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What does a pre-boil gravity tell me?

What does it mean, and what do I do with that information?
Seems like something I should know, and something I should be doing, yet, here we are.

It tells you how much denser your wort is than water as a result of dissolved sugars. This is an indication of approximately how much sugar is available to the yeast (approximately because wort contains some types of sugars that yeast can’t break down).

Once you have both post-boil gravity and post-fermentation gravity you can roughly calculate how much alcohol is in your beer.

Assuming you’re doing AG batches, the pre-boil gravity will give you information on your mash+lauter efficiency which you can use to pinpoint any problems (or to confirm that you’re golden). You can also make last-minute adjustments to your recipe - for instance, if you’re gravity is too high, you might want to shorten the boil a little or increase the hops.

ok, so…
We did a porter and the target og was 1.063, we came up short at 1.051. we didn’t take a preboil grav. reading, but, if we had, how could we have used that info to help us up the gravity (besides adding more fermentables to the boil)

I don’t know how we managed to miss by that much, I think we sparged (fly) too fast, if we had taken a preboil grav. reading we would have known that we were off (or at least that the REASON we were off) was in the Mash process?

If the pre-boil is too low, you can either add more fermentables or increase the boil time and/or the boil vigor (a really rolling boil will evaporate more water than a simmer) to reduce the volume and raise the OG.

A low pre-boil means that you had incomplete conversion in the mash or your lauter efficiency was impacted, by sparging too fast or by channeling for instance, or a combination of both.

So. What is the best way or ways to raise or lower your OG if you miss your mark too much? Is there a simple way to calculate your additions of fermentables if you fall short? Just wondering best ways to mend the problem.

You also need your volumes (pre-boil, and a predicted post-boil based on your evaporation rate). Say if you have six gallons of 1.040 wort at pre-boil, and you evaporate one gallon over your hour boil. The math: 40 points/gallon X 6 gallons = 240 points. Those 240 points in 5 gallons yields a gravity of 48 points per gallon (240 divided by 5) or 1.048. If that is below your target, you can add DME to add more points. If its too high you can increase your volume.

Extract additions is one easy way. Adding sugar will do it, but I believe that can dry it out and diminish body. You could also boil it down until you hit your mark, but it could take forever depending how far off you are.

I think you get 9 gravity points for every pound of extract added to a 5 gallon batch.

[quote=“Gr8abe”]ok, so…
We did a porter and the target og was 1.063, we came up short at 1.051. we didn’t take a preboil grav. reading, but, if we had, how could we have used that info to help us up the gravity (besides adding more fermentables to the boil)[/quote]
Another option a pre-boil gravity gives you is the ability to adjust your recipe to fit the gravity that you did get. For example, if your gravity is low on a Robust Porter or IPA, you can adjust the hops to make a Brown Porter or Pale Ale. Alternately, you can figure out how long of a boil it would take to hit the gravity you want and correct the hop schedule to accommodate that.

If you take a pre-first runnings gravity, you can figure out if you had complete conversion and extend your mash or increase the temperature to make sure that you will hit your desired gravity. That won’t fix a channeling problem, but rules out incomplete conversion as the potential source of low OG.

I know everyone’s eyes (including mine) glaze over when numbers come up but it’s really not that bad. Here’s how I figure out how much to add.

The easiest way to work with specific gravity is by using points. Simply drop the leading “1.” from the preboil gravity reading, and use the 3 digits after the decimal - so a specific gravity of 1.060 is equivalent to 60 points.

First, how many gravity points came from your mash?
Gravity x Pre-boil Volume = Total Gravity Points
60 x 6.5 = 390

Next, how many gravity points were we hoping for?
70 x 6.5 = 455

Finally, use the calculation below to find out how much to add considering that DME is worth roughly 44 PPG and LME is worth 38 PPG

Amount to Add = (Gravity of Target - Gravity of Mash)/44 for DME or 38 for LME

1.48 lbs of DME = (455 - 390) / 44

Up until now, I haven’t taken a pre-boil gravity reading. I never really thought to. After reading this thread, I think I will start, beginning this weekend with my Fat Tire AG clone. There was some really good info posted here. Thanks to all who shared.


To me, a preboil gravity reading is incredibly useful. Here’s how I think about it:

ANY accurate gravity reading at any point in the boil, coupled with an accurate volume measurement, interestingly enough, will give you pretty much the exact same information, unless there are wort losses in the process.

Here’s an example of how I use it…

Let’s say I’m going for a 5G batch of a 1.050 beer. I measure my preboil gravity at 1.040 and volume of 6G. The simple math is that 6*40 (volume times “points per gallon”) gives me the measure of sugars that are in my wort, which will remain constant throughout the process.

In this case, that’s 240 points. When I boil that down to my desired final gravity, that will give me a gravity of 1.048 (the same 240 points, divided by the current volume of 5G).

So that’s pretty close, but I could always boil a bit longer (long enough to get 4.8 galllons post-boil) if I wanted it exact. Of course, you have to watch the boil time of your flavor/aroma additions if you decide to do this, so it helps to make any decisions along these lines before you add those hops. For bittering hops it effectively makes no difference unless your final volume is skewed enough to really alter the IBU’s just because you’ve got the same hops in higher or lower volume.

In my case, I almost always add boiled top-off water toward the end of my boil to make my final volume right, which means I intentionally target a postboil volume lower than what I really want before the final top-off. I feel like this gives me the most control and flexibility, but this is not necessary if you’re confident in the consistency and accuracy of your boil-off rates.

Basically, the earlier in the process you get info on gravity, the more options you have. More fermentables, increase or decrease final volume to dilute or concentrate the final wort–these are your tools.

By the way, I batch sparge and I use a refractometer for all my gravity measurements, which makes it very quick and easy to take as many as I want. I take gravity readings on the first runnings, second runnings (which can tell me if I can or want to sparge more), full volume preboil (just for sanity check), postboil, and whenever else I feel like it :slight_smile: .

Hope this helps.

I’m thinking I need to get a refractometer.

I love my refractometer. Some people don’t like them because they don’t directly tell you the amount of sugar in your wort, but they use the light-refracting properties of wort as a proxy, more or less.

They require some calculations to give you FG (or any gravity once fermentation has started) data based on the measurement results, but I’ve never had a problem with this at all.

I have not used my hydrometer in 5 years, but I’ve done countless refractometer readings–with a volume of just a few drops of wort each, basically at any temperature up to boiling as long as the refractometer has ATC!

Another top-5 all-time equipment purchase for me :cheers:

hi everyone!
I brewed an IPA the other night,
11# 2row
1# caraMunich
.25# choco

I mashed a 122(10), 148(75), and 170(10)
I sparged for 60 minutes.
and took a preboil gravity before sparging (I think that might be part of my question here…)
the PBG was 1.080 (at about 70 degrees). Which if my math is correct, should indicate that my final gravity should be 1.104, am I right so far?

however when I took my starting Grav. it was 1.070 right on the nose. I KNOW I did something wrong, since my pbg should be lower than my starting grav. Right?

So what am I messing up here?
any help would be hugely appreciated since this math thing is really pissing me off!

You should take your pre-boil volume after sparging, not before. Your sparge runnings will be much lower and will bring your total pre-boil gravity down.

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