Adding water after the primary

I made a rookie mistake, and assumed that my primary carboy was 6 gallons. Turned out it was 5. Anyway, i only had about 4 1/4 gallons that was fermenting. My question is, while i am racking to my secondary, can i add water to get the volume up to 5 gallons, or should i just let it be? Also, if i leave it alone, will it just have stronger flavors, ABV, etc, or will it be undrinkable? Thanks.

Adding water when siphoning to a secondary is a common thing...personally I would boil it first just to be safe-make sure to let it cool before adding. Another way to do it, but probably not the best way is to just adjust your priming sugar water volume. Usually I just use the amount that I am short to boil with the priming sugar.

I've done this once. I had a brain-fart and only had 4 1/4g ferment, when the recipe was for 5g. I had already overshot my OG by assuming I would get low efficiency, then I got super high efficiency, so the beer wasn't what I wanted at only 4 1/4g.

I boiled and cooled the extra galllon I needed (boiled to drive off most oxygen, not to sanitize really) and added it when I racked. The beer has never tasted oxidized or anything, so I think its fine to do.

Generally though, you should try to remember to top up before fermentation as to not take any unncessary chances...

Thanks for the help guys.

If it was me, I would taste the beer. If it's good (but flat) as is, I wouldn't add any additional water at all. Why water down perfectly good beer?

If it's too strong, or too flavorfull, then you can top up with some boiled water if you want to, though.