Flat homebrew is almost never a mystery once you check the right things in the right order. Most cases trace back to one of four causes, and working through them in this order will find the problem faster than guessing.
1. Not enough time, not enough patience
This is the most common cause by far. Bottle carbonation takes two full weeks at room temperature (68–75°F) to develop properly — sometimes longer in a cooler space. If you're opening bottles at the one-week mark and finding them flat, that's not a problem to troubleshoot, that's just early. Leave the rest of the batch alone for another week before assuming something went wrong.
Where you store the bottles during that window matters more than most first-time brewers expect. Yeast is a living organism, and its activity slows down noticeably as the temperature drops. A closet that sits at a warm 72–75°F can carbonate a batch to full fizz inside the standard two weeks. The same beer stored in a cold basement or an unheated garage corner running 60–65°F can easily take three to four weeks to reach the same carbonation level, and below about 60°F the process can crawl to a near standstill. If your conditioning spot runs cold, don't measure your timeline against the two-week rule of thumb — measure it against a warm room's two weeks, then add roughly 50% more time for every 10°F below that. Moving the bottles somewhere warmer for the final stretch, rather than assuming the batch failed, fixes more "flat beer" cases than any other single change.
2. Not enough priming sugar
Carbonation comes from a measured dose of sugar added at bottling time, which the remaining yeast consumes and converts into CO2 inside the sealed bottle. If you eyeballed the amount instead of measuring, or used a recipe's sugar amount for a different batch size, you may simply not have added enough. There's no fix for bottles already capped with too little sugar — but for future batches, use a priming sugar calculator based on your actual batch volume and desired carbonation level, not a flat guess.
Batch size is the detail that trips people up most often here. A priming sugar amount written for a 5-gallon batch will under-carbonate a 5.5-gallon batch and over-carbonate a 4.5-gallon one, and it's easy to lose track of your actual final volume between topping off the fermenter, leaving trub behind during racking, and normal evaporation during the boil. Before you calculate priming sugar, it's worth actually measuring how much beer you're bottling — a sanitized measuring pitcher or marked bucket works fine — rather than assuming it matches the batch size printed on the kit box. That single measurement removes most of the guesswork the rest of the calculation depends on.
3. Dead or exhausted yeast at bottling time
If fermentation ran long, ran at a stressful temperature, or the yeast was already old when pitched, there may not be enough active yeast left to consume the priming sugar. This is more common in higher-alcohol beers, where yeast is under more stress by the time fermentation finishes. If you suspect this, a small pinch of fresh dried yeast added at bottling (a technique brewers call "bottle-priming with fresh yeast") can restart carbonation in an already-bottled batch — sanitize your hands and the yeast packet's exterior, open each bottle briefly, and add a few grains before recapping.
4. A leaking seal
Check your caps. A cap that isn't fully crimped, a cracked bottle, or a bottle that wasn't properly cleaned around the rim can let carbonation escape slowly over the two-week conditioning period. This shows up as flat beer with no sediment change and normal color — the carbonation simply had somewhere to go. Recapping is not a fix once a bottle has been conditioning for two weeks; the yeast has likely already finished its work and vented through the leak.
You don't have to guess whether a specific bottle is actually leaking. Mix a small amount of dish soap into water, wet a paper towel or sponge with it, and wipe it over the top of the cap and the crimped edge where the cap meets the bottle's glass lip — the same principle as checking a bike tire or a gas fitting for a leak. If gas is escaping, you'll see tiny bubbles forming right at the seam within a few seconds, even from a slow leak that wouldn't be obvious any other way. No bubbles means the seal is intact and the flatness has another cause; visible bubbling confirms the seal is the problem and tells you it's worth double-checking your capper's crimp on the rest of that batch, since a single bad crimp on your capper's alignment often affects more than one bottle.
Ruling out what it isn't
Flat beer is not usually caused by fermentation temperature, hop schedule, or grain bill — those affect flavor and aroma, not carbonation. If your beer tastes correct but has no fizz, the cause is almost always somewhere in the bottling and conditioning steps above, not earlier in the brew day.
It's also worth ruling out your own perception before you tear apart a whole batch. Pour into a clean glass rather than drinking straight from the bottle — a glass with any leftover soap film or oil residue from a dish sponge will kill carbonation bubbles on contact, making a genuinely carbonated beer look and taste flat. Give the glass a hard rinse with hot water and no soap, or use a dedicated beer-clean glass, before you decide a batch has a real problem. And pour it with a bit of purpose — a gentle trickle down the side of a tilted glass won't release much carbonation even from a well-conditioned beer, while a normal pour into an upright glass will show you what's actually there.