Scaling down a 5 gal recipe to 4

Im about to brew my first batch, it's going to be a 5 gal recipe. The only fermenter I could get my hands to is a 5 gal carboy. I've read in other threads about scaling down a 5gal recipe to 4 gal since i dont want to overfill my fermenter. Sorry if its a dumb question, but how do you scale down a recipe? Would you consider ok to fill it to 4.5gal and install a blowoff? Or scaling down? Thanks for all the help!

Or you could just make it stronger and do 4.5. You may or may not get alot of blowoff though, i lucked out a few times when i almost maxxed out my feremter but having a behaved ferment,luckily.
Other wise a good brew calc. will work or someone could do it for you if you put up your recipe.

The easiest way to go to a 4 gallon batch would be to multiply all the ingredients by 4/5 (except yeast- use the full amount of yeast).

You scale it down by changing all the ingedients by the same amount. If you want 4 gallons remove 20% from all ingredients.

Would it be ok to just make the recipe to 4 gal, then add a gal of water one it slows a bit?

Thanks everyone for the replys!

DMartin Thats a good question too, seems reasonable, anyone have any opinions on that?

DMartin said:

Would it be ok to just make the recipe to 4 gal, then add a gal of water one it slows a bit?

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Honestly never thought about that... I guess one of the big concerns would be making sure that you aren't vigorously dumping the water on top and splashing the wort all over the place. Then there's the sanitation issues. Every time you tinker around with your wort after it has cooled you're increasing your chances of inviting unwanted visitors in. Third trouble that I can think of off the top of my head is that if you siphoned water into an already fermented batch I'm not sure how it would all mix together. When you add top off water to a partial boil batch for example, you pitch your yeast which stir the wort around while their doin their normal business. I mean, I suppose it'd probably mix in just from the motion of siphoning considering thats basically how priming sugar is mixed in before bottling.

Short answer. Good question, Apparently I can't really definitively answer it no matter how long I argue out loud about it with myself, but, it sounds sketchy IMO.

Anyone else wanna weigh in?

I'd say that keeping it simple on your first batch is for sure going to be the most beneficial. Follow the recipe you've got (adjusting for fermenter volume in your situation) and use the brew knowledge that you've got and keep it a pretty simple, by the book, brew process... at least until you get a few batches under your belt and feel comfortable with the entire process and with the equipment that you have.

If you want to add water to stronger wort to dilute it, you could: Boil water vigorously for 10-15+ minutes (sanitize it and remove oxygen), cover, let it cool to room temps & siphon into your bottling bucket, then quietly rack beer on top with priming sugar.

Just do the recipe as is and don't fill your fermenter all the way

I do it all the time. Just reduce everything by 20%, or even better just let your brewing software do the scaling.

I have a couple of 5 gal carboys that I very rarely use for secondary, so I put them to good use for 4 gallon batches.