If you're reading this within 36 hours of pitching yeast, your homebrew is probably fine. The airlock isn't a reliable measure of fermentation. The hydrometer is. Take a reading; check it again in 48 hours. If the number drops, fermentation is happening — even if the airlock never bubbled once.
"Why isn't my airlock bubbling?" is the most common panic in homebrewing. Almost every brewer has stared at a still airlock at 3am wondering if the batch is ruined. In nine cases out of ten, it isn't — the airlock just isn't telling you what you think it's telling you. This article walks through the six real causes, in order of how likely each one is, plus how to actually verify what's going on.
What the airlock actually tells you
The airlock isn't a fermentation meter. It's a one-way valve that releases pressure from inside the fermenter. If pressure builds up faster than it can escape elsewhere, the airlock bubbles. If pressure builds up slowly — or escapes elsewhere — the airlock doesn't bubble, even though fermentation is happening just fine.
Fermentation can be active and producing alcohol without producing visible airlock activity. Conversely, a bubbling airlock isn't proof of fermentation either — temperature changes, barometric pressure shifts, or even bumping the fermenter can produce a few bubbles. The only reliable test for fermentation is a hydrometer reading. Everything else is interpretation.
The airlock isn't a fermentation meter. It's a pressure valve. They look the same when working; they're very different when not.
The six causes, in order of likelihood
What's happening
CO2 is escaping around the lid instead of through the airlock. Fermentation is happening — you just can't see the evidence. This is by far the most common reason for "no bubbling" reports.
How to check
Press firmly down on the lid — you may hear a tiny hiss as the seal tightens. Look at the krausen line on the inside of the fermenter (if you can see it through a translucent fermenter) — if foam is present at all, fermentation has been happening. Smell near the airlock — if you can smell yeast or beer aroma, gas is escaping somewhere.
What to do
Press the lid down evenly until it sits flush. On most plastic fermenters, the lid needs to be seated by working around the rim in sections, not pushed straight down. You don't need to over-tighten — just firm and flush. If the airlock still doesn't bubble after sealing, fermentation may still be happening (see the next causes), or it may be near complete.
What's happening
If it's been 7-14 days since pitching and the airlock stopped, fermentation has likely completed. Active CO2 production tapers off rapidly once the yeast runs out of available sugars.
How to check
Take a hydrometer reading. For a Hazy Pale Ale, finished gravity (FG) typically sits at 1.010-1.014. If your reading is in that range and the airlock stopped recently, you're at the natural end of fermentation rather than a stuck ferment.
What to do
Wait 48 hours and take another reading. If the two numbers match, fermentation is genuinely done and you can start planning to bottle. If they don't match, fermentation isn't quite finished — give it more time.
What's happening
Yeast slows dramatically below its preferred range. For most ale yeasts, that's below about 16°C. The yeast is still alive but barely active, which means almost no CO2 production and no airlock activity.
How to check
Read your stick-on adhesive thermometer. If it shows below 16°C, the wort is too cold. Common in Australian winter brewing in unheated rooms or garages.
What to do
Gently warm the surrounding air. Move the fermenter to a warmer room, wrap it in a blanket, or place a brewing belt or seedling mat around it. Warm the wort gradually — avoid sudden 5-10°C jumps that shock the yeast. Within 12-24 hours of reaching 18-20°C, you should see activity resume.
What's happening
Fresh yeast doesn't start producing visible CO2 instantly. The yeast first needs to rehydrate (if dry), multiply, and consume oxygen before switching to anaerobic fermentation. The "lag phase" typically lasts 12-36 hours and can extend to 72 hours in cooler conditions or with older yeast.
How to check
Look at the timestamp on your pitching. If less than 36 hours have passed, you're likely fine and the yeast just hasn't woken up yet.
What to do
Wait. Resist the urge to open the fermenter or pitch additional yeast. Most "stuck" fermentations at the 24-hour mark spontaneously start within the next 24 hours.
What's happening
If the yeast was past its expiry date, stored badly (heat-damaged liquid yeast), or pitched onto wort that was too hot (above 30°C kills yeast cells rapidly), most of the cells may be dead. The few survivors can take days to multiply enough to produce visible activity, or never reach a viable population.
How to check
Check the date on the yeast packet. If liquid yeast, check whether it was kept refrigerated. Check the wort temperature at the time you pitched — if you remember pitching onto warm wort, this is the likely cause.
What to do
If 72 hours have passed with no activity and no gravity drop, pitch a fresh packet of dry yeast directly into the fermenter. Sprinkle on top, swirl gently to disperse, seal, and wait. Most "dead yeast" situations recover within 24 hours of a re-pitch with fresh yeast.
What's happening
The airlock isn't installed correctly — pushed in the wrong way, missing water, or with the cap on backwards. Less common because most modern fermenter kits are foolproof here, but worth checking.
How to check
Look at the airlock. Sanitiser solution should sit in the U-bend at the lower line. There should be a small plastic float or cap on top that lets gas push through. If the airlock looks dry, has solution above the top line, or the cap is missing, you've found the issue.
What to do
Refill with sanitiser solution to the mark on the side. Make sure the cap is on top. Reinstall in the grommet on the lid.
How to actually verify fermentation is happening
If you've worked through the six causes above and you're still not sure whether anything is fermenting, here's the only reliable test.
- Sanitise your hydrometer, test jar, and a small siphoning tube or turkey baster.
- Briefly open the fermenter and draw a small sample — about 200ml into the test jar.
- Float the hydrometer in the sample and read the gravity.
- Compare to your original gravity (OG) reading on brew day. If the current reading is lower than OG, fermentation has been happening.
- Reseal the fermenter and wait 48 hours.
- Repeat. If the reading drops further, fermentation is still active. If the reading is the same, fermentation has finished (or is genuinely stuck).
Two hydrometer readings 48 hours apart give you the answer to almost every "is something happening" question in homebrewing. They take five minutes and require no special skill. They're more reliable than any amount of staring at the airlock.
Most "not bubbling" cases are: loose lid, finished ferment, or fewer than 36 hours since pitching. Take a hydrometer reading to know for sure. The hydrometer doesn't lie; the airlock often does.
When to actually worry
Genuinely worrying situations are rare. The combination that indicates a real problem:
- 72+ hours since pitching
- Lid confirmed sealed (no air escape)
- Wort temperature in the correct range (18-20°C)
- Hydrometer reading shows no drop from OG
If all four of those are true, you have a stuck ferment that's not going to start on its own. The fix is a fresh yeast pitch, ideally with rehydration first. Sprinkle 11g of dry yeast directly onto the wort, swirl gently, reseal. You should see activity within 24-48 hours.
For a full breakdown of common homebrew problems beyond just bubbling, see our common homebrew mistakes guide. For the broader brewing process and why each stage matters, see how to brew from a fresh wort kit.