Why Is My Beer Flat? Carbonation & Priming
Bottle-conditioned beer needs 2–3 weeks at room temperature (65–70 °F) to carbonate — if you chilled it early or checked at one week, it’s just not done yet. The standard priming rate is about ¾ cup (≈4 oz / 130 g) of corn sugar per 5 gallons, targeting ~2.4 volumes of CO₂. Truly flat beer usually means too little priming sugar, too cold a conditioning spot, or tired yeast.
You bottled a week ago, cracked one open, and it poured flat. Before you conclude the batch failed, know that carbonation takes time — and “flat at one week” is completely normal.
Give it 2–3 weeks, warm
Bottle conditioning works by feeding the yeast a small, measured dose of priming sugar at bottling. The yeast eats it and produces CO₂ that dissolves into the beer. This needs about 2–3 weeks at 65–70 °F. Conditioning in a cold basement or garage stalls it — the yeast slows right down. A common mistake is refrigerating bottles too soon; chilling pauses carbonation. Test one bottle at two weeks, another at three, until it’s where you want it.
How much priming sugar?
The classic rule of thumb is ¾ cup (about 4 oz / 130 g) of corn sugar (dextrose) per 5 gallons, which lands around 2.4 volumes of CO₂ — right for most American ales. But the ideal amount depends on the style and on how warm the beer was after fermentation (warmer beer retains less residual CO₂, so it needs more sugar):
- British ales, milds, dry stout: lower, ~1.7–2.2 volumes.
- American pale ale, IPA, amber, porter: ~2.2–2.6 volumes.
- German wheat & many Belgian styles: high, 3–4+ volumes.
Don’t eyeball it across styles — use a priming sugar / carbonation calculator that factors in your batch size, target volumes, and beer temperature. Too little sugar gives flat beer; too much risks over-carbonation and, in extreme cases, bottle bombs.
If it’s still flat after 3 weeks
- Too cold. Move the bottles somewhere warm (68–72 °F) and wait — this revives most “flat” batches.
- Too little priming sugar, or it wasn’t mixed evenly through the bottling bucket, so some bottles got shorted.
- Exhausted yeast, common in high-alcohol beers or after long aging. You can add a few grains of fresh dry yeast per bottle when priming a big beer.
- Poor seals — bad caps or a weak capper let CO₂ escape.
Frequently asked questions
How long does bottle carbonation take?
About 2–3 weeks at room temperature (65–70°F). Colder storage takes longer. Test a bottle at two weeks and again at three until the carbonation is where you want it.
How much priming sugar per 5 gallons?
Roughly ¾ cup (about 4 oz or 130 g) of corn sugar for standard American ales, which yields around 2.4 volumes of CO₂. Adjust up for wheat and Belgian styles and down for British ales, ideally with a carbonation calculator.
Can I fix flat beer that is already bottled?
Often yes. First move the bottles somewhere warm for another week or two — cold is the usual culprit. If they were genuinely under-primed, you can uncap, add a tiny measured amount of sugar, and re-cap, though warming first is easier and lower-risk.