Adjusting Calculation for gravity reading (Added top-up Water)

[brandonlovesbeer]

My most recent batch of beer boiled down more than I was expecting. From 7 gallons down to (Final volume after chilled) 4.8gal.

I took a gravity reading right before putting it in the primary. 1.072

Before pitching the yeast, I added an additional gallon of water. But I did not know how to adjust the gravity reading after the water addition.

I don't need an exact number. Just a ball park.
But if you have a formula so that I can calculate (this and for future reference) that would be great.

Basically, there's 20.8% more water in the fermenter than when I took the initial gravity reading.

Thanks

 

[Black Island Brewer]

Gallons x gravity points = total gravity points
Adjusted gravity = total gravity points divided by adjusted gallons

4.8x72=345.6
345.6/5.8 gallons=59.58 or 1.059

 

[McKnuckle]

First, sum the volumes that were added and call that Vt (total volume)

New Gravity = (V1/Vt * G1) + (V2/Vt * G2)

In your example:
Vt = 4.8 + 1 = 5.8
New Gravity = (4.8/5.8 * 72) + (1/5.8 * 0) = 59.59 (1.060 rounded)

 

[brandonlovesbeer]

You guys are awesome! I knew there must have been a way.
Now I can add this formula to my ever-expanding Homebrew notebook.

 

[Easycreeper]

Knowing the math behind what you're doing is great since you won't always have access to the internet, but this is handy when you do:

http://www.brewersfriend.com/dilution-and-boiloff-gravity-calculator/