Why Isn’t My Beer Fermenting?
Most healthy fermentations show visible signs within 12–48 hours, but a slow start is normal and rarely means anything is wrong. A quiet airlock is not proof that fermentation has failed — CO₂ often escapes around a loose lid instead of through the airlock. The only reliable test is a hydrometer: if gravity has dropped from your original reading, it’s fermenting.
Nothing rattles a first-time brewer like pitching yeast, checking the airlock a few hours later, and seeing… nothing. Take a breath: a slow start is one of the most common experiences in homebrewing, and the overwhelming majority of the time your beer is completely fine.
Lag time is normal
After you pitch, yeast doesn’t start converting sugar to alcohol immediately. It spends a lag phase absorbing oxygen and nutrients and multiplying to a population large enough to ferment your whole batch. This typically takes 12–24 hours, and with older yeast, cold wort, or an under-sized pitch it can stretch to 48–72 hours. Seeing no activity six hours in tells you almost nothing.
The airlock is a liar
New brewers treat airlock bubbling as the heartbeat of their beer, but an airlock only bubbles when the fermenter produces more CO₂ than can escape anywhere else. Plastic bucket lids and many fermenter gaskets don’t seal perfectly — so the CO₂ quietly leaks around the lid and the airlock never moves, even during vigorous fermentation. A still airlock is not evidence of a problem.
How to actually check
There are two trustworthy signs:
- Krausen — a foamy, tan head of bubbles on top of the beer. If you see it, you’re fermenting. Full stop.
- A gravity drop — take a hydrometer reading and compare it to your original gravity (OG). Any meaningful drop means the yeast is working. This is the definitive test.
When it really is stuck — and what to do
If it’s been 72+ hours with no krausen and no gravity change, work through the usual suspects:
- Too cold. The single most common cause. Ale yeast wants roughly 65–70 °F (18–21 °C). A fermenter in a cold garage or basement can drop the yeast dormant. Move it somewhere warmer and give it a day.
- Under-pitched or old yeast. Expired or badly stored yeast may lack the viable cells to get going. Pitching a fresh, healthy packet usually restarts it.
- Under-aerated wort. Yeast needs oxygen up front. If you didn’t aerate, gently rousing the fermenter can help.
Raising the temperature to 68–70 °F and gently rousing the yeast back into suspension fixes the large majority of genuinely stalled batches. For a deeper dive on high final gravity, see our guide on stuck fermentation.
Frequently asked questions
How long should I wait before worrying?
Give it at least 48–72 hours before considering fermentation stuck. Many healthy batches take 24–48 hours just to show visible activity, especially with cooler wort or older yeast.
My airlock never bubbled but there is foam on top. Is that OK?
Yes. Foam (krausen) is a definitive sign of active fermentation. A silent airlock usually just means CO₂ is escaping around the lid instead of through the airlock.
Can I just pitch more yeast?
If it has been more than 72 hours with no gravity change and warming the fermenter did not help, pitching a fresh, healthy packet of yeast is a reliable fix. Check temperature first, since cold is the most common cause.