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So I brewed my second all-grain batch today, and, like with my first all-grain batch, I misjudged the final volume and ended up with slightly under 4 gallons in my carboy. Does anyone have any good resources for approximately how much will boil off? I guess I'm undershooting because I don't want to be too low on my gravity with too much volume, but at the same time going through a brew day and cheating myself out of over a gallon of beer isn't efficient for time or money.

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I've been brewing all grain for six years and I'm finally getting the right volume into the keg at the end of the brew. You can get close with calculations.

Mash

The grain absorbs about one pint per pound. Divide the weight of your grain by eight to get the number of gallons it will absorb. You won't lose any water during sparging because the grain has already absorbed as much water as it can hold.

It is important, however, to figure out how much sweet wort you leave behind in the mash tun. My cooler with manifold tun couldn't suck up all the liquid in the bottom. I'd leave a quart or more in it.

Boil

Measure the pre-boil volume. It is an important aid in figuring the final gravity and hop utilization.

Don't blast the wort. It's sufficient to maintain a 6-10% evaporation rate according to the pros I talk to. Listen to the Brew Strong episode on the boil for the whos and whyfores.

Hops and trub will absorb a small amount of water, especially in an IPA.

Measure the post-boil volume. Keep in mind that boiling water takes up 4% more volume than water at room temperature. In other words, if you have 5 gallons of wort at a boil, it will seem like 5.2 gallons. Adjust accordingly.

Account for any wort left in the kettle when all is said and done. I was wasting almost a quart before changing my pick up tube.

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It doesn't matter what other people get. Everyone's equipment is different, and everyones environment (climate, humidity) is different. In 90minutes I boil off between 1.25 to 1.5 gallons. Equipment variances are just to great person to person, especially when it comes to the boil right. What one guy calls a rolling boil could be someone else's idea of a simmer. etc etc.

You would really do yourself a better service if you figure out what your boil off rate was yourself rather than looking for an equation. Just look at what volume you started with and where you ended up. Then make adjustments for next time.

The least talked about but most important part of all grain brewing is probably volume measurements. You need to accurately be able to measure the water going into the mash, the wort coming out of the mash (grain absorption), the amount in the kettle pre and post boil (evap rate), and the amount making it into the fermenter (brewhouse efficiency).

Take the time now in your early all grain brewing career and make some sort of measuring stick for your kettle. Do it before you brew your next batch and it will make the process much less of a mystery going forward.

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Thanks for the solid advice, especially the measuring stick idea. I brewed a Pliny the Elder clone today and hit my gravity and volume exactly. Excellent! –  WhatsBillDoing Jul 18 '10 at 22:30
    
Glad it helped. –  brewchez Jul 19 '10 at 12:16
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Though your beer is stronger than it would have been.. :)

Here's a pretty good calculator for figuring out how much water to start with, which is a huge plus: http://www.brew365.com/mash_sparge_water_calculator.php

Do you know how fast you boil off water? Aka what your evaporation rate is? If so, you can do some fun math and figure it out.

Start with total gravity. Total gravity is how much sugar is in your wort, regardless of volume. Add 1000 gallons or boil it to a syrup, total gravity remains the same.

Take a reading (at room temp) with your hydrometer (if you have one. If not, get one.)

I like to take the 1.0 out of the reading, so 1.050 becomes 50. If you need a formula for that, it's ( gravity reading - 1 ) * 100

Multiply that number times the number of gallons you currently have. So, 6 gallons at 1.050:

50 * 6 = 300

300 total gravity points

If you know you boil off at 1 gallon per hour, then at the end of the boil, you'll have 5 gallons, with a gravity of 1.060:

300 gravity points / 5 gallons = 60

If you want to get to a gravity of, say, 1.060, and want to know how much water to boil off (simple version):

300 gravity points / 60 original gravity = 5 6 gallons (current volume) - 5 gallons (desired volume) = 1 gallon

This can get more complex, obviously. If you want to boil it down so that at the start of your timed boil you're at enough volume to end with 5 gallons, you have to know your evaporation rate. But, you can still figure it out.

I hope that helps, if not, please comment and I'll try to expand on it.

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How does all that gravity calculations equate to boil off rate??? Start with the amount of water you have at the end of 60 or 90 minutes you have a new volume. Figure the percent and that's HIS boil off rate. I think all the gravity calcs here confusion the original issue. But they are valuable calcs to know regardless. –  brewchez Jul 12 '10 at 11:49
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How does all that gravity calculations equate to boil off rate??? Record your starting volume and your finishing volume pre and post boil. Subtract the post from the pre- and that's how much water you evap in your boil time (60 or 90 minutes) Figure the percent if you like. I think all the gravity calcs here confusion the original issue. But they are valuable calcs to know regardless –  brewchez Jul 12 '10 at 11:51
    
I guess I was more answering how to figure out how much to boil off to achieve the gravity you want, assuming you know your boil off rate. I didn't even think to explain that part! ha, oops. +1 sir. –  hookedonwinter Jul 12 '10 at 14:22
    
Thanks PJ, the calculations were helpful here! –  WhatsBillDoing Jul 18 '10 at 22:29
    
I just built a calculator for this, actually: pjhoberman.com/tools/volume_adjustment.html –  hookedonwinter Jul 19 '10 at 14:41
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