at that rise. At 25 percent in a 6.5 gallon fermenter, that limit is 5.2 gallons, which means the standard 5.5 gallon batch is already over it by 0.3 gallons. That is not a disaster — it is a blowoff tube instead of an airlock, decided in advance rather than discovered on the floor at two in the morning.
| Vessel | Batch | Headspace | Krausen at 25% | Verdict |
|---|---|---|---|---|
| 6.5 gal | 5.0 gal | 1.5 gal | 1.25 gal | Fits, with 0.25 gal to spare |
| 6.5 gal | 5.5 gal | 1.0 gal | 1.38 gal | Over by 0.37 gal |
| 6.5 gal | 6.0 gal | 0.5 gal | 1.50 gal | Over by 1.0 gal |
| 7.9 gal | 5.5 gal | 2.4 gal | 1.38 gal | Fits comfortably |
Read down the last column and notice how quickly the margin disappears. Half a gallon more beer costs half a gallon of headspace and adds an eighth of a gallon of krausen, so the overflow widens by 0.625 gallons for every extra half gallon of beer — a gallon and a quarter of extra overflow for every extra gallon in the vessel. That compounding is the mechanism behind the observation that fermenters get dramatically messier over a narrow range of fill levels.
Rate, and why the average is the wrong number
Spread 469 litres over three days and the average is 156 litres a day, or about 109 millilitres a minute. That sounds gentle, and it is, and it is not what your airlock has to cope with. Fermentation is not steady: there is a lag phase where almost nothing happens, an acceleration into a vigorous phase where most of the sugar goes in a day or two, and a long slow tail. Peak output several times the average is ordinary, which puts the busiest moment somewhere over 300 millilitres a minute on this batch.
That is the number that decides whether a given exit is adequate, and this page deliberately does not tell you what exit to use. It gives you the rate and the overflow volume; what tube, what fitting and what vessel to put on the end of it is equipment territory, and the honest answer is that people size it by experience rather than by calculation.
Where the gas number comes from, and its limits
The extract actually fermented is worked out from the two gravities using the standard real extract relation, converting each to Plato and taking the difference between the original extract and the real extract. That gives the mass of sugar consumed. Multiplying by the stoichiometric ratio gives the CO2 mass, and the ideal gas law converts it to a volume at whatever temperature you enter.
Two honest caveats. Yeast diverts a few percent of the sugar into building more yeast rather than into ethanol and CO2, so the true gas figure is a little below the stoichiometric one — treat the number as a ceiling. And some of the CO2 stays dissolved in the beer rather than leaving, which at fermentation temperatures is a modest fraction but not zero. Neither correction changes the conclusion. For the default case the gas is 124 times the headspace, and running the calculator across ordinary vessels, fills and gravities the ratio spans roughly 26 to 570 times — never remotely close to something the empty space could absorb. Fill a large vessel a third full with a weak wort and the ratio drops into single figures, which is the one case where the arithmetic stops being alarming. The exit matters more than the empty space in every other case.
Questions people ask
How much headspace does a fermenter need?
That depends on how much krausen your beer throws on your vessel, which is why the calculator asks rather than tells. What it can do is turn your own observation into a limit: at a 25 percent rise, a 6.5 gallon fermenter takes 5.2 gallons before the foam reaches the brim, and at a 40 percent rise the same vessel takes 4.64. If you have never measured it, watch one batch, note where the krausen peaked as a fraction of the beer depth, and you have a figure for your equipment that is worth more than any general recommendation.
How much CO2 does a fermentation produce?
For an ordinary five and a half gallon batch going from 1.052 to 1.012, about 859 grams, which is roughly 469 litres or 124 gallons of gas at room temperature. The figure scales with the sugar fermented rather than with the volume, so a strong beer makes considerably more per gallon than a weak one. The comparison worth carrying away is that for that batch it is 124 times the headspace, and across ordinary vessels and fill levels the ratio does not drop below about twenty-five times, which is why the vessel depends on the gas leaving continuously rather than on having room for it.
Airlock or blowoff tube?
The calculator answers the volume half of that question — whether your krausen figure exceeds your headspace — and stops there, because what to attach is equipment rather than arithmetic. What is worth stating is the failure mode: an airlock that clogs with krausen becomes a seal, and a fermentation behind a seal builds pressure with nothing to relieve it. The hazard is the blockage rather than the choice of fitting, and a blowoff jar that fills up and submerges the tube end is blocked in exactly the same way an airlock is.
Does a bigger blowoff jar help?
It holds more krausen before it becomes a problem, which is the only thing it does. The calculator compares your overflow volume against the jar so you can see whether the jar is the constraint. What it cannot see is the liquid you start the jar with, which occupies part of it, or how much foam collapses on the way, which is usually a lot. Treat the overflow figure as the amount of liquid that could arrive rather than the amount that will.
Why is the gas figure called a ceiling?
Because it assumes every gram of sugar goes to ethanol and carbon dioxide, and some of it does not — yeast uses a portion to build more yeast, which is a few percent of the total. Some CO2 also stays dissolved in the beer rather than leaving through the airlock. Both corrections push the real figure below the calculated one, and neither is large enough to change any decision you would make on the basis of it. The number is there to establish the scale of the problem, which it does whether it is off by five percent or not.