5 gallons wort + 2 gallons water = ?

[BrewmeisterSmith]

I'm going to be getting 5 gallon of wort from a brewery for a local competition. I want to end up with 5.5 gallons in my fermentor. If the 5 gallons of wort is 1.050 and I add 2 gallons of water to bring up the volume, I'll need add X amount of DME to get the final gravity to 1.065 for the IPA that I'm making. The water will obviously bring the pre-boil OG down. Does anyone have some advice that will help me figure out the math on this?

 

[thadass]

DME is approx 1.040-1.043 per LB per gallon, so it would work out like this:

50 gravity units per gallon (1.050) x 5 gallons = 250 GU total from the brewery. 250 / 5.5 expected final volume = 45.45 GU, or 1.045, so 20 GU per gallon short of your target at the final volume.

If DME provides on the low end 40 GU per lb per gallon, we would like about half that, which is 0.5lb x 5.5 gallons = 2.75lb DME to raise it to 1.065 at a 5.5 gallon volume.

This means you can also expect your pre-boil gravity to be (65 GU/gal x 5.5 gallons)/7 gallons = 357.5/7 = 51, or 1.051 pre-boil gravity at 7 gallons.

 

[BrewmeisterSmith]

If anyone else is wondering. I came up with a simple solution to figuring this out. I poured my 5 gallons of wort into the boil kettle. Took a refractometer reading to verify that it really was what the donor said it was. 1.047 I believe. Then, I added enough water to get to my pre-boil volume. Took another reading. Then added enough DME to get me to my starting pre-boil gravity according to my beersmith recipe. I ended up hitting my target and confirming that my refractometer really does work!!