### Author Topic: Calculating ABV after adding yeast starter to a stalled ferment  (Read 559 times)

#### BonJoey

• 1st Kit
• Posts: 1
##### Calculating ABV after adding yeast starter to a stalled ferment
« on: January 26, 2023, 02:59:57 pm »
Looking to figure out how to calculate my new ABV. OG was 1.045, looking for FG of 1.005, stalled out at 1.014. Added yeast starter, which brought gravity to 1.019. Any help would be most appreciated. Thanks.

#### dmtaylor

• Official Poobah of No Life. (I Got Ban Hammered by Drew)
• Posts: 4774
• Lord Idiot the Lazy
##### Re: Calculating ABV after adding yeast starter to a stalled ferment
« Reply #1 on: January 26, 2023, 04:24:01 pm »
If your FG changed from 1.014 to 1.019, then you added 5 points to the gravity.  You can add the same 5 points to your OG, so now your effective OG has gone from 1.045 to 1.050.  From there, the ABV depends on whether the new yeast starter did anything to reduce the gravity further.  In any case, it will be:

ABV = (OG - FG) * 131
Dave

The world will become a much more pleasant place to live when each and every one of us realizes that we are all idiots.

#### jverduin

• Cellarman
• Posts: 70
##### Re: Calculating ABV after adding yeast starter to a stalled ferment
« Reply #2 on: January 26, 2023, 05:24:04 pm »
Looking to figure out how to calculate my new ABV. OG was 1.045, looking for FG of 1.005, stalled out at 1.014. Added yeast starter, which brought gravity to 1.019. Any help would be most appreciated. Thanks.
It would be a bit of a guessing game unless you have more info. The reason being is that a starter’s gravity could be somewhere between its OG and FG when you put it in.

If you know your starter’s OG and volume, it’s much easier….

New OG = ((original OG in gravity points * original volume) + (original OG of starter wort in gravity points * original volume)) / beer volume + starter volume

If this was a 5 gallon batch OF 1.045 with a half gallon starter at 1.060 it would calc like this:

New OG = ((45 * 5) + (60 * 0.5)) / 5.5 = 46.3 or rounded 1.046

Once you hit final gravity you can calculate the abv with the new OG and the measured FG.

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#### Skeeter686

• Assistant Brewer
• Posts: 114
##### Re: Calculating ABV after adding yeast starter to a stalled ferment
« Reply #3 on: January 26, 2023, 05:49:34 pm »
You wouldn't happen to have both a refractometer and a hydrometer, would you?  I believe that if you get both of those readings, you can determine ABV without needing to know OG.

Someone more knowledgeable can please correct me if I'm mistaken.

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#### dmtaylor

• Official Poobah of No Life. (I Got Ban Hammered by Drew)
• Posts: 4774
• Lord Idiot the Lazy
##### Re: Calculating ABV after adding yeast starter to a stalled ferment
« Reply #4 on: January 28, 2023, 06:30:34 am »
FYI -- the resolution to this one that we reached on the other thread linked below is that the Tilt was not reading accurately. My guess is there was yeast or hop schmutz stuck to the Tilt. The other two readings are consistent at approximately 1.011 plus or minus a point. Didn't achieve his FG goal of 1.005 (impossible with Kolsch yeast and recipe), but DID achieve his ABV goal of ~5.0%.