First, the timing question

Bottle conditioning takes at least two weeks at room temperature. If you opened a bottle at one week and it tasted flat, that's not a problem — the carbonation isn't finished yet. Give it another week before deciding anything is wrong.

Flat homebrew is one of the more dispiriting outcomes in brewing. The beer was meant to be done, you waited the conditioning period, you cracked the first bottle expecting that satisfying hiss — and got a quiet pour with no foam. This article walks through how long carbonation actually takes, the three real causes of under-carbonation, and how to rescue bottles that aren't fizzing the way they should.

How long carbonation actually takes

The most common "flat beer" report turns out to be impatience. Bottle conditioning isn't instant. Here's what's actually happening inside the bottles:

  1. At bottling, you mixed roughly 130-150g of dextrose into your beer (priming sugar).
  2. The yeast still in suspension in the beer eats that sugar over the following 2 weeks.
  3. As the yeast eats the sugar, it produces CO2. The bottle is sealed, so the CO2 can't escape — it dissolves into the beer.
  4. Once the priming sugar is fully consumed, the beer is at its target carbonation level.

This process takes roughly 14 days at 18-22°C. Some specific timelines:

If you opened a bottle at day 7 and called it flat, the most likely explanation is timing. Wait another 7 days at room temperature and check again.

In Short

If your beer has been bottle-conditioning for less than 14 days at room temperature, it's probably not flat — it's just not ready yet. Wait another week before troubleshooting.

When timing isn't the issue

If you've waited the full 14-21 days at room temperature and the beer is still flat, you have an actual carbonation problem. Three causes account for almost all cases.

Cause 01 · Most likely
Priming sugar issue at bottling

What's happening

Either you used too little priming sugar, you forgot to add it entirely, or the sugar didn't mix evenly through the batch — meaning some bottles got most of it and others got almost none.

How to check

Try several bottles. If they're all flat, you under-primed or forgot to prime. If some are properly carbonated and others are flat, the sugar didn't mix evenly. The right amount for a 20-litre batch is 130-150g of dextrose, dissolved in boiled water before adding to the bottling vessel.

What to do

If you forgot priming sugar entirely or used too little, see the rescue section below. The fix is to re-prime the existing bottles, which is fiddly but possible.

Cause 02 · Common
Bottles too cold during conditioning

What's happening

Yeast becomes inactive below about 15°C. If your bottles have been conditioning in a cool spot — a winter garage, a cool basement, a fridge — the yeast may not be working fast enough to carbonate the beer in a normal timeframe.

How to check

Touch a bottle. Are they noticeably cold? If you're in southern Australia in winter and your bottles are in an unheated room, this is very likely. Storage at 12-15°C will eventually carbonate beer but takes 4-6 weeks instead of 2.

What to do

Move the bottles to a warmer location (20-22°C is ideal). Wait another 2 weeks. Most "cold conditioning" cases resolve at this point. Don't refrigerate bottles until carbonation is genuinely complete — cold conditioning slows the process to a crawl.

Cause 03 · Less common
Not enough viable yeast in the beer

What's happening

If you long-conditioned the beer in the fermenter before bottling, or used a highly flocculent yeast that settled out very thoroughly, there may not be enough yeast cells left in suspension to consume the priming sugar. The sugar is there, the temperature is right, but the workforce is missing.

How to check

This is harder to diagnose visually. Indicators: you fermented for 3+ weeks before bottling, the beer was unusually clear at bottling, you used a yeast strain known for high flocculation (English ale strains, some hazy strains), or you cold-crashed the beer before bottling.

What to do

The fix is to add fresh yeast to each bottle. Open carefully, sprinkle a few grains of dry yeast (a tiny pinch — less than 0.1g) into each bottle, re-cap, and condition for another 2 weeks. Tedious but effective.

How to rescue under-carbonated bottles

Two methods, depending on what went wrong.

Method 1: Re-prime each bottle

If you forgot priming sugar or significantly under-primed, you can add sugar to each bottle individually. This is fiddly but it works.

  1. Sanitise a small spoon or measuring cup.
  2. Open one bottle. The flat beer won't gush; expect a quiet pop.
  3. Add the priming sugar. For a 375ml stubby, about 2-3g of dextrose. For a 640ml longneck, about 4-5g. Eyeballing is fine — a quarter-teaspoon for stubbies, a half-teaspoon for longnecks.
  4. Recap immediately. Use fresh caps, not the ones you removed.
  5. Condition at 20-22°C for 2 weeks. Carbonation should now develop normally.

Sanitation matters more here than it usually does because you're opening sealed beer and exposing it briefly to air. Move quickly with each bottle.

Method 2: Add fresh yeast to each bottle

If priming sugar was added at bottling but the yeast was too settled to use it, add new yeast.

  1. Have a packet of dry yeast ready. Champagne yeast or a robust ale yeast both work.
  2. Open one bottle.
  3. Add a tiny pinch of yeast. Honestly, just a few grains visible to the eye — certainly under 0.1g. Too much yeast adds yeast flavour to the beer.
  4. Recap with a fresh cap.
  5. Condition at 20-22°C for 2 weeks.

For a 30-50 bottle batch, you'll use only a few grams of yeast total — the rest of the 11g packet can be stored sealed in the fridge for next time.

Don't combine both rescues at once

If you're not sure whether the problem is priming sugar or yeast, only add one or the other — not both. Adding fresh sugar AND fresh yeast risks over-carbonation when both work together. Try the most likely cause first; if it doesn't work after 2 weeks, then try the other.

Most "flat beer" reports turn out to be impatience. The second most common is just a temperature too cold for the yeast to work.

The opposite problem: over-carbonation

Worth flagging for context. Some readers searching about "flat beer" actually have the opposite problem developing — bottles that are about to be wildly over-carbonated. If your "flat" bottles are showing:

...then your beer isn't flat at all — it's over-carbonating because either you over-primed or you bottled before fermentation was actually finished. This is a dangerous situation that can produce bottle bombs. We cover this case in detail in our bottle bombs guide.

Preventing flat beer next batch

Three habits prevent almost all carbonation problems.

Use a priming sugar calculator

Free online calculators take three inputs: batch volume, beer temperature at bottling, and your target carbonation level for the style. Output is the exact gram amount of dextrose to add. Don't eyeball this — over-priming is dangerous and under-priming is annoying.

Stir the priming sugar through the bottling vessel evenly

Pour the dissolved priming sugar into your bottling bucket, then siphon the beer in on top. The act of siphoning gently mixes the sugar through. Don't pour beer into the bucket first and then add sugar on top — you'll get uneven distribution.

Condition at room temperature

18-22°C consistently for the full 2 weeks. Don't move bottles to the fridge until carbonation is complete. Don't store them somewhere that gets cold at night.

For broader troubleshooting context, see our 10 common homebrew mistakes guide. For the broader brewing process this builds on, see how to brew from a fresh wort kit. For if you suspect your beer is over- rather than under-carbonating, see our bottle bombs guide.