I brewed Megalodon today and had a situation come up. I started with 5 gallons of water for my brew and thanks to all of the malt, hops, and corn sugar in this recipe, ended with about 4 3/4 gallons of wort following my chill. I took a OG from my brew kettle which came in at a whopping 1.100. Here’s where it gets interesting. I didn’t add water to my brew kettle and racked my wort directly to a cake from a previous brew. The previous cake was actually pretty big and took up some volume in the carboy. I have my carboys marked at 3, 3.5, 4, 4.5, 5, 5.25, 5.5, 5.75, and 6 but not marked low enough to figure out the volume of the cake. I took a guess at how much water to add based upon what I thought the volume of the cake was. Following that, I shook the hell out of my carboy to mix the additional water and to aerate. When I took the next measurement, the OG was 1.095. I added about 1/10 of a gallon after that, shook it up again, put the airlock on, and put it in my basement. Here’s my questions: where can I find a calculator for figuring how much H2o to add to my wort to bring it to the OG i’m aiming for? Is an OG measurement accurate if it’s taken AFTER pitching my yeast?

The relationship of gravity vs. volume is very simple. If you take just the numbers to the right of the decimal place in the gravity, you get something called gravity units (GU). So for a 1.100 OG beer, it’s got 100 GU. The key thing to keep in mind is that GU times volume always stays the same after the mash. Mathematically:

V x GU = V x GU = V x GU…

It doesn’t matter how many times you do this. Add water as many times as you want (or boil water off) but the two factors multiplied will always stay constant.

So, for example, if you started with 4.75 gallons at 1.100, and say for instance you would like to bring this down to 1.085, you can do the math like this:

100 (4.75) = 85 (GU)

Solving for GU, you need to do the math and divide both sides by 85. 100 x 4.75 / 85 = 5.59

This means that to get from 4.75 gallons to 5.59 gallons, you would need to just subtract the two, 5.59 - 4.75 = 0.84 gallons of water need to be added.

You can do the same kind of math for any other gravity that you want.