Can I add top-off water after fermentation?

So I just figured out that the gallon labels on my primary fermenter pail are most likely wrong. That means my fermenting beer is about 4.2 gallons whereas I thought it was 5 gallons. I was wondering if I could add 0.8 gallons of distilled water after fermentation (right before bottling I suppose) to get the 5 gallons of beer that the recipe was designed for...

personally i wouldn't risk it, just bottle it up and enjoy the fruits of your labor.

+1. You absolutely CAN add water. I'm not there to stop you. But as was previously posted, its probably not worth it.

When you take a gravity sample, taste it. And then ask yourself if it would be better with approximately 20% water added to it. Or even try that- add some water to the sample and stir it and drink.

My guess is that it won't be better with the added water, but it very well could be. Then you could decide whether to just add more water to your priming solution, boiled and cooled, so you don't oxygenate the beer at that point.

It really shouldn't be about the quantity but the quality and you'll find as you brew, until you really set into your process, your final volume will vary sometimes.

Like Yopper said, taste the beer and let your taste buds determine your next move. IMO, it's usually not worth messing with. Take good notes and figure out how to correct it next batch,