One of the topics that seem to come up quite often in various Facebook groups is the subject of bottle bombs. This happens when a bottle of beer explodes, normally in a quite spectacular fashion, as it is no longer able to contain the pressure inside the bottle. It is not unknown for the neck of a bottle to end up embedded in the ceiling, or for it to start a chain reaction where one bottle exploding is enough to trigger an explosion of the bottle next to it and so on. Whilst the end cause of the bottle failure is simply too much pressure for that bottle to contain, there are many ways to get to that point, and that will determine what you need to do to either fix the problem now or prevent it in future.

In the normal bottling process, the beer is left to finish fermenting and this is usually checked by taking 2 gravity readings of the beer, at least 24 hours apart, then seeing if both readings are the same. If there is any difference between the two readings, you should leave it another 24 hours and then try to take a reading again. You should only consider bottling your beer when the gravity has remained stable for at least 24 hours and it is somewhere around the expected final gravity, or FG, for the beer being brewed. This information is normally given in the instructions for kits or is calculated by brewing software for your recipes.

If you have had a bottle bomb, then you will probably have found out how much mess a bottle of beer can make when it sprays the contents all over the place and have just gone through a lengthy process of cleaning up, but there may be some good news. Just because one bottle has blown up, it does not necessarily mean the entire batch is going to. Depending on the cause, it could be a single bottle affected, or you could have more of them to look forward to, in either case, until you are sure of the actual cause, you should take great care moving any other bottles of the same batch of beer as they could also explode at the slightest knock or movement. Here are the most common causes of bottle bombs.

Over priming

The most common cause of failure is that there are too many fermentable sugars in the bottle at the time of bottling. This may be due to the addition of too much priming sugar or by bottling the beer before it had finished fermenting. In both cases, you have too much fermentables in the bottle and as the yeast keeps on working its way through them all, it will continue to produce CO2 and alcohol, the more fermentable material available to the yeast the more CO2 produced, up until a point where either the yeast dies off, the fermentation finishes or the bottle explodes. In the cases where the bottle does not explode this can still exhibit itself in the form of a gusher, where you open the bottle and are immediately presented with a fountain of foamy beer shooting out of the bottle.

This is one of the easiest causes to avoid in future. You just need to make sure that you are within a couple of points of your expected FG before bottling and ensure you do not add too much fermentable when priming your beer. have a good priming calculator available which will let you pick your beer type, batch size and the type of priming fermentable you are going to use. It will then tell you how much you need to use for your batch of beer.

The only remedial action you can take to try and reduce the impact of over priming is to let some of the pressure out of the bottle. You can do this by carefully lifting the edge of the cap, without actually removing it and release a little bit of pressure that way, or you can chill the bottles cold first then do it. In either case, you may have to do this several times to release all the excess pressure in the bottle, but there is a very real risk that another bottle could explode whilst you are handling it to do this. Chilling it down helps to reduce that risk slightly but you should never hold a bottle of beer you are trying to de-gas with your bare hands, just in case it explodes. What’s more, don’t have your face directly over the bottle when you de-gas and seriously consider eye protection.

Besides the risk of a further explosion, other less dangerous risks are ending up releasing too much pressure and having a flat beer, or having a gusher when you finally serve your beer. Depending on how much time, money and effort went into your batch of beer, it can sometimes be much simpler and easier to just brew a new batch rather than spend a lot of time trying to save your beer.

Uneven priming

If you try to measure out a small amount of priming sugar into each bottle individually, it is very unlikely that you are going to get the same amount in every bottle, Some may get more or less where some sugar was left on the spoon or funnel you used and it ended up in the next bottle instead, or it may just not have been measured as carefully as the others. If you dose all your bottles individually and find the carbonation level varies from one bottle to the next, this could well be an indicator of uneven priming. Some bottles can be very highly carbonated and explode where others may be fine or even flat.

Whilst you can prevent this by very careful measuring and careful attention to the dosing of each bottle, ensuring the full measure of priming sugar goes into each bottle, there are a couple of much simpler ways of preventing this from becoming a problem. One option is to take a pre-measured dose of priming sugar for each bottle either in the form of Carbonation Drops or some people have reported success using sugar cubes, however, the later can contain a varying amount of sugar from one cube to the next, or you can do what is called batch priming. To batch prime, you will need a sanitised bucket that is capable of holding the full batch of beer, this is normally in the form of a second fermenter with a tap and bottling cane fitted to allow for easier bottling. This is referred to as your bottling bucket. You mix up your full amount of priming sugar as usual, and add it to the bucket, before syphoning or draining your beer gently into the bottling bucket in a swirling motion to mix the fermentable through the whole batch of beer. A gentle stir with a sanitised spoon also helps to ensure even distribution through the whole batch, but be gentle, you don’t want to be mixing lots of oxygen into your beer now the initial fermentation stages have finished. You then proceed to fill each bottle from the bottling bucket as you would from the fermenter. This ensures a much more even dosing of each bottle.

As uneven priming leads to each bottle having different carbonation levels, it is hard to know which bottles are at risk of exploding and which not. You can try releasing some pressure from each bottle, but be prepared to have a higher number of under carbonated bottles when you serve them at a later stage if you go through and let the pressure out of them all. 


No matter how careful a brewer is, there is always some risk of infection. Whilst normal brewing yeast may seem such a simple thing, it is actually very complex in the way it works and exactly what sugar molecules it can use. Once it has finished fermenting there can be several types of sugars left in the beer that are fermentable, just not by normal brewers yeast. If your beer becomes infected at any stage, depending on what has caused that infection, it may be able to make use of those remaining sugars and continue to break them down, producing more gas and pressure inside the bottle. This tends to cause gushers more often than bottle bombs, but it can still cause a bottle to fail if the pressure gets too high.

Whilst preventing infections is as simple as ensuring proper hygiene and sanitation, sometimes it is hard to see where you are going wrong. Firstly, it is always important to remember that if something is not clean it cannot be properly sanitised. If there is any dirt left on a surface, then the sanitiser cannot get beneath it to properly sanitise the surface so you can have bacteria or wild yeast left behind, even when you have given your equipment a good going over with sanitiser. Make sure you use a good cleaner before sanitising or at the very least a combined cleaner sanitiser to ensure your equipment is properly clean and sanitised. Check your equipment carefully and regularly, over time surfaces can become scratched or damaged and then it becomes practically impossible to properly clean and sanitise them, leaving nasties in the scratches which then get into your beer.

Depending what your beer has become infected with, and if the source of the infection was inside a single bottle, fermenter or something else that contacted the full batch, you may have no resort but to throw the whole batch away and start again. Just make sure that you either take great care cleaning and sanitising all your equipment and bottles before re-using them or replace them. Just remember that whilst some types of yeast or bacteria can be introduced to beer on purpose, wild yeasts or some types of Brettanomyces for example, with accidental infections, it is unlikely that the results will be very palatable or they could potentially be harmful to your health.

Weak or damaged bottles

Over the years, I have heard of people sourcing their bottles in many different ways, from buying them new from homebrew shops, through to rummaging, sometimes with permission, in the bottle skips at their local pub. If you have ever heard how pubs treat empties, then it probably wouldn’t surprise you to hear that quite a few of the bottles in the bottle skips have been thrown in there and have been chipped or got hairline cracks in them from bad handling. This is something that can affect bottles from any source if they have not been cared for and handled properly. Some other bottles just aren’t designed to handle the pressure produced when carbonating beer as they may have previously only held flat or very lightly carbonated beverages. Whilst a far rarer occurrence, some new bottles can also potentially have a weak spot which hasn’t been discovered yet. In all of these instances, it is only going to affect the weakened bottle and not your whole batch.

In the main, this issue prevents itself from happening again as the weak or damaged bottles have blown up, so they are unable to get reused anyway. If you are reusing bottles, especially ones that may have had a hard life, take care to inspect each bottle before use and ensure it is totally clean and sanitised. Any chips or cracks, can not only weaken the bottle, causing it to fail, but they can also cause bad sanitation and infection as in our previous issue. If a bottle is damaged in any way, just recycle it and use a different one. Using damaged bottles is like playing Russian Roulette. You may get away with it this time and maybe next one, but if you keep playing you will lose eventually and cleaning up that sticky mess will not be much fun.


Sarah is an experienced home brewer, a qualified BJCP beer judge, organiser of the Welsh National Homebrew Competition and sometimes Brewtuber under her home brewery name, Daft Cat Brewing