I know that I said I'd be doing the Technical Tuesday column on the impact of Sodium levels in brewing water this week, but I think it's going to get put off again. Instead, I'm going to detail another calculation regarding hitting target original gravity.
In contrast to the previous post about boosting OG by adding DME, this is going to show you how to calculate the amount of water you'll need to decrease OG.
(Note: This method assumes that you have already cooled your wort and are planning on adding water to the fermenter. Of course, you should never fill a bucket or carboy so much that you don't have at least a gallon or two or headspace left for the krausen. Additionally, it is advisable to add the cleanest water possible. This will lessen your chance of infection by bacteria or wild yeast that may be present in your water source. Boiling water and cooling it rapidly is a good method for making sure your top-off water is clean.)
You're almost to the end of your brewday. Your cooled wort is in the fermenter and you're taking a hydrometer reading for your records. Oh horror of horrors, you're ten points high! You know that you can dilute the wort you have to reach your desired OG, but how much water shoul you add?
Here are the numbers that we need at the outset:
1.) Gallons of Wort
2.) Original Gravity in GUs (pre-water addition)
3.) Desired original gravity (in GUs)
4.) Total Gravity Units [Original Gravity (in GUs)*Gallons of Wort]
(A quick word about GUs: You can convert specific gravity to GUs by multiplying OG*1000, then subtracting 1000.)
Let's say you have 5 gallons of 1.060 wort. However, you wanted an OG of 1.050. First find the Total Gravity Units (GUs).
[Original Gravity (in GUs)*Gallons of Wort] - 60*5 = 300.
Here's the calculation you'll use:
[Total Gravity Units/Desired OG (in GUs)] - Gallons of Wort. That's it!
[300/50] - 5
6-5 = 1 Gallon.
You'll require an additional gallon of water in order to bring your gravity down to the desired 1.050.
Here it is one more time with a new set of numbers.
1.) Gallons of Wort = 3
2.) Original Gravity in GUs (pre-water addition) = 77
3.) Desired original gravity (in GUs) = 54
4.) Total Gravity Units [Original Gravity (in GUs)*Gallons of Wort] = 231
[231/54] - 3
4.28 - 3 = 1.28 Gallons.