I've had a few batches get a little too carbonated in the bottle. Currently, I have a delicious stout that fills two glasses on pour, and needs some time to settle down. That is, of course, if you open it over the sink and don't create a mess all over the kitchen.
I'm pretty sure I know why this is happening, but as it's happened more than once, I'd love some input as to possible reasons why. And no, I'm not shaking the bottles. It's not a violent explosion, more of a steady... plume.
There is only one real answer, absent any off smells or flavors and that's too many fermentables. Infections can cause gushers too, but there would be other signs.
You are either adding too much priming sugar (corn sugar, DME, what have you) or you are not letting your beers reach terminal gravity, which is the point when they go dormant due to lack of fermentables. You need to be using your hydrometer to get a starting gravity and then use it a few more times to be sure that the beer is done. I usually call it good when the gravity remains steady without dropping for 48 hours.
There is no "right number" for determining the final gravity of the beer. That is determined by the effectiveness of the yeast, the style of the beer and a myriad of other factors. Most recipes do include a final gravity number, so you can know what you are shooting for. Unless its a big beer, I usually wait until the gravity is under 1.015 before bottling for a standard ale yeast.
If you are getting uneven carbonation, meaning some bottles that explode or gush, or some that are under-carbed, then you are not mixing the priming sugar in thoroughly enough in the bottling bucket. I created an entire batch that was either flat or near bottle-bomb level by not carefully stirring in my priming sugar and making sure it was evenly distributed.
This is one of the reasons I'm glad I keg my beer now. I had this happen a bunch of times when I used to bottle.
From what I've always heard, it is most likely due to either a.) Too much priming sugar, or b.) Too many fermentable sugars left--in other words, fermentation wasn't complete.
Another cause could be bacterial infection, but my sanitation was always very good and there were no flavors, odors, or other evidence to indicate infection. If it tastes and smells good, and you don't see any visual evidence of contamination like a ring around the inside of the bottle neck or mold on the cap, than it probably isn't due to infection.
So, I'm gonna go with "too much priming sugar". I never used a calculator, but something like this might help you out in future batches:
Yeast needs 2 things to make alcohol and carbonation: o2 and fermentable solids. One simple thing I did that made a surprising amount of difference was to make sure that while I was bottling, I left the lids off for a little while after filling the bottles. This allows the yeast to replace the oxygen sitting in the pocket above with CO2. Less O2 for the yeast to use, less explosive openings.
