Pressure and temperature, and nothing else
How much CO2 a beer holds is set by two things: the pressure of gas above it and the temperature of the liquid. Nothing else in the keg matters to the equilibrium — not the beer, not the volume, not how long the gas has been connected. Given enough time at a fixed pressure and temperature, the beer arrives at one particular carbonation level and stays there.
The relation used here is the standard fit that most brewing software uses:
volumes = Pabs × (0.01821 + 0.09011 e−(T−32)/43.11) − 0.003342
with the pressure in absolute psi and the temperature in degrees F. Rearranged for pressure, it says that 2.5 volumes at 38 degrees F needs about 11.2 psi on the gauge. The same 2.5 volumes at 48 degrees F needs about 16.4 psi, and at 68 degrees it needs about 29. That temperature sensitivity is the whole reason a keg that poured properly in a cold fridge foams when the fridge is warm: the pressure that held 2.5 volumes at 38 degrees holds far less at 50, and the difference comes out of solution as foam.
Where the gas actually goes, which is mostly not into the beer
People size a CO2 cylinder by thinking about carbonation and are then surprised by how fast it empties. Run the default five gallon keg through. Dissolving 2.5 volumes into 18.9 litres takes about 93 grams of CO2 — that is the part everyone accounts for. Purging a quarter gallon of headspace three times at 15 psi costs about another 10.5 grams. And then every drop of beer that leaves the keg is replaced by gas at serving pressure, which for a full keg at 11 psi is about 33 litres of gas, or 61 grams.
So of the roughly 164 grams a keg consumes, only about 56 percent is carbonation. The dispensing gas is the second biggest term and it is invisible because it leaves slowly. A five pound cylinder holds 2,268 grams, which at these settings is thirteen kegs — and thirteen is about what people report, which is a reasonable sign that the accounting is right.
| Where the gas goes | Grams, default keg | Share |
|---|---|---|
| Dissolved in the beer | 92.7 | 56% |
| Replacing beer as it pours | 61.1 | 37% |
| Purging headspace, three times | 10.5 | 6% |
What that table leaves out is leaks, and leaks are the reason real cylinders empty faster than any calculation. A seal that loses gas slowly enough that you never hear it can still empty a cylinder over a few weeks, and it will not show up anywhere except in the number of kegs you got.
Balanced lines are the pressure arithmetic nobody does
A draft system pours properly when the resistance between the keg and the tap uses up the pressure the keg is under. Set 11 psi on the regulator and connect it to a two foot length of wide tubing and the beer arrives at the tap still at most of that pressure, comes out of solution on the way, and pours as foam. The fix is not to turn the regulator down — that undercarbonates the beer over the following days — it is to make the line long enough.
The arithmetic is a subtraction. Start with the keg pressure. Take off half a psi for every foot the beer has to climb, because a vertical column of beer costs pressure to lift. Take off whatever you want left at the tap for the pour itself. Whatever is left is what the line has to absorb, and dividing it by the tubing resistance in psi per foot gives the length. For the default case that is 11.2 psi less 1 psi of lift less 1 psi at the tap, leaving 9.2 psi, which at 2.2 psi per foot is about 4.2 feet of line.
The resistance figure has to come from the tubing specification rather than from this page, because it varies by a large factor with bore and with material, and a figure quoted for one product is meaningless for another.
What this page will not do
It will not tell you what carbonation level a beer should have. Carbonation is a recipe decision and it belongs to whoever wrote the recipe; the calculator takes whatever number you give it and reports an equilibrium. It also states no procedure for filling, purging, pressurising or connecting anything, and no rating for any component. A keg is a pressure vessel, the fittings are rated parts, and what any particular assembly is good for comes from the people who made it.
Two hazards are worth naming plainly and leaving there. Kegs and cylinders store real energy under pressure. And carbon dioxide is heavier than air and displaces oxygen, so a cylinder discharging into a closet, a walk-in or an unventilated cellar is an asphyxiation hazard that arrives without warning.
Questions people ask
What pressure do I set to carbonate a keg?
Whatever the equilibrium relation gives for the temperature the keg actually sits at and the carbonation level your recipe calls for. The two inputs are not interchangeable: 2.5 volumes needs about 11 psi at 38 degrees F and about 16 psi at 48, so a keg in a warmer fridge needs meaningfully more gauge pressure for the same result. Set the pressure from the temperature the beer is at rather than from a number somebody quoted, because the number they quoted was for their fridge.
Why does my beer pour as foam even though the carbonation is right?
Almost always because the line is too short for the pressure, so the beer arrives at the tap still under most of the keg pressure and breaks out of solution as it opens. The temptation is to turn the regulator down, which pours better today and slowly flattens the beer over the following week as it re-equilibrates to the lower pressure. The durable fix is line length: work out how much pressure the line has to absorb after the lift and the pour allowance, and use enough of the tubing you have to absorb it.
How many kegs does a five pound CO2 cylinder do?
At the default settings here — five gallons, 2.5 volumes, 38 degrees, three purges, dispensed at serving pressure — the arithmetic gives about thirteen, and that is roughly what people report in practice. The figure moves with all of it: a higher carbonation level, a warmer keg, more purging or a partly empty keg dispensed over a long time all cost more gas. What ruins the estimate entirely is a slow leak, which no calculation can see and which is the usual explanation when a cylinder does far fewer kegs than it should.
Does the calculation change for a half barrel or a sixtel?
Only through the volume, which you enter directly. The pressure for a given carbonation level at a given temperature is identical regardless of keg size, because it is an equilibrium between the gas and the liquid rather than anything about the vessel. What scales is the gas: dissolved CO2 and dispensing gas both go up in proportion to the beer, while purging goes up with the headspace. Enter the actual beer volume and the actual headspace and the totals follow.
Is priming a keg with sugar the same as force carbonating it?
It reaches the same place by a different route and the arithmetic is different, so it lives on a different page. Priming works out a weight of fermentable sugar whose fermentation produces the CO2, which means it depends on how much CO2 the beer is already holding and therefore on the warmest temperature the beer has reached. Force carbonating works out a pressure and lets the equilibrium do it. The priming sugar calculator covers the first; this page covers the second. Neither one tells you which to use.