Yes, beer is carbonated. Most of that carbonation comes from carbon dioxide (CO2) that yeast naturally produces during fermentation, then either gets trapped in a sealed container or supplemented afterward with force carbonation.
How Beer Actually Gets Carbonated
During fermentation, yeast consumes sugar and produces two byproducts: alcohol and CO2. If that fermentation happens in an open container, the CO2 just escapes into the air. To carbonate the beer, brewers need to trap that gas, or add more of it, before the beer is served.
There are two main methods. Natural carbonation seals the beer (in a bottle, cask, or keg) with a small amount of unfermented sugar still present, so the yeast keeps working in the sealed container and the CO2 it produces has nowhere to go except into solution in the beer. Force carbonation skips that step entirely: the brewer chills the finished beer and pumps CO2 gas directly into a sealed keg under pressure, which dissolves into the liquid according to Henry’s Law (the colder and higher-pressure the environment, the more gas stays dissolved).
| Method | How it works | Common use |
|---|---|---|
| Natural (bottle conditioning) | Priming sugar added before sealing lets yeast produce CO2 in the bottle | Homebrew, Belgian ales, some craft bottles |
| Natural (cask conditioning) | Beer finishes fermenting inside the sealed cask itself | British real ale, cask ale |
| Force carbonation | CO2 gas injected directly into a sealed, chilled keg | Almost all commercial draft and canned beer |
Most fermentation on its own only produces 1.2 to 1.7 volumes of dissolved CO2. Standard packaged beer is carbonated to 2.4 to 2.8 volumes, so even naturally-conditioned beer usually needs the extra push from a sealed, pressurized container to reach a normal fizz level.
Why Some Beers Use Nitrogen Instead
Nitro beers, most famously Guinness, use a gas blend that is roughly 70 to 75% nitrogen and 25 to 30% CO2, not pure CO2. Nitrogen bubbles are much smaller than CO2 bubbles, which creates the dense, creamy, long-lasting head and smoother mouthfeel nitro beers are known for. Pure nitrogen alone cannot carbonate a beer, since it does not dissolve into liquid the way CO2 does, which is why every nitro beer still needs some CO2 in the gas mix.
For a real-world example, our Guinness Draught review covers exactly how that nitro pour and cascade effect works in the glass.
Carbonating Your Own Homebrew
If you are bottling at home and want a controlled, repeatable carbonation level instead of guessing with priming sugar, a carbonator cap lets you force-carbonate directly in a PET soda bottle using a standard CO2 setup.
For a properly rigged setup instead of a one-bottle cap, our homebrew recipes hub links out to full keg-and-CO2 force-carbonation processes for finished batches.
How to Tell If Your Homebrew Carbonated Properly
Give bottle-conditioned homebrew a full 2 weeks at room temperature (68 to 72 F) before checking, since carbonation is a chemical reaction that needs time, not just pressure. Chill one test bottle for 24 hours, then open it over a sink: a healthy pour should build a rocky white head that settles within a minute or two and leaves visible lacing on the glass as you drink. A flat pour with no head usually means not enough time has passed or too little priming sugar was used; a gusher that foams over instantly usually means too much priming sugar, or bottling before fermentation had actually finished.
FAQs
Does all beer have the same amount of carbonation?
No. Standard lagers and ales run 2.4 to 2.8 volumes of CO2, British cask ale is much lower and softer, and nitro beers like Guinness use a nitrogen-CO2 blend that feels less fizzy but has a denser, creamier texture.
Can beer go flat?
Yes. Once opened, dissolved CO2 escapes into the air over time, which is why a beer left open or improperly resealed loses its carbonation and starts to taste flat.
Is nitrogen or CO2 responsible for beer’s fizz?
CO2. Nitrogen alone does not carbonate liquid the way CO2 does, it is added alongside CO2 specifically to create smaller bubbles and a creamier texture, not to replace the carbonation itself.
Why is homemade beer sometimes over-carbonated?
Usually because too much priming sugar was added before bottling, or the beer was bottled before fermentation was fully finished, leaving extra sugar for the yeast to convert into CO2 inside a sealed bottle.
