Forced Carbonation Calculator

How it's worked out

One chart underneath everything. Dissolved CO₂, temperature and pressure sit in a fixed relationship — Henry's law, fitted to the standard carbonation chart as volumes = k(T)·P_abs − c, with k falling as the beer warms. Rearranged, the pressure to hold a carbonation is P_abs = (volumes + c) / k(T), and the regulator reads that minus the local air pressure. It's the very same equation behind the priming sugar and line-balancing tools, which is why a keg at 2.5 volumes and 38 °F lands on 11.2 psi in all three.

Set and forget. Hold that pressure on the keg and the beer drifts to the matching carbonation on its own. At rest in a corny keg it's slow — figure one to two weeks — because the only place gas crosses into the beer is the still surface at the top. Nothing's wrong if it takes a while; it's diffusion doing patient work. There's no overshoot risk, because the beer can never pass the carbonation that pressure holds.

Burst, and the ceiling. Raise the pressure and you steepen the gradient, so gas dissolves faster — but the target the keg is climbing toward is now the carbonation that burst pressure would hold, which is far past where you want to be. Leave 30 psi on at 38 °F and the beer heads for 4.3 volumes and a gusher. So burst is a race against a clock you can't read precisely: a rough day or two, then drop to serving pressure and taste. The honest, foolproof alternative is to agitate at serving pressure, where the ceiling is your target and overshoot is impossible.

Reading a keg. Let a keg settle at one pressure and temperature long enough to equilibrate and it tells you its carbonation directly through the same chart — a useful way to find out where an unknown keg actually sits. To bring an over-carbonated keg down, vent the headspace and let it re-equilibrate at the lower serving pressure; at rest that's slow, so venting repeatedly over a day, or venting and rousing, gets there faster.

Spunding. Carbonating from fermentation itself: a spunding valve caps the fermenter at a set pressure and lets the rest escape, so the beer saturates to whatever that pressure holds at fermentation temperature. Because the beer is warm, the pressure runs higher than it would cold — 2.5 volumes at 68 °F needs about 29 psi on the valve. The gravity estimate is approximate: each apparent point of remaining gravity yields about 0.99 g of CO₂ per liter — close to 0.5 volumes — from the alcohol that point makes and the carbon dioxide that comes with it, and you need enough to saturate the beer and pressurize the headspace, so a few points above terminal is usually plenty. Seal a touch early rather than late.

Altitude. The regulator reads gauge — pressure above the outside air. To hold a given carbonation you need a fixed absolute CO₂ pressure, so where the air is thinner you set a slightly higher gauge number to reach it: about 2.6 psi more in Denver than at sea level for the same beer. Small, but it's there, and it's switched off until you ask for it.