A hydrometer is the only reliable way to confirm fermentation has finished. If you don’t have one, a few signs together can tell you it’s likely complete. When in doubt, give it more time, as bottling too early is the most common cause of over-carbonated or burst bottles.
Why airlock bubbles aren’t reliable
Slow or stopped bubbling doesn’t always mean fermentation has finished:
- Leaks: if the lid or grommet isn’t perfectly sealed, gas escapes there instead of through the airlock, so fermentation can be active with no bubbles at all.
- Temperature: a drop in temperature can slow or pause yeast activity, which may restart when it warms up again.
- Dissolved gas: a brew can keep releasing dissolved CO₂ after fermentation ends, so bubbling may continue slowly even when it’s finished.
Treat the airlock as a rough guide only, never as the deciding factor.
Signs fermentation is likely finished
Look for all of these together, not just one:
- Activity has stopped for several days. No bubbling for at least 3–4 days at a steady temperature.
- The brew is clearing. The liquid is becoming clearer, and a layer of yeast sediment has settled at the bottom of the fermenter.
- For beer, the foam has dropped. The foamy layer (krausen) on top has fallen back into the beer.
- It tastes less sweet. Take a small sample with clean, sanitised equipment. If it tastes noticeably less sweet than at the start and doesn’t change over a few days, the yeast has likely finished. Meads and sweeter wines may still taste sweet when finished.
- Enough time has passed. Check the recommended fermentation time in your kit instructions. Mead often takes several weeks or longer, so allow extra time.
If you’re unsure, wait another week. Extra time rarely does any harm.