Easy Gravity Adjustments for All-Grain Brewers

As I’ve said before, I’m a very analog person when it comes to taking notes and making adjustments while brewing. This runs contrary to the rest of my life, which is lived in front of a computer, but brewing is a chance for me to escape to the garage for several hours and read the Sunday paper while I sip a cup of hot coffee.

While my strike water heats, I typically draft up a sheet like this:

Brew Day Sheet

Typically, the only adjustment I need to make is to gravity or volume. I’m not sure if I just don’t brew often enough to have my boiloff rate pegged, or if the humidity really makes such a drastic difference, but the variability seems high with my crappy burner, so I often find myself doing some napkin calculations on whether or not I need to add some DME or boil longer in order to reach the desired specific gravity for a given batch. The calculations are really simple, but perhaps not obvious:

Let’s say for a given recipe, we’re trying to reach a post-boil gravity of 1.060 with a volume of 6 gallons.Our target points is the product of the number of gallons and the desired points:

6 gallons * 60 points per gallon = 360 total points

Now let’s say we ran off 7.5 gallons of 1.046 wort, and compute how many points we yielded:

7.5 gallons * 46 points per gallon = 345 total points

So we were 15 points short overall. We have two options at this point: boil longer, ending up with less than 6 gallons of wort, but reaching the correct gravity, or add some dry malt extract (DME) to make up for the lower-than-expected efficiency and keep our 6 gallon post-boil volume target.

Figuring out the required post-boil volume is simple division:

345 points / 60 points per gallon = 5.75 gallons

This is useful, because boiling off an extra quart might be just fine depending on your system. The DME route is a bit more complicated, but if one pound of DME yields 43 points per gallon (Briess Golden Light) and we need to add 15 total points:

 15 points / 43 points per gallon per pound = 
    0.34 pounds (about 5.5 ounces)

Hopefully this takes some of the mystery out of doing gravity corrections on the fly. There’s surely dozens of smartphone apps to help out with problems like this, but for those of us that enjoy a more unplugged brew day, it’s great to know how to just hammer things out on paper.