Calculating pre-boil volume for full boil?

I've been at this for about a year now, I've been trying to figure out how do a full boil and end up with the appropriate amount (and therefore the right OG). I never quite seem to get it right. I know I should "relax, don't worry, have a homebrew", but I'm a bit of a perfectionist.

A couple weeks ago I brewed a fairly high gravity scotch ale and ended up with about an extra half gallon in the fermenter. It should have been an OG of 1.079, but I ended up with 1.070. I used 6 gallons of water for my boil.

Last night, I brewed an oatmeal stout. I apparently over corrected by using 5.5 gallons of water. I ended up about a half gallon short in the fermenter. The OG should have been 1.059, but I ended up with 1.051. I anticipated this happening about half way through the boil and was able to boil up a half gallon of water on the side that I had ready to add at the end. But it still annoyed me that I had to bother with that.

I see 2 variables at play here:
1.) Because of the difference in OG, my pre-boil volume was considerably more with the Scotch Ale because of additional extract.
2.) Boiling Temp.

Is there a formula to help me calculate how much water I should start my boil with that will get me more consistent results?

Your gravity really shouldn't affect your boil off rate.

#2 is probably closer to the issue. Your boil will always be the same temp, but the more heat you put into the system the more you will boil off. Of course the amount of time will also change the amount you boil off. Temperature can also affect this if one boil was inside on the stove and the other was outside on a turkey fryer for example.

There's no good formula that I know of that would be useful, boil of rate is something that is more experimental by keeping all those other factors the same.

conpewter said:

Your gravity really shouldn't affect your boil off rate.

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I may have misspoken, but I wasn't referring to preboil gravity, I was referring to pre-boil volume. The quantity of extract you have to add to reach the gravity will effect the total volume of the boil, right? That's why when you use a 30 qt turkey fryer like me you damn near overflow the pot when you are adding so much extract to a high gravity beer.

6 gal of water + 6# of DME is going to produce a smaller volume than 6 gal of water + 8# of DME. Does this make sense?

Ah I understand now. If you add the water and the extract all at the beginning of the boil you can add the extra, then add water until you are at 6 gal (or whatever you determine your pre-boil gravity should be) If you do late extract addition, which is a good method, then you need to make sure your pot can handle it as mentioned. Keeping most other factors the same you'll have a different boil off rate for differing volumes of water. So if you are making a big beer and you need to boil with less water, and top up in the fermenter you'll have a different boil-off rate.

I don't know exactly how to calculate it. But your pre-boil total volume should always be the same given that you don't change how you do the boil. The hard part is that you will need to adjust your water to DME ratio to get that volume.

When I have done brews with 6 pounds of DME or LME, I have used 5.5 gallons. It usually is right on at 5 gallons after aeration. Sometimes it will be slightly over or slightly under. By slightly under I have had to add .25 gallons to bring it up to the 5 gallons and the gravity reading has been fine.

Coincidentally, I stumbled across some information about this last night. I just got my copy of "Brewing Classic Styles" in the mail yesterday and I was reading the introductory chapters by Palmer. He mentions that 3# of extract and 3 gal. of water is not the same as 3# of extract topped off to 3 gallons total volume and does not produce the same gravity. He provides a rough estimate that 12# of DME adds one gallon of volume, and 4# of LME adds a quart of volume.

I also read (which makes perfect sense) that you lose water that is soaked up by your steeping grains. I used about 1# more steeping grains in the 2nd batch because of the oats for the oatmeal stout. Palmer doesn't give a way to estimate this loss.

Long story short, 6 gal of water does not equal a 6 gallon boil. It sounds like the best method is to steep your grains, add extract, and then bring your wort to the desired preboil volume (or preboil gravity). Then add your hops and time your boil. Obviously late extract additions need to be accounted for.

I see a brewing software purchase in my near future.

Thanks all!

Fill your pot with one gallon, then measure with a stainless, or all plastic ruler. Add a gallon, and repeat until you get the whole volume filled and measurements written down. Now you can at any time calculate your volume in the pot.

I see a brewing software purchase in my near future

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There are free ones too. I use Qbrew. works nice.

i bought a pot with quart markings on the side -

but guess what? they're innacurate, lol. i dumped in two brand new, sealed 2.5 gallon water jugs. read 22 quarts. i mean, come on? if you're gonna make markings, make them accurate!

KNOW YOUR PREBOIL GRAVITY REQUIRED.

If you know your boil off rate, you know the preboil gravity required to get your OG. If you get the preboil gravity right, you dont need to worry about volume. Just keep sparging and bringing wort from the mash until you hit your preboil gravity in the boil kettle. Then boil away at your previously known and consistent boil off rate and... voila... OG perfection.

PreboilVolume = FinalVolume / (1 - Boil Off Rate%)

OG*FinalVolume = PreboilGravity*PreboilVolume

Use gravity units when using this equation - example 1.051 is 51 grav units.

Plug in the OG you want, the final volume you want, and the known boil off rate %, usually around 18% on my system. Everything else can be calculated by the above equations. Solve first for preboil volume in the top equation - then solve for preboil gravity in the bottom equation - and get to brewin!

Pangea