The single most reliable answer is this: fermentation is finished when your gravity stops changing. Not when a certain number of days have passed, and not when the airlock stops bubbling. The only instrument that actually measures fermentation is a hydrometer or refractometer — everything else is a guess.
Open BrewLog →Take a gravity reading, then take another two or three days later, and a third a few days after that. When two or three consecutive readings are identical (within measurement error, say ±0.001), fermentation is done. A typical ale might read 1.013, then 1.011, then 1.011 again — that last unchanged pair is your green light.
Time is only a rough guide. A healthy, warm-fermented ale can finish in 4 days; an under-pitched big beer or a cold lager can take three weeks. Bottling on a calendar instead of a measurement is the classic cause of bottle bombs: leftover sugar keeps fermenting in a sealed bottle, builds pressure, and the bottle can burst. Always confirm with gravity before you package.
A quiet airlock means CO₂ production has slowed — but airlocks bubble from any pressure change, including temperature swings, and a slightly loose lid or bucket seal will never bubble at all even during a vigorous ferment. Bubbling tells you nothing precise. Trust the gravity, not the glug.
Stable gravity confirms fermentation stopped; comparing it to your target final gravity tells you whether it stopped in the right place. If your readings are stable but sitting well above where the yeast's attenuation says it should land — for example holding at 1.025 when you expected 1.010 — you may have a stuck fermentation rather than a finished one, and it's worth warming and rousing before you call it.
Once gravity is stable for several days and in the expected range, you're clear to cold-crash, dry hop, keg, or bottle. Many brewers give it a couple of extra days at fermentation temperature first so the yeast can clean up green-beer compounds like diacetyl and acetaldehyde.
This is exactly what BrewLog is built for: log each dated reading and it shows the trend flattening and flips a batch from fermenting to done when gravity goes stable. No more guessing, no more bottle bombs. Free, offline, private to your browser.
Track gravity to done →