How to use the force carbonation pressure calculator
- Choose whether you want the regulator pressure for a CO2 level, or the CO2 level a pressure will give.
- Enter the beer temperature and pick Fahrenheit or Celsius. Use the temperature of the beer in the keg, not of the room.
- Enter the CO2 volumes you want, or the gauge pressure you plan to use, and pick psi, kPa or bar.
- Read the pressure to set, the two other pressure units and the tables of other CO2 levels or other temperatures.
Formula
P = -16.6999 - 0.0101059 T + 0.00116512 T^2 + 0.173354 T V + 4.24267 V - 0.0684226 V^2
P is the gauge pressure in psi, T the beer temperature in degrees Fahrenheit and V the volumes of CO2
To find V from a pressure, solve the same equation for V (a quadratic) and keep the root between 0 and 31
The equation is a fitted curve for beer near sea level, not a law of physics. Gauge pressure means pressure above the air around you, so 0 psi on the gauge is atmospheric pressure.
A keg at 38 F aiming for 2.5 volumes
At T = 38 and V = 2.5 the terms are -16.6999, -0.3840, +1.6824, +16.4686, +10.6067 and -0.4276. They add to 11.246 psi, so the tool shows 11.2 psi, which is 78 kPa or 0.78 bar. The side table shows that the same beer at 2.0 volumes needs 6.0 psi and at 3.0 volumes needs 16.5 psi.
Run it the other way: with 10 psi on the gauge at 38 F the quadratic gives 2.38 volumes. A warmer beer needs more pressure for the same level. At 50 F and 3 volumes the answer is 23.8 psi, and at 3 C (about 37.4 F) and 2.2 volumes it is 7.8 psi.
Why temperature matters as much as pressure
A cold liquid holds more dissolved gas than a warm one at the same pressure. That is why the same regulator setting gives a livelier beer at 36 F than at 46 F, and why a keg that warms up will slowly push CO2 back out of the beer into the headspace. Set the pressure for the temperature the keg will actually sit at.
Dissolving the gas takes time. Holding the pressure steady for a few days brings the beer to the level in the table, while shaking the keg or raising the pressure speeds it up but is harder to control. If you are still planning the batch, the Gravity Dilution and Boil-Off Calculator and the ABV Calculator cover the earlier steps.
Reading the zero and the limits
For cold, lightly carbonated beer the formula can give a negative number. That means the beer would hold that much CO2 even with no pressure at all, so the tool shows 0 psi and says why. The inputs stop at 32 to 80 F, 0.5 to 5 volumes and 60 psi, where the fitted curve was built to be used. Outside those ranges the tool will not guess.
Assumptions and limits
- The beer is at one steady temperature and has reached equilibrium with the gas above it.
- The gauge reads pressure above the surrounding air at about sea level. At high altitude the same gauge number holds a little less CO2, and this tool does not correct for that.
- Pure CO2 is used. A mixed gas, a long gas line or a regulator that is off in calibration will change the real result.
- The result is a starting point for a recipe or a keg, and tasting the beer is the real test.
Frequently asked questions
What does a volume of CO2 mean?
One volume is the amount of CO2 gas that would fill the same space as the beer when measured at standard conditions. A beer at 2.5 volumes holds gas worth two and a half times its own volume.
Does this work for soda, cider or sparkling water?
The fitted curve was made for beer. Other drinks dissolve gas a little differently because of sugar, alcohol and acid, so treat the numbers as a rough guide and adjust by taste.
How long should I leave the keg on gas?
Without shaking, the beer usually needs several days to reach the level for its pressure and temperature. The time depends on the keg size and how cold it is, which this tool does not model.
Why is the pressure shown as 0.0 in some cases?
Low volumes in cold beer give a negative result from the formula. A negative gauge reading would mean a vacuum, so the tool shows zero and explains that the beer already holds that much CO2 at normal air pressure.