Home Brew Fermentation: How to Tell When Your Beer Is Ready to Bottle in 2026

Fermentation is the part of home brewing where you do the least and where it matters most. Yeast turns sugar into alcohol and carbon dioxide, and your job is to give it a clean, steady environment and then wait until it has finished. The most common beginner mistake is not a recipe error but impatience: bottling too early. This guide explains what is happening in the fermenter, the temperature to aim for, why bubbling in an airlock proves little, and how to use a hydrometer so that you know, rather than guess, when to bottle. It is desk research, not hands-on brewing: we have not brewed the kits mentioned, and the figures come from the maker’s own instructions, which you should always follow for your own kit. We leave prices out because they vary.

Quick picks

What is happening in the fermenter

Once you add the yeast, it begins feeding on the sugars in the wort and producing alcohol and carbon dioxide. Fermentation usually starts with a burst of activity, often a foam on the surface, and then slows as the sugar runs out. The beer then looks calmer, but it is not yet finished: the yeast carries on working quietly, cleaning up and finishing off the last of the sugar.

That is why the time on a kit’s label is a guide and not a promise. A cooler room, an older yeast or a stronger beer can all make fermentation take longer than the leaflet suggests.

Temperature: keep it steady

Coopers’ basic instructions for its kits say to try to ferment within a range of 20 to 24 degrees Celsius, and to avoid opening the lid during that time. Other makers give their own ranges for their own yeasts, so use the figure on your kit’s instructions.

Practical points:

• Steady beats perfect. Large swings in temperature stress the yeast more than a slightly off-target but steady temperature does.
• Use a thermometer. A stick-on strip or a simple thermometer in a spare container of water beside the fermenter tells you what your room is doing.
• Choose the spot with care. A cupboard away from radiators, sunlight and draughts is usually better than a kitchen.
• Cold slows the yeast. If the room is cool, fermentation can look finished when it has only slowed. Warming the fermenter a little can restart it.

Why the airlock proves little

Many people judge fermentation by watching the airlock. Bubbling does tell you that gas is escaping, but gas also escapes when dissolved carbon dioxide comes out of the beer, and a leaky lid can let gas out without passing through the airlock at all. Slow bubbling can continue after fermentation is done, and a quiet airlock does not prove it is. If you bottle on the strength of the airlock alone, there is a risk of leftover sugar creating too much pressure in the bottles. A hydrometer is the reliable check.

Using a hydrometer

A hydrometer is a weighted glass float that measures how much dissolved sugar is in the liquid. It floats higher in a denser, sugary liquid and lower as the sugar is converted to alcohol. You read the number where the surface meets the stem.

Coopers’ instructions give the process in plain terms: when activity inside the fermenter seems to have subsided a bit, measure the specific gravity with a hydrometer and take a new measurement every day. Fermentation has finished once the reading is stable and unchanged over two days.

Tips for reliable readings:

• Sanitise everything. Anything that touches the beer, including the hydrometer and the jug, should be sanitised.
• Degas the sample. Bubbles of carbon dioxide cling to the float and push it up, which gives a false reading. Pouring the sample between two clean glasses a few times helps.
• Read at eye level. Look at where the liquid surface meets the stem, and read it the same way every time.
• Check the temperature. Hydrometers are calibrated to a stated temperature, and readings drift if the sample is much warmer or colder.
• Write it down. Note the date and reading each time, so you can see the pattern.

If you want to check your hydrometer, put it in plain water at the temperature printed on it. It should read 1.000.

When to bottle

The brew is ready to bottle once the reading has stabilised. Coopers says the beer is ready once the gravity has stabilised over a couple of days. Compare the number with the final gravity expected for your kit, if the instructions give one. If your reading is well above that and has stopped moving, the yeast may have stalled, and that is a different problem to solve before you bottle.

If you are unsure, leave it a little longer. An extra few days in the fermenter does the beer no harm, while bottling too early can cause real problems.

Common mistakes

• Opening the lid to look. Each time you lift it, you risk letting in unwanted microbes. Resist the urge.
• Using the airlock as a timer. Use it as a rough signal that things are alive, not as proof.
• Skipping sanitising. A small contamination can ruin a batch.
• Bottling to a deadline. Plan for the beer to be ready when the hydrometer says so, not when you would like it to be.

Frequently asked questions

How long does fermentation take? It depends on the kit, the yeast and the temperature. Use the instructions for your kit as a guide and let the hydrometer decide.

Do I need a hydrometer? It is the only way to be sure fermentation has finished. Most starter kits include one.

Can I move the fermenter during fermentation? Better not to. Disturbing it can stir up sediment and risk contamination. Choose the spot before you start.

Why does my beer look flat and still? Active foaming slows down, but a still surface does not mean the beer is finished. Check the hydrometer.

What if my reading never reaches the expected number? The fermentation may have stalled, often because of cold. Warm the fermenter slightly, give it a gentle swirl without opening it widely, and measure again over the next days.

The short version

Keep your beer at a steady temperature within the range your kit suggests, leave the lid alone and measure with a hydrometer rather than watching an airlock. Bottle once the reading has been stable for a couple of days. Read your own kit’s instructions first, since this guide describes general practice and one maker’s wording.