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Consistently Low OG despite good Efficiency

I seem to be missing some. For my past few all grain brews I have measured my efficiency and it averages to be about 80%. However my OG after my last hop addition consistently come out consistently low. I am adjusting for heat. In response. I add the appropriate amount of DME and usually nail my target. On the other hand, I don’t like having to add DME after my last hop addition because I am afraid this will change my aroma since theses are typically short boil times but I add 10 minutes of boil time after adding the DME.

Any suggestions?

Adjust your boiloff rate. Maybe you aren’t boiling off as much volume as you think. Efficiency calcs depend on volume measurements just as much as gravity measurements. How do you measure volume, pre-boil and post-boil?

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How are you calculating your efficiency? Are you stirring the pre-boiled wort well before taking a BG reading? Sugars will stratify in the BK before boiling. Are you using a hydrometer or refractometer to do your gravity measurements. If refractometer, have you calibrated it with distilled water?

I usually use the efficiency calculator on (surprised beersmith doesn’t have one on the app). I don’t think it is boil off time since I measure preboil. My kettle has volume gage. So, I should be pretty accurate there.


What is your total batch volume immediately prior to the boil? What is your volume immediately after the boil? This matters a lot.

13 pre/11post typically

You’re sure that’s not 12.75 pre and 11.5 post? It’s got to be accurate.

Now take your pre-boil gravity, and look just at the gravity units (GU) to the right of the decimal. So for a 1.065 beer, the GU is 65.

Okay, a bit more mathematics…

If you intended to have 13 pre/11 post with a post-boil OG of 1.065, then you would have to have 1.055 pre-boil because 55 * 13 / 11 = 65. You use the ratio of volumes to get the gravity you want with the 2-gallon boiloff that you have.

But if your actual volumes are something like 12.75 and 11.5, then look what happens for the same 1.055 pre-boil:

55 * 12.75 / 11.5 = 61 or a post-boil OG of 1.061.

Oh no, my gravity is a little low again.

You need to know all your volume measurements exactly if you want to nail your post-boil gravity.


Well explained @dmtaylo2. In addition have you confirmed the measurements in the BK are accurate?

This is great info. I will continue to refine my calculations and use this post next time I calculate my OG. I guess this leads me to another question.

How did you come up with the 1.055 (GU of 55) to multiply by my ratio boil off ratio?

The 1.055 was just an example, selected in this case because it is evenly divisible by your pre-boil value of 11 and I wanted to make the math easier to see. It gets more complicated when you start measuring in quarts or tenths of a gallon. The same methods however will apply for any other gravities as well.

The concept to keep in mind is that you start with one volume, boil off some, end up with a post-boil volume… but, the volume times the GU will always stays the same no matter how much or how little you boil. 55 * 13 = 65 * 11 = 715 = 143 * 5 = 73.27 * 9.76 Whatever you can possibly think of, the volume times GU is always the same for your particular batch of beer before and after the boil.


So am I assuming that pre-boil gravity should be lower than post because of the boil off volume?

I think that is what you are saying…

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Pre-boil is lower than post because of boil off.

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dmtaylo2… you are waay better at math than I am. Sometimes I need this stuff explained to me a couple different ways. I grabbed this off of a blog and compared it to what you were saying. Now it all makes perfect sense. Thanks!

You had a 5 gallon batch with a OG of 1.062. If you multiply the 62 times the number of gallons, you have 310 points in your wort. For a preboil reading, you likely had 7 gallons of liquid, which probably had a gravity reading of 1.044 (310/7).

So, if you know what your preboil liquid level is, you can get a gravity reading and estimate what your gravity will be postboil the same way.

If you had gotten a 1.044 reading preboil on 7 gallons, you would have measured 308 points in your wort (7 times 44). By dividing the 308 by 5, you’ll get your postboil gravity, 1.062.


Thanks Dave! I can do the math too, but it takes sooooo long for me to type stuff out, that I don’t reply… Sneezles61

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