Can I top off 12 hours after brewing?

.... and the reason I ask is that, yes, I'm stupid, and made a mistake today.

Can I boil a gallon of water tonight, let it reach room temp, and carefully pour it into my primary fermenter tomorrow morning? Currently the airlock is bubbling away happily so fermentation is definitely active (I used a starter). There's just over 4 gallons in the bucket (for a 5 gallon recipe)

thanks.....

I have never done it so take it for what it's worth but I would probably try it. It seems I read on here some where about some one brewing a concentrated batch and cutting it with water at bottling time for some reason and they reported that it worked out ok. Maybe some one with some experience with this can provide more insight. Bottom line is that it may not be ideal but it's probably not going to ruin the beer. Hope this helps

edit to add: You may want to store the water in some quart canning jars with lids in place to minimize O2 exposure. If there is still a good fermentation the yeast should take care of the little bit of O2 that may make it in.

I just boiled the water and am letting it sit (covered by the same pot cover that was on when I boiled) to get back to room temp, which is what my IPA is currently sitting at as well. Will that be okay?

Again, Im not the expert here but I think you should be just fine. If there is still a good active fermentation going on all the better. But don't blame me if it don't work.....lol

I'm in the same boat, I just plain forgot to top off until I opened the fermenter after a week and thought "that sure ain't five gallons." So I topped off when I transferred to secondary, we'll see how it goes.

I dont see why not, all you are doing is adding bacteria free water to dillute your wort.