Brew Tips: Compensating for Missed OG

Today’s topic is hardly a secret or new idea in the brewing world, but one a lot of new and even relatively experienced all-grain brewers overlook: correcting for missing your pre-boil gravity.  It took me reading about it three or four times before I started to implement some of these tips, so hopefully this post will finally get you into it.

It can be very difficult for even experienced brewers to regularly hit all the important numbers when brewing, especially if its a new recipe or the equipment or process has changed (I like to throw in a step mash instead of my normal single infusion sometimes).  Many brewers only measure the final wort after boiling, at which point the best they can do is take good notes and make tweaks to the process next time.  A better way is to measure pre-boil gravity and take some corrective action.

First of all, you’ll need a method of measuring at least gravity; volume can be approximated if you know your system very well.  A refractometer is preferred, as the smaller volume can be much more quickly cooled to a reasonable temperature than an entire hydrometer sample tube (remember, ATC doesn’t mean you should throw boiling wort on your refractometer!).  Some way of measuring wort volume in your boil kettle also helps.  A site glass is nice, but I just use a long spoon with calibrated markings.


Photo by CEphoto, Uwe Aranas / CC-BY-SA-3.0

Whatever your method, take a volume and gravity reading after you finish running your mash off and record these numbers.  For example, we’ll say its 6 gallons (24L) at 1.050 (in this post, I will not be converting the metric exactly, just taking round numbers for each; metric will be in parenthesis).  Take the gravity reading, subtract one, and multiply by 1000 (ie throw away the 1. from and keep the xxx).  Now, multiply this value by the volume to get total gravity: 6*50=300 GU  (24*50=1200 GU), where GU is English or metric gravity units.

Total gravity, unless you add more sugar, will now stay the same: when water is boiled off, the volume decreases, but the specific gravity increases.  So, you can predict your post-boil gravity by simply dividing total gravity by your target volume.  This assumes you know your average boil-off rate.  Say the final batch is to be 5 gallons (20L), with a boil-off rate of 1 gal/hour (4 L/hour) for one hour.  You can expect your post-boil gravity to be 300/5=60 or 1.060 (1200/20=60 or 1.060).  If that’s what you are shooting for, great!  Its nice when things work out.

If not, you can take action now to correct ityou have plenty of time to figure things out while the wort comes to a boil.  First, find your desired total gravity and then you can take action.

Gravity too high:

Say you want a 1.055 OG beer: 55*5=275 GU (20*55=1100 GU)  So, you are 300-275=25 GU (1200-1100=100 GU) too high.  Your best option at this point is to simply add more water to the boil kettle and make more beer.  Simply divide the GU you have by the GU you want and multiply by your original target volume to see how much beer you can make: 300/275*5=5.45 gallons (1200/1100*20=21.81 L).  So, add 0.45 gallons (1.81 L) of water to your brew kettle and proceed as normal.  I like to use excess sparge water, as it is already warm and treated.

You’ll also need to recalculate your hop amounts for the excess volume.  Just multiply by the same factor as before: 300/275*(hop weight), (1200/1100*(hop weight)).  If you have a solid bittering charge, you can simplify things and only adjust this addition.

Gravity too low:

Say you want a 1.065 OG beer: 65*5=325 GU (20*65=1300 GU).  So, you are 325-300=25 GU (1300-1200=100 GU) short of your goal.  You have a few options at this point.  The best is to add some malt extract to make up the difference.  Using DME with an extract of 40 points/lb/gal (334 points/kg/L), you’ll need to add 25/40=0.625 lb (100/334=0.30 kg).  If you go this route, you’re doneno hop adjustments are needed.

If you still have some sugar left over in your mash tun, you can also continue sparging and add the runnings to your kettle.  You’ll have to re-measure your gravity and volume at this point and decide which course of action to take now.  You’ll need to boil off additional water (see below), and perhaps still need to add DME, adjust your batch size, or adjust your hop rates and live with the result.  Take care not to over-sparge, or your beer could become astringent from tannins in the grain husks.

If you’re willing to make a smaller batch of beer, you can also boil longer and reduce the total volume produced.  You can find the final batch size similarly to the above water calculation: 300/325*5=4.61 gal (1200/1300=18.46 L)  So, you need to boil off an additional 5-4.61=0.39 gal (20-18.46=1.54 L).  Assuming an evaporation rate of 1 gal/hour (4 L/hour), you’ll need to boil for an additional 0.39/1*60 min=21.6 min (1.54/4*60 min=23.1 min).  Add the extra time to the start of your boil, before hop additions, so your hop rate calculations do not get too complicated.  Again, adjust your hop rates by 300/325*(hop weight), (1200/1300*(hop weight)).

Whether your gravity is too low or too high, you can also choose to only adjust your hop rates to maintain your originally intended IBU/GU ratio.  If you aren’t concerned with exactly hitting the intended numbers, this will at least maintain nearly the same balance.  Especially if you aren’t too far off your mark originally, this alone may be enough to make the difference unnoticeable.

You can find some additional examples in Daniels’ Designing Great Beers, chapter 6.

Water expands slightly when it gets hot.  If you measure the volume of the same mass of water when its boiling, it will have a volume that’s about 4% more than its volume at room temperature.  This will throw off the above calculations a bit.  My advice: ignore this factor and you won’t miss it.

Dennis, Life Fermented Blog


About Dennis
Home brewer, home chef, garage tinkerer. Author of Life Fermented blog.

2 Responses to Brew Tips: Compensating for Missed OG

  1. brulosopher says:

    Great info! I believe this issue indicates some of the importances of 2 things: knowing your boiloff rate and measuring your pre-boil SG. As you mention in the Gravity Too High section, “You’ll also need to recalculate your hop amounts for the excess volume,” which you can’t really do at the end of the boil; however, if your pre-boil SG is higher than target and you know your boiloff rate, you’re able to effectively calculate how much extra water to add at that time and, subsequently, any extra hops you might add. Cheers!

    • Dennis says:

      Yep, you hit the nail on the head. I don’t know why it took me so long to start measuring pre-boil SG, but its an easy thing to do to really improve the consistency of your beer and come closer to brewing the same beer as someone else with their recipe.
      – Dennis

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