CO2 Levels

Is your beer in bottles or kegs flat or overcarbonated?  I see questions about these problems in beer brewing magazines and beer blog sites all the time. I even know a Professional Brewer that has struggled with this problem. I have had this problem and I know other homebrewers also struggle with this problem as well. The answers from experts are somewhat complicated at times and are often difficult to understand.


Carbonating kegs can be forgiving. If it is too flat or too carbonated the remedy is easily to correct.  By adjusting the CO2 pressure, a flat draft beer can be corrected.  An overly carbonated beer can be fixed over a period of a couple days by releasing pressure and then raising the pressure to a proper serving pressure. Specifics on procedures to correct both over and under carbonation will be discussed later in this article. However, according to, an overly carbonated keg should be addressed as soon as possible because the longer the keg is over carbonated the more the taste of the beer might be affected, even after correction. In other words, a keg of beer should not be over carbonated for more than a week or so, but correcting this in a few days would be ideal. A keg that does not have any leaks should hold pressure for many months if aging for that long is desired. This does not really answer the underlying issue of proper carbonation so that approximate or  proper carbonation is achieved the first time.  I mentioned a ProBrewer having this issue for his Micro Brewery, so he consulted with one of the 2 biggest Breweries in the state of Arizona. They told him that even they didn't fully understand the carbonation process.  The theory and math follows at the end of this article, titled Messy Details.

Keg charging pressure for serving is also very important.  According to Kegworks and other published experts-a keg CO2 pressure of 10-12 psi is ideal for most beers and ales and a pressure of 25-30 psi for stouts and other nitrogen reliant beers and ales. recommends 12-14 psi for serving most beers and ales. I would use 12 psi as it seems to be the the sweet spot to start out as a best known practices (BKM) for serving.

Here are recommended procedures and charts for carbonating at different temperature and pressures for all styles of beer.    

High Alcohol by Volume beers such as Imperial Stouts, Imperial and double Imperial IPA, Barley and Wheat Wine are more resistant to carbonation. Carbonating these beers requires higher CO2 pressure for carbonation in the 20 psi range and 12 psi or higher for serving.

Many beer brewers I have interviewed also stress that a ¼” delivery line be a minimum of 4 feet to reduce foaming is necessary, 6 feet may work better. A line of this inside diameter and length acts as a resistor in an electronic circuit to slow the flow gradually of beer without greatly affecting the line pressure. In a kegerator (a converted refrigerator or freezer) with a horizontal faucet -the line stays cold and the faucet stays somewhat cold which reduces foaming. Using a dispensing tower or a length of line that does not stay cold in the tower will generate foaming if the line and tower will not stay cold.  Also the beer that comes out first will be considerably warmer than beer that which dispenses seconds later. This will create more foam.  A tower can be cooled by using an old recycled computer microprocessor fan to blow cold air up from a small refrigerator or chest freezer through a length of ½” or so tubing. Long runs  in a bar or brewpub utilize cooling glycol lines and use insulation to keep the long lines and tower cooled.

BKM for less than high ABV beers

First and foremost exercise patience. Don't expect to be able to start serving a keg immediately after filling a keg. It takes time to diffuse CO2 into your beer. Some recommendations are to:

Charge your keg to 12 psi and purge by manually operating depressing the gas quick disconnect fitting  to a low pressure, but not zero. Do this three times to reduce as much unwanted oxygen as possible.

Recharge the keg to 12 psi and ideally wait a month before fully activating the keg. Periodically recheck the keg to maintain 12 psi.  Sample anytime you feel curious but for the most part no adjustment will be needed for the month, but keep the keg at 12 psi. This method works successfully for many brewers most of the time.  

Use a 4 foot or longer dispense line kept cool for sampling or keg activation.  The long ¼” line should reduce foaming.

After a month determine whether the beer is flat or too foamy. If flat-raise the pressure to 20 psi ½ a day, then reduce keg pressure to the 12 psi delivery pressure, and test, repeat as necessary.  It should take roughly about 7 seconds in a standard ale pint glass to fill and leave about an inch of foam. Any fill time much faster or much slower than 7 seconds is either too fast or too slow. Increase or decrease the delivery pressure to achieve satisfactory results.  Some pressure may have to be manually relieved if filling is too fast, either manually or using the relief valve on the regulator after lowering the regulator setting.  If there is a little more or less foam than 1 inch, be patient and observe over a few pints or so for consistency.

BKM for high alcohol beers

Follow steps 1-3 for general beers except in step 2 use 25-30 psi for about 1-2 days and test. Repeat if still flat.

For serving pressure start at 20 psi and draw a pint. You may find that 25-30 psi will be necessary but try to use the 7 second rule to judge carbonation.


Improper carbonation in bottles is unforgiving.  Most breweries that sell bottles do not have a carbonating problem in bottles because they fill from a pressurized tank and fill their bottles under pressure in an automated machine.  However the fill tank, like  a keg must be carbonated properly.  The homebrewer carbonates with a process that is referred to as bottle conditioning. Homebrewed beer, and some breweries like Moylans and Sierra Nevada beers, as well as some Belgian beers, when added to a bottle still contains yeast that could process more sugar into alcohol.  Due to health of the yeast or environmental conditions and fermentation issues-that fermentation prematurely halts. The recipe will usually specify the amount of sugar needed and will suggest the type of sugar needed for priming-usually corn sugar.  I bring this up because any type of sugar will work, but because each type of sugar ( dextrose, glucose, and sucrose ) has different molecular weights, a different gram weight of that sugar will be needed for priming.  Dry glucose or dextrose is corn sugar, dry sucrose is table sugar. Glucose is made from starches like corn or ripe fruit. We do not discuss liquid sugars as the amount of moisture ( percentage of water ) muddies up the equation.   The amount of moisture or liquid versus wort ( sugar ) can vary greatly and does not fit the formulae.


Calibrate the carboy or priming bucket by placing a piece of tape on the side, adding 1 liter at a time so that you can place volume marks on the tape.  Liters work best in calculations.  24 or more liters may be necessary for a 5 gallon batch.  After 16 liters it would be helpful to have graduations in 1/10 liters. Calibrate and mark your vessels for at least 4 liters greater than the expected batch volume if using larger batch sizes such as 10 and 14 gal.( 37.85 -52.99 liters ).

Knowing your beer volume and the amount of sugar required, coupled with an accurate weight of your priming sugar, will yield more predictable results. If you have the exact quantity, in liters specified in the recipe, then use the quantity of priming sugar specified. If the beer volume is higher or lower the adjust the priming sugar accordingly. But be as precise as possible. Sugar weghts should resolve down to a 1/10 of a gram, especially if individual bottles are being primed.

Yeast viability is important. If an extended secondary fermentation has been used, then a small pinch of live working yeast added to the bucket may be considered or necessary.Home brewers who bottle their beers suffer with the problem of over or under carbonation all the time. Before getting into great technical issues, the one recurring theme from experts is that proper carbonation starts with the very careful weighting of sugars and measure of volume of liquid in liters for adding the proper amount of priming sugar.  Carbonation is expressed in volumes.  A volume is equal to 2 grams per liter of CO2. his will be discussed in the following section.  Note that quite often many home brewed Belgian beers are over carbonated according to Randy Masher in his book Radical Brewing regarding Belgian brewing.  Open the bottle, pour the beer, and let I settle down. I don’t know the reason but I suspect it is the sugars and or yeast.

The Messy Details

If you are creating your own recipe you will have to determine, on you own, how much priming sugar to use. If you are not going to guess and are interested in the messy formulas, then here is some of it. The following is an example from Brew Your Own Magazine in December 2015.

Example: A beer containing 5.3 gal. or 20 liters.  Here is the mysterious part. It is assumed that there is 3.2 grams of CO2 per liter. I don't know where this number comes from so I did some research and found that this number comes from a chart that gives a different number depending on fermentation temperature. Here's a chart of CO2 volume versus fermentation temperatures.  The higher the fermentation temperature the lower the CO2 volume.  Given: 2 gr. CO2/liter =1 volume therefore 3.2 gr./liter divided by 2 gr./liter=1.6 volumes.  Recipes usually suggest between 2.6-3.3 volumes for the finished beer.  In this example 5.2gr./l or 2.6 volumes will be the target. since 3.2 gr./l was the initial CO2 level and 5.2 gr./l is the target, then 2 gr./l must be dissolved into each liter of finished beer. Which means 20liters x 2 gr./l =40gr. CO2 are needed. From the data in the charts for Glucose ( Corn Sugar ) 2.05 gr. of Glucose yields 1 gr. CO2.  40 gr. CO2 X 2.05 gr. Glucose/gr. CO2=77.6 gr. Glucose needed to properly carbonate your beer.

W(gr./mole)/W(gr.CO2 yield for specific sugar)=yield of sugar in gr.

W(gr./mole Glucose or Dextrose)= 180 gr./mole      

W(gr./mole Sucrose)=384 gr./mole

Glucose C6H12O6  has a molecular weight of 180 gr. per mole. When fermented yields 2 molecules of CO2 and 2 molecules of methanol.   This yields 88 gr. CO2.

180/88=2.05gr. glucose. So 2.05 gr. Glucose or Dextrose yields 1 gr. CO2.

Sucrose C12H22O11 has a molecular weight of 384 gr.per mole. When fermented yields 4 molecules of CO2 and 4 molecules of alcohol.


Weigh and measure carefully. Be precise and be accurate. Modify priming sugar based on actual volume of beer. You will need a scale that resolves to a ½ gram.

Follow the recipe. Ensure whether or not that there is the proper volume in liters. Adjust the suggested sugar weight or volume up or down to match the actual number of liters in the bottling vessel.

Follow Best Known Methods.

Be patient.

Jim St. Germain

Jim St. Germain is president of St. Germain Cellars and the resident hops enthusiast. When he isn't evangelizing IPAs (75+ IBUs!), he enjoys a nice glass of pinot noir or good whiskey, neat.