A beer sitting at 1.030 that should be at 1.012 is genuinely worrying, and the advice you will find is a list of six causes in no particular order, usually ending with "or the yeast hit its alcohol tolerance". Most of that list is real. The ordering is not, the tolerance answer is arithmetically impossible for an ordinary beer, and the fix recommended most often is one the best source on the subject explicitly tells you not to do.
First: is it actually stuck?
Two readings, three days apart. If they match, fermentation is over whatever the number says, and you do not have a stuck fermentation — you have a beer that finished higher than the recipe hoped, which is a different problem with a different answer. Our attenuation calculator compares where you landed against your yeast's published range, and the hydrometer guide covers taking the reading so the two are comparable.
Brew Your Own gives a useful field rule: a final gravity of 15–30% of the original gravity's points is normal, and more than a third means something is wrong. That is worth translating, because it is the same thing our calculator prints — a third of the OG is 67% apparent attenuation, at any gravity. Below that number, keep reading. Above it, your beer is fine and the recipe was optimistic.
Why "it hit the alcohol tolerance" is almost never the answer
This is the most repeated explanation on every forum thread, and it is the easiest to rule out. A yeast quits on alcohol when the beer reaches its tolerance, so working backwards, here is the original gravity your beer would have needed to start at before alcohol could possibly be what stopped it:
| Stopped at | 8% yeast | 10% yeast | 12% yeast |
|---|---|---|---|
| 1.015 | OG 1.076 | OG 1.091 | OG 1.106 |
| 1.020 | OG 1.081 | OG 1.096 | OG 1.111 |
| 1.030 | OG 1.091 | OG 1.106 | OG 1.121 |
The OG each stall would require, from this site's engine — the same stall calculation the mead and cider pages use, on beer's dextrin floor of 1.010. Read it as: unless your OG was at least this high, alcohol is not your problem.
A beer stalled at 1.020 needs to have started at 1.081 before even a weak 8% yeast could be alcohol-stalled — and at 1.111 for a 12% one. An ordinary 1.050 pale ale stuck at 1.020 has made about 3.9% alcohol. Nothing sold as brewing yeast gives up at 3.9%. If your OG was under about 1.080, cross tolerance off the list entirely and look at the six things that actually stop fermentations. (Above 1.080 it becomes real: an 8% yeast in a 1.081 wort genuinely does stop around 1.020.)
What actually stops a fermentation
In rough order of how often it is the answer:
- The fermenter got cold. Far and away the most common. Ale yeast slows badly below about 18 °C (64 °F) and an overnight drop in an unheated garage will flocculate a batch that was working fine. This is also the easiest to fix and the easiest to miss, because the beer looks finished.
- Not enough live yeast went in. An old liquid pack, no starter, or a big wort pitched at the rate for a small one. The viability calculator shows what a date costs you and the pitch-rate calculator what the batch actually needed.
- The wort was never that fermentable. A mash at 156 °F, a pound of crystal malt, or extract you did not choose the fermentability of. This is not a stall at all — the yeast ate everything it could and stopped. The attenuation calculator's section on what moves the number is the one to read, because no amount of fresh yeast will eat a dextrin.
- No oxygen at pitching. Yeast needs it to build cell walls before it can ferment. Skipping it shows up as a long lag and a fermentation that runs out of steam two-thirds of the way down.
- Nothing to eat but sugar. Beers with a lot of simple sugar or fruit are nutrient-poor; malt normally supplies what the yeast needs, and adjuncts dilute it.
- The yeast flocculated early. Some English strains drop bright while there is still sugar left. This is the only cause on the list that rousing fixes — which matters for the next section.
Should you rouse it?
Here the two most-read pages flatly disagree. Home Brew Answers: "the simple solution is to rouse the yeast to get it back into suspension by stirring up the wort". Brew Your Own, about a confirmed-stuck beer: rousing "may cause all sorts of oxidation problems...without actually doing anything to improve attenuation."
They are both right, about different beers, and the distinction is the whole answer. Rousing puts settled yeast back in suspension. Go back to the list above: that is a treatment for cause six, and for nothing else. If your yeast flocculated early, stirring works. If the wort is unfermentable, if there was never enough yeast, or if the beer is simply cold, stirring does nothing — and a beer at final gravity has no CO₂ blanket left to protect it, so you are splashing oxygen into flat beer and buying cardboard and diacetyl for nothing.
If your two readings are still moving, even by a point, nothing is stuck and none of this applies. Warm the fermenter a few degrees, give it a gentle swirl, and wait — a slow fermentation still has active yeast and a CO₂ blanket, so the oxidation risk that makes rousing a bad idea later is not there yet. The warning above is about a beer whose gravity has genuinely stopped moving.
How to restart one that really is stuck
In this order, stopping as soon as the gravity starts moving:
- Warm it to the top of the yeast's range — 20–22 °C (68–72 °F) for most ales — and give it two or three days. This alone fixes the majority of real stalls, costs nothing and risks nothing.
- Swirl the fermenter gently, but only if you have reason to think flocculation is the cause: a famously flocculent strain, or a beer that dropped bright unusually early. Rock it, do not stir it open.
- Pitch actively fermenting yeast, not a fresh packet. This is BYO's krausening recommendation and it is the one that works: a starter already at high krausen, added to beer that has alcohol in it, because dormant yeast dropped into a hostile environment mostly dies. A 1-quart starter is the minimum for a 5-gallon batch; 2 quarts is better.
- Accept the beer. If it will not move, it is a fuller beer than you planned, not a ruined one — but read the next section before you package it.
The part nobody warns you about: bottling it
Every page treats a stuck fermentation as a disappointing beer. The real risk is what happens if you bottle one and it wakes up. All those unfermented points are sugar, and in a sealed bottle sugar becomes pressure. Here is what is still in there:
| Bottled at | Sugar left | CO₂ if it restarts | vs bottle rating |
|---|---|---|---|
| 1.015 | 13 g/L | 4.3 volumes | 1.4× |
| 1.020 | 26 g/L | 7.7 volumes | 2.6× |
| 1.030 | 52 g/L | 14.5 volumes | 4.8× |
| 1.040 | 78 g/L | 21.3 volumes | 7.1× |
The extract that would disappear on the way to a 1.010 finish, counted as fermentable sugar and run through this site's priming stoichiometry, with no priming sugar added at all. An upper bound — some of that extract is not fermentable — but the conclusion does not need the precision. Standard bottles are comfortable to 3.0 volumes. Pressures at these levels are far outside the range the carbonation fit was built for, so the table does not print one.
A beer bottled just ten points high carries 26 g/L of sugar — about 4.4 times the 5.9 g/L you would have primed with — and if it restarts in the bottle it makes 7.7 volumes against glass rated for 3.0. Normal carbonation is 2.4 volumes, about 26 psi. This is not a flat-beer problem; it is a case of grenades in a warm cupboard. Never package a beer whose gravity you have not seen hold steady for three days, and if you already have, chill the case immediately and open them slowly somewhere the glass cannot hurt anyone.
How to not have this happen again
All six causes are decided before the yeast goes in, which is why BYO's article is mostly about prevention. Pitch enough healthy yeast for the gravity you are actually brewing; aerate the wort well; hold the fermenter at a stable temperature rather than a lucky one, and hold it there for the first 72 hours; and mash where you meant to. A fermentation that starts strongly almost never stops early — the stalls are nearly all decided on brew day, and diagnosed a fortnight later.