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I've seen some tables online like the ones at American Homebrewers Association and Brewhaus Canada that list Potential ABV for different values of Original Gravity. My intuition tells me that these tables are developed using an estimated conversion rate for an average sugar content based on the Original Gravity.

It seems to me that most of these tables do not agree with each other. How are tables like these developed? Are they based on observations and experience, or is there a calculation that can be performed to determine Potential ABV using the Original Gravity (and a knowledge of the ingredients used)?

Disclaimer: I'm aware that Potential ABV is a poor estimate of Actual ABV. I'm mainly interested in the theory behind the concept.

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There's 4 main parameters that are going to go into potential attenuation:

  1. The yeast's inherent attenuation range.
    • While most yeasts are in the 75-77% apparent attenuation range, some (like Wyeast 3711 in particular), have nearly 90% attenuation.
  2. The relative "strength" of the yeast going into the fermentation.
    • This is going to be a function of the number of cells pitched, the cell vitality, the amount of the various yeast nutrients in the wort and available oxygen.
  3. The mix of fermentable and unfermentable sugars in the wort.
    • Mash temperature plays into this balance; the lower mash temp range promotes shorter, more fermentable sugars, while higher mash temps tend toward less fermentable sugars, thus lower attenuation and more residual sweetness.
    • Adjuncts (rice, simple sugars, honey) also affect this balance, of course.
  4. The temperature of the fermentation.

If I wanted a quick rule of thumb for abv based on OG: last-two-digits-of-OG * .75 * .135. Eg. for a 1.040 OG beer: 4.05% ABV. This represents 75% attenuation, then the "standard" .135 multiplier for ABV. Adjust this based on any outliers based on the criteria above: using an outlier yeast, adding sugar to the wort, outlier mash temp or outlier fermentation temp.

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