# How do you calculate how much extract to add if you don't hit your target gravity pre-boil?

Planning to do my first all-grain BIAB. If miss my target gravity after mashing, how do I figure out how much extract to add before I start the boil in order to reach that OG?

I would imagine pre-boil is the ideal time to add it because adding it after the boil could cause problems. I'd like to know how to calculate the amount of extract needed including water volumes. I'd prefer to use DME because I'm guessing I won't need a ton and once you open an LME container/bag it doesn't stay fresh too long. If there's a calculator for this online, links would be cool. I also have BeerSmith, though I haven't learned how to use all of it's features yet.

This is similar to this question, except that one is asking about post-boil, and I'm interested in pre-boil.

(TOG - GR * (BV/FV)) / (45/FV) = lbs of DME to add pre-boil to hit target OG

• TOG = Target Original Gravity in Points
• GR = Gravity Reading in Points
• BV = Boil Volume (This is what you are taking your reading from)
• FV = Final Volume (i.e. 5 gallons)
• 45 = # Gravity Points you get per lb of DME per gallon

So lets say you are making a 5 gallon smash beer with 10lbs of 2-row, you estimate your target OG by assuming 80% efficiency, and after sparging you got 6 gallons of wort in your brew pot. Equation would be...

``````(59 - 37 * (6/5)) / (45/5)
(59 - 37 * 1.2) / 9
(59 - 44) / 9 = 1.666
``````

So in the above scenario, you would toss in 1 and 2/3 pounds of Light DME pre-boil to ensure you hit your target gravity.

I'd love to provide a link to a calculator that does this equation for you, but I am not aware of one that is out there.

• Perfect, exactly what I was looking for. I guess I should code it up and stick it on a website at some point.
– paul
Jun 29 '12 at 20:51
• This worked well. I was shooting for 1.064 on my TOG. I put the numbers into the formula and it said add about 1/2 lb DME. After boiling and topping off with water, I ended up at 1.060, which I thought was pretty good for my first attempt at all-grain BIAB, though the efficiency was still only in the high 60s-low 70s I think.
– paul
Jul 2 '12 at 22:33

I do this all of the time. And it involves no math (that I have to do, anyway).

First of all, the day before (or sooner), I have my recipe in the brewing software. I make sure everything looks good (the pre-boil gravity, the post-boil gravity, volumes, etc).

After sparging, I take a pre-boil gravity and volume. If I'm ok on volume and low enough on gravity, I run downstairs and open the recipe in the brewing software. I change the mash efficiency (or something) to make the receipe look like what I ended-up getting for my gravity. I then add DME to the recipe in various amounts (trial and error) until I get back to the target pre-boil gravity. And a positive side-effect is that now your recipe software is more in line with your equipment.

• Very practical answer. I have BrewSmith and my recipe is in there, so I may try this method as well to see how it compares to using the equation hartski posted.
– paul
Jun 29 '12 at 21:19