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In the Book "Brewing Classic Style" there are a set of conversions given to decrease the boil volume. The question I have is how to determine the pre-boil gravity? The only set of recipes I've ever seen with that given are in that same book. Thanks for any help.

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I use a brewing software called Beer Alchemy (a Macintosh software). There are plenty of software out there for PC. ProMash and BeerSmith are two of the bigger ones. There is also an online tool called BeerTools.com. I never used it, but I believe this will also give you preboil gravity. I am sure there are some mathematical calculations you can do, but I am lazy and use the software.
If you don't have Designing Great Beers by Ray Daniels, now's the time to pick it up. He has a whole section in the beginning that talks about the concept of gravity units (GUs). This concept will really help you nail your original gravities.

The concept relates specific gravity to volume. Here's the gist...

Fist let's talk "gravity points". This is just stating the specific gravity a different way. You just drop the 1 and get rid of the decimal point. That is, a specific gravity of 1.065 has 65 gravity points.

To calculate gravity units you just take your gravity points and multiply by the volume of wort in gallons. Let's say I'm making an IPA and the recipe calls for 6 gallons of wort at the end of the boil with a gravity of 1.065. That would mean that I need 390 gravity units. 6 gallons x 65 gravity points = 390 GUs.

It's important here to understand that GUs don't change with volume. That means that my pre-boil GUs should also equal 390.

Okay, so how do I figure out what my pre-boil gravity should be? Well, I know that I boil off 1.25 gallons per hour and this recipe calls for a 60 minute boil. That means I need 7.25 gallons of wort pre-boil. Since my GUs don't change with volume, I now have 7.25 gallons times ??? gravity points = 390 GUs. That's the same as 390 GUs divided by 7.25 gallons or about 54 gravity points. (390 / 7.25 = 53.8) So my pre-boil gravity should be about 1.054.

You're looking to decrease the boil volume so let's make a 3 gallon batch of the same 1.065 IPA. We need 195 GUs (3 x 65). If you lose a gallon of wort during the boil, you'll need to start with 4 gallons. (195 / 4) = 48.75. Let's round that up and your pre-boil gravity shoud be about 1.049.

Hope that makes sense. Of course, Ray Daniels does a much better job explaining it and also shows you how to adjust your pre-boil gravity if you're off by a few points. I haven't missed my OG since!

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