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.

4 Answers 4


(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, 2012 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, 2012 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, 2012 at 21:19

I'm sure there are some easier ways of doing this, but I enjoy doing my brew calculations by hand! You need to figure out the total gravity "points" of your wort. After you've pulled the bag and rinsed it, take a hydrometer reading. You will most likely need to correct for temperature, most hydrometer's are calibrated to 15C, by 20C they are off by 1 point so at 70C it will be very inaccurate. I recommend cooling a sample as much as possible (I put it in my freezer) and then using a correction factor if it is not 15C. There are several sites online that will correct for you, or give you the correction factor. (http://www.howtobrew.com/appendices/appendixA.html) Once you have a reading on your hydrometer, let's say 1.035, you need the total volume of pre-boil wort. Let's assume 6.5 gallons. Multiply "35" (your hydrometer reading without decimals, called gravity points) by the volume, 6.5 gal to give you 228 total points. The total points of your brew will stay the same until you add the yeast. So by figuring our what your target pre-boil SG is, you can figure out how much your off by. Example, your shooting for a final, post boil volume of 5 gallons of wort at 1.055, or 275 points. By simpley dividing your brews total points by the pre-boil voume (6.5 gal) you know what your pre-boil SG should be (42.3 pts or 1.042). So, lets say your BIAB, pre-boil, comes in at 1.035 rather then 1.042. Multiply 35 gravity points, by the volume, 6.5 gal. to give you 228 total points. Subtract your target points 275, by 228 to give you a difference of 47 points. DME yields 40 pts, per pound, per gallon. About 1.2 pounds (544g) in 6.5 gallons will give you about 47 extra gravity points, helping you meet your target.

  • This is a very good answer and very helpful, just had to give it to hartski for the preciseness and conciseness of the equation!
    – paul
    Jun 29, 2012 at 20:50

I use the Beer Calculus tool over on Hopville.com. I start off my all-grains with my ingredients and about 80% efficiency for most beers (a little lower for wheat beers though). As I am boiling my wort, I collect a sample at the point where I've got about 5 gallons, near the end of the boil. I chill that sample down and test it's gravity. If the gravity of the sample is lower than my target gravity on Hopville, then I know my efficiency is down, so I'll turn down my efficiency setting on the recipe until the projected gravity matches the sample's gravity. From there, I can add back to the recipe some LDME until the target gravity is where I want it to be.

Whenever I've done this, the measured gravity of the final wort going into the carboy is pretty much exactly the gravity that Hopville predicted once I add in the extract.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.