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I've prepared a 1qt (~1L) yeast starter according to instructions from Wyeast.com. I didn't measure its gravity, but in theory it should be about 1.040.

I am brewing a 5 gallon batch of high gravity stout @ 1.076 target original gravity.

So, after pitching the starter, the total volume in the fermenter will be 5.25 gallons.

How do I calculate a more accurate gravity reading from the blended starter and wort?

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4 Answers 4

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If the starter fully fermented, most of the 1.040 should be gone; normal yeast attenuation is around 75%, so you should have 1L of 1.010 beer in the starter vessel.

1L of 1.010 beer into 19L of 1.076 OG wort would reduce the OG down to about 1.073. (76 points * 19L + 10 points * 1L) / 20L = 72.7 points = 1.073.

Though if possible, you should try to cold-crash the yeast, decant off most of the starter liquid, only retaining enough liquid to swirl the yeast back up into a slurry, and pitch that.

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    I think you should use the original gravity of the starter, not the final gravity, when doing the weighted average. Otherwise your ABV calculation will be off for the completed beer.
    – FishesCycle
    Commented Jan 19, 2014 at 16:56
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    Oh! Use an average formula. Makes sense now. But I think @Tobias-Patton is right that you should use the OG for both wort contributions.
    – gfullam
    Commented Feb 7, 2014 at 4:23
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The easiest way to determine the effect of starter gravity is to decant the starter so the amount is negligible. In addition, I've found that it makes better beer.

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Average it!

First convert your values to Plato as specific gravity isn’t linear for mass of sugar in a volume vs gravity points. This allows you to use simple algebra to average different beers. If the two gravities are very close then not converting will mean you will be off by only a few points, but if the gravities are vastly different your calculated value will be off by quite a bit.

Multiply your starter volume and wort volume by their original Plato gravities respectively to produce numbers that can be combined to derive an average gravity reading from the blend.

Divide the sum of the gravity-volume products by the sum of wort volume:

( ( OG1 × V1 ) + ( OG2 × V2 ) ) ÷ ( V1 + V2 ) = SG

Where...

  • OG1 is the original gravity of your starter wort in Plato.
  • V1 is the volume of your starter wort.
  • OG2 is the original gravity of your wort in Plato.
  • V2 is the volume of your wort.
  • SG is the specific gravity of the blend in Plato.

Finally convert back to specific gravity from Plato using an appropriate table or online calculator.

If blending more than two parts:

( ( OG1 × V1 ) + ( OG2 × V2 ) [ + ( OGn × Vn ) ] ) ÷ ( V1 + V2 [ + Vn ] ) = SG

In this case:

( ( 1.040 × 0.25 ) + ( 1.076 × 5 ) ) ÷ ( 0.25 + 5 ) =

( 0.26 + 5.38 ) ÷ 5.25 =

5.64 ÷ 5.25 =

1.074 specific gravity of blended starter and wort

Kudos to @jsled and @tobias-patton for providing the clues to solve this problem.

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  • Holy bold text Batman!
    – Scott
    Commented Feb 7, 2014 at 5:07
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If you add your entire starter to the wort, you're doing it wrong. Like Denny says, chill it, decant it, and then add the yeast slurry to your wort. No calculation necessary.

Adding a massive starter to a beer changes the recipe in unknown ways and potentially passes along any off flavors that might exist in your starter.

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