# Adjusting ABV calculations to adjust for a big starter

### Help Support Homebrew Talk:

#### TAK

##### Well-Known Member
I have a big beer in primary at the moment and I'm contemplating the calculations for ABV. I had a rather large starter, so I want to get a sense of how much I should adjust for the dilution from that starter. The starter brings gravity and alcohol to the batch, so it's not like dilution with water.

Is this method and math sound?

I assume that I should adjust my OG for the gravity points contributed from the starter. FG of the entire batch from that adjusted OG should give me an accurate ABV. Correct?

OG before starter = 1.121
Volume before starter = 4.6 gal
Starter OG = 1.037
Starter Volume = 0.375 gal (does not include volume of slurry)

So....

(1.121 - 1) * 1000 * 4.6 = 556.6 gravity points*gallon from wort
(1.037 - 1) * 1000 * 0.375 = 13.9 gravity points*gallon from starter
556.6 + 13.9 = 570.5 total gravity points*gallon
4.6 + 0.375 = total gallons of volume
1 + ((570.5/4.975) / 1000) = 1.115 adjusted OG

If my FG = 1.025, than I calculate...

11.75% AVB from adjusted OG, as opposed to
12.6% ABV without adjusting for the starter.

Is this an accurate way to adjust ABV for a big starter?

#### Bigscience

##### Well-Known Member
I think what he's getting at is he's diluting out his wort with a large starter that will also bring with it some alcohol.

#### Qhrumphf

##### Stay Rude, Stay Rebel, Stay SHARP
ABV has nothing to do with the starter. ABV is calculated from OG and FG.
http://www.brewersfriend.com/abv-calculator/
But if you're pitching an extra liter into a 5 gallon batch, and the OG of the starter is lower (or in some cases higher) than the full batch, you will be altering the gravity.

Usually I cold crash and decant off spent wort if I'm adding a large starter, so I end up adding only ~500-600 ml of yeast slurry, and don't worry about it. But if I'm pitching the whole thing, I let my software do the work for me, and I just adjust the batch size for the starter volume, and add the DME weight used for the starter into the grain bill.

The math you're doing looks right to me, so yeah, I'd say that's pretty accurate.

#### Yooper

##### Ale's What Cures You!
Staff member
Mod
For most bigger beers with a bigger starter, I decant the spent wort as to not dilute the beer with unhopped oxidized spent wort.

What we've got there is 4.6 gallons of beer that started at 1.121, but we don't know what it ended at as it was diluted with .375 gallons of finished beer and we don't know that FG of the starter.

To be really accurate, you'd have to know the FG of the starter.

But you can get "close enough" by making some assumptions. I'd assume the starter would finish at 1.010-1.012, and be approximately 3.3% ABV.

So, that'd be .375 gallon at 3.3% ABV.

The 4.6 gallons that started at 1.121 finished at 1.025(?) but there was some of the spent wort in that starter. Not really enough to matter, though, but it would change the FG a small amount. That would be 4.6 gallons of 12.9% ABV. I don't know of many yeast strains that go to 12.9% ABV, though.

Anyway, if it's 4.6 gallons of 12.9% beer + .375 gallons of 3.3% beer, that is 4.975 gallons of 12.1%. check my math- I drink, you know! :cross:

#### WoodlandBrew

##### Well-Known Member
Your logic and math look good to me.

#### kh54s10

##### Supporting Member
HBT Supporter
I believe you will get some idea of the ABV with your calculations but the only way to be accurate is to take the OG of the beer with the added starter then the FG and run it through the calculators.

#### WoodlandBrew

##### Well-Known Member
Actually using the OG with the added starter will give you an inaccurate result because some sugar has been converted to alcohol by the starter.

#### Yooper

##### Ale's What Cures You!
Staff member
Mod
Actually using the OG with the added starter will give you an inaccurate result because some sugar has been converted to alcohol by the starter.
Right. And the FGs would be different. That's why I did the ABV separately, as certainly the starter's FG was NOT 1.025. We don't know what it is, so I had to make the assumption that it was 1.012 or so.

#### WoodlandBrew

##### Well-Known Member
The way I see it is the fermentables are all known. The adjusted OG accounts for all of them even though the beer never had that OG. It's like calculating ABV when sugar is added to a beer after some fermentation has occurred.

OP
T

#### TAK

##### Well-Known Member
Thanks for all the feedback.

The way I see it is the fermentables are all known. The adjusted OG accounts for all of them even though the beer never had that OG. It's like calculating ABV when sugar is added to a beer after some fermentation has occurred.
Right, this is what I was thinking.

At first I was thinking along Yooper's route, calculating ABV separately for each and taking the weighted average. But then I realized that I cannot get the FG of the wort separately from the starter. If the reading is 1.025, that includes any effects from the starter, volume, alcohol, and unfermentables.

I don't know exactly what my starter was, but I added a calculated amount of DME and water to what should have been 1.037. Adjusting the OG for the weighted average of the wort and the starter, and then taking a single FG of the whole thing, should affectively capture a weighted average ABV as well. I don't think that it should matter that the starter attenuated separately. The starter's FG should be captured within the FG of the whole batch.

OP
T

#### TAK

##### Well-Known Member
Or, another way to put it is, I can know the following values:

a) OG of my wort, and volume
b) OG of my starter, and volume
c) FG of the whole batch
d) FG of the starter

I cannot ever know the FG of my wort alone, because the affect of the starter can't be isolated.

So, (d) doesn't seem useful. But if you aggregate (a) and (b), and calculate ABV from (c), all fermentables should be accounted for.

#### FRS

##### Well-Known Member
If you can borrow a refractometer you can use your hydro FG and refractometer FG to determine ABV without having to know anything about the OG, or worrying about how to compensate for the starter.

#### oskhelly

##### Member
Don't worry about anything except the OG and volume of both to get the combined OG

OG wort 1.121 - 4.6gal
OG starter 1.037 - .375gal
To find the OG of both the starter and wort combined divide the wort 4.6 by the starter volume .375 = 12.266... units the same size as the starter. Now you can average the two OG's
12.266 x 1.121 = 13.75...
Add starter OG 1.037 to 13.75 = 14.788
Now divide by the total number of units of volume which is 12.266 units in the wort and the 1 starter unit to get 13.266 total units
14.788 divided by 13.266 = 1.1147 OG of combined wort and starter.
Now what gravity the beer finishes at is all you need, to know ABV
Note I rounded some numbers

#### Wyrmwood

##### Well-Known Member
For most bigger beers with a bigger starter, I decant the spent wort as to not dilute the beer with unhopped oxidized spent wort.
I decant all my starters, even if they made with wort. Adding in the wort from the starter affects volumes and will have at least some impact on the flavor, diluting it to some degree, albeit perhaps negligible. The purpose of the starter is to increase yeast volume not beer volume

OP
T

#### TAK

##### Well-Known Member
If you can borrow a refractometer you can use your hydro FG and refractometer FG to determine ABV without having to know anything about the OG, or worrying about how to compensate for the starter.
Details here about using a refractometer and hydrometer in combination:
http://woodlandbrew.blogspot.com/2013/02/abv-without-og.html
Oh, man. I'm geeking out. Ran the numbers for some other beers I already know, and the formula here gets it right about the same, only using FG.

OP
T

#### TAK

##### Well-Known Member
Don't worry about anything except the OG and volume of both to get the combined OG

OG wort 1.121 - 4.6gal
OG starter 1.037 - .375gal
To find the OG of both the starter and wort combined divide the wort 4.6 by the starter volume .375 = 12.266... units the same size as the starter. Now you can average the two OG's
12.266 x 1.121 = 13.75...
Add starter OG 1.037 to 13.75 = 14.788
Now divide by the total number of units of volume which is 12.266 units in the wort and the 1 starter unit to get 13.266 total units
14.788 divided by 13.266 = 1.1147 OG of combined wort and starter.
Now what gravity the beer finishes at is all you need, to know ABV
Note I rounded some numbers
Yup, same math as OP, phrased just a bit differently. Combined OG is 1.115 after rounding to 3 digits.