Caleb MorganBy Caleb Morgan

Updated November 2026

To put an airlock on a fermenter, sanitize the airlock and stopper, push the stopper firmly into the hole in the lid, fill the airlock to the marked line with water or sanitizer, and then insert the airlock into the stopper. Done right, the airlock lets carbon dioxide escape while keeping oxygen and wild microbes out of your beer or wine. The whole job takes about five minutes once your gear is clean.

Gather the Right Parts Before You Start

Most standard fermenters use a three-piece airlock: a bubbler tube, a cap that holds the water, and a rubber or silicone stopper that fits the lid. Check that the stopper’s hole matches the diameter of the airlock’s stem. A stopper that is too small will leak, and one that is too large may crack the lid when you push it in. Keep a spare stopper on hand, since they wear out over time.

Shopping for one? See our guide: FastFerment Conical Fermenter 7.9 Gallon Review (2026).

Step-by-Step: Sealing the Fermenter

  1. Sanitize the airlock, stopper, and the inside of the lid with a no-rinse sanitizer. Follow the sanitizer’s contact time on the label.
  2. Fill the airlock with water or sanitizer up to the fill line. Filling it too high can push liquid back into the fermenter when gas pressure changes.
  3. Snap the airlock cap onto the bubbler tube so the water stays sealed inside the tube.
  4. Press the stopper into the lid’s hole. Push it in until it sits flush, but do not force it. Wet the stopper slightly with sanitizer to make insertion easier.
  5. Push the airlock assembly into the stopper until the stem is seated firmly.
  6. Check the seal by looking for the water level in the airlock to stay steady and for gas to escape in bubbles.

Checking That the Airlock Works

Within 12 to 48 hours of pitching yeast, you should see bubbles moving through the airlock. Bubbling is a sign of active fermentation, not a guarantee that the seal is perfect. If you see no bubbles at all after two days, press gently on the lid to check for leaks. Sometimes the lid isn’t fully seated, or the stopper is slightly out of alignment. If the bubbling slows and then stops, fermentation is usually finishing rather than failing.

Many homebrewers expect the airlock to bubble constantly. In reality, bubble rate varies with temperature, yeast strain, and how much gas has already escaped. Watch the seal and the gravity readings instead of the bubbles alone. Taking readings with a hydrometer gives you a more accurate picture of progress.

Common Mistakes to Avoid

  • Skipping sanitization on the stopper and airlock. Even a clean-looking airlock can carry bacteria that spoil the batch.
  • Filling the airlock above the line, which can draw liquid back into the beer.
  • Forcing a stopper that doesn’t fit, which can crack the lid or leave gaps.
  • Opening the lid during active fermentation to check bubbling. Each opening risks contamination and oxygen exposure.
  • Using an airlock on a fermenter that is overfilled. Foam can clog the airlock, so a blow-off setup may be safer for vigorous fermentations.

A three-piece bubbler airlock is the standard choice for most fermenters, and a three-piece design is simple to clean. Look for a model whose stem matches your stopper size.

A drilled rubber stopper in the size your lid requires is the other essential. Keep a spare on hand in case the first one cracks. View on Amazon

A no-rinse sanitizer makes the sealing step quick, because you can apply it to the airlock and stopper without a rinse that might introduce contaminants. View on Amazon

FAQ

Should the airlock be filled with water or sanitizer?

Either works. Water is common and inexpensive, while sanitizer adds a small margin of protection if liquid gets pulled back into the fermenter. Just avoid anything that could affect flavor or be harmful to yeast.

What if my airlock is not bubbling after 24 hours?

Check that the lid is sealed and the stopper is fully seated. Fermentation may simply be slow due to temperature, so confirm the wort or must is in the correct range for your yeast before worrying.

Can I use an airlock without a stopper?

No. The stopper creates the seal between the airlock and the lid, and without it gas escapes around the airlock instead of through the water, which exposes the batch to oxygen and contamination.

Conclusion

Putting an airlock on a fermenter comes down to clean parts, a snug stopper, and the correct water level in the airlock. Once it’s sealed, check for bubbles and look for steady activity over the next few days. With a reliable seal in place, you can let fermentation do its work while you focus on the rest of the brew.

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Caleb Morgan

About Caleb Morgan

Caleb Morgan is part of our editorial team and reviews and edits our home brewing guides, checking product details against manufacturer specifications and owner feedback.