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Help a newbie Partial Mash brewer

Hi all,

I am so confused! Extract brewing is numerically easy…add the extract to the boil and the OG is almost always hit. No worries. The brewing programs available (i.e., BeerSmith) seem to work well for AG brewing. I am equipment-limited to partial mash at this point (5 gallon brewpot on the stovetop), but really enjoy utilizing the palette of grains available to the partial masher. The ‘conversion’ feature of BeerSmith from an AG recipe to PM never gives me an addition of LME/DME to make up for the added water in the fermentor.
So I am left with attempting to devise my own recipes. I have a 5-gallon Rubbermaid round cooler with a SS false-bottom and a ball-valve. I want to mash as much grain as possible for a 3.5-4 gallon boil, but I seem to be having trouble grasping the concept of quantity of extract to use to reach the desired OG.
I guess what I am asking is how do you take an AG recipe and convert it to a PM? In BeerSmith, I can remove a quantity of base malt and add an extract and adjust the amount to equal (or nearly equal) the original OG. But that recipe is generally constructed using a larger boil volume than I am capable of at this point.For some reason, adjusting the boil volume down and the addition of top off water to the fermentor does not alter the recipe quantities?
This may not be rocket science, but it is science. I have read more than I care to admit to be at this state of confusion. It would seem to me that adding 1-1.5 gallons of water at SG 1.000 to the fermentor would lower the SG of the 1.062 wort extracted from the MLT. But by what factor?
I apologize for what I feel should be an obvious answer, but I’ll be damned if I can figure it out.
Thanks in advance for your patience with a newbie-


Don’t over think it. The only volume that really matters is your finished batch size (ie 5 gallons) whether you do full or partial boil. Assume an efficiency for your partial mash (70% is probably a good starting point), and calculate the gravity contribution of the grains based on that, as if they were the only fermentables in a 5 gallon batch. Then, make up the difference between there and where you want to be with extract. Add the extract toward the end of the boil, and when its all done top up to 5 gallons just like a regular extract batch.

Also, Beersmith I believe lets you specify batch size and boil volume separately, which may help you.

If you’re trying to figure out how adding water will affect your gravity, it’s actually pretty simple. You just have to go with the idea of “total points.” As long as you know your actual volume and actual gravity, you can figure out how much water to add to hit your desired gravity.

Let’s say you have 3 gallons of wort at 1.062. You have 3 (gal) x 62 (gravity points ) = 186 gravity points total. If you add 1 gallon of pure water, then you still have 186 gravity points, but now you have 4 gallons of wort.

So, 186 points/ 4 gallosn = 46.5 points per gallon, or an O.G. of 1.0465

I hope that makes sense to you.

But if you want to boost gravity, like you said, then you’re adding extract. DME provides about 45 points per pound per gallon.

So for each pound of DME you add to your wort, you’ll be adding 45 TOTAL gravity points. One pound of DME would raise one gallon of wort by 45 points. Keep in mind the concept of total points. So if you add just one pound of DME into a 5 gallon batch, you’ll be raising the entire gravity by 45 total points, or (45 points/5 gallons) = 9 gravity points per gallon.

So that explanation turned out like the first pint of altbier I just pulled - About as clear as mud! Hope it helps though.

the above site has a lot of useful calculators. One is a “new SG calculator” or “new volume for target OG”.

You will need to do a little backward calculating.

Example: you can boil 4 gallons, so you will end with 3 after a 1 hour boil. You want a OG of 1.050. Playing with the calculators, you need to have a “ending boil” SG of 1.085

Then play with your recipe calculator to have the grain/extract needed for a 4 gallon boil, ending with 3 gallons to reach the 1.085.

When you get a OG above 1.065-.70, efficiencies start to drop off. So the bulk of your fermentables above that should be the extracts. So I would use the grain needed to reach 1.065 and then add the extracts to reach 1.085.

to see if the grain/water will fit in the cooler, see “can I mash it” here.

If I lost you let me know. I will try to explain if differently.

Thanks for the replies! I’m currently at work and need to sit and ruminate on the responses.
Reminds me of my old Calculus classes…staring at equations until magically POOF! the answers and procedures became clear.
Of course, aging doesn’t help… :slight_smile:


Thanks to all of your input, I think I am getting a handle on this. I was thinking of brewing this recipe for a Belgian Dubbel. Any and all feedback would be greatly appreciated!

Boil Size 3.5 gal Total Batch size: 5.25 gal


Munich - 10°L (Briess) 1.4 1.035 Mash
Aromatic Malt - 19°L (Castle) 1.4 1.036 Mash
Special B – 147°L (Castle) 1.4 1.031 Mash
Biscuit Malt – 24.5°L (Crisp) 1.3 1.036 Mash
CaraPils/Dextrine – 1.5°L (Briess) 0.6 1.036 Mash
Candi Sugar, Dark (Hard) - 275°L 0.9 1.046 Boil
Briess DME Pilsen 5.0 1.043 Boil
Hallertauer 1.0 oz Boil

Mash @ 150 for 60-90 minutes in 1.9 gal water (1.25 qt/lb)
Batch sparge w/ 2 gal 168 water
Collect 3.5 gallons form mash/sparge. SG of wort into kettle 1.037

Add Candi Sugar: Boil 60 mins
Add .5 oz Hallertauer: Boil 60 mins
Add .5 oz Hallertauer: Boil 30 mins
Add 6.6 lbs Light DME: Boil 15 mins
Add 1/2 tsp yeast nutrient: Boil 15 mins

Pitch 2 packs Wyeast WY3787 Trappist High Gravity

Top up fermenter with 2 gallons and wait impatiently!

If I calculated this correctly my OG should be approx. 1.069.

Please let me know if my numbers make sense. All calculation were done using (lbs of malt)*gravity / volume * efficiency (estimated at 65%). Obviously the extract was calculated at 100% efficiency.

Am I getting close here??

Seems I neglected to add the 5 lbs Pilsen DME to the boil ingredients. It should be the Pilsen DME instead of the 6.6 lbs Light DME

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