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I have advanced to a very prolific brewer and have mastered many aspects of home brewing. For some reason I seem to be unable to master the issue of Specific Gravity. Try as I may, I just do not grasp it and as a result always keep my beer in fermentation longer than necessary. Any suggestions to help a slow learner?

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    What do you do to measure S.G. now? Do you have a specific problem with a hydrometer or how to use it? Where do you feel you are going wrong?
    – barking.pete
    Aug 9, 2017 at 22:18
  • What kind of beer are you brewing and how long are you letting it ferment?
    – nullability
    Aug 10, 2017 at 17:30
  • Mostly I use a hydrometer, not a refracometer. I brew a truly eclectic group of beers using both all grain and extract. Most recently Plinus the Wise, Kolsch, American Amber, and Ferocious Hop
    – Rory
    Aug 11, 2017 at 0:51

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Your question is very general, but I can point you in the right direction.

Specific gravity (SG) is the ratio of the density of a substance to the density of a reference substance. In homebrewing we measure sugars (fermentable or not) in wort.

We measure it using an hydrometer or/and a refractometer.

Here are a few good articles on how to measure gravity :

Why do we need to measure it :

  1. To determine potential alcohol level in wort
  2. To determine when fermentation is finished

To determine that fermentation is completed, when aim to get a stable SG 3 days in a row (unless we suspect a stuck fermentation, which is another question).

To determine the potential alcohol, we need the original SG or OG (measured before fermentation) and the finishing SG or FG (measure at the end of fermentation). There are 2 (popular) ways to calculate it:

  1. ABV = 132 * (OG-FG)
  2. ABV = (OG-FG) / 0.00753

I would recommend that you start with an hydrometer which is not expensive and easy to use. Later if you want more options, you can buy an refreactometer wich costs at least twice as much (depending on the quality).

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  • Thanks, Philippe. I will just have to get better with my math.
    – Rory
    Aug 11, 2017 at 0:55
  • I use the first formula, example: (OG=1.053 and FG=1.013) ABV=132 * (1.053-1.013)= 5.28
    – Philippe
    Aug 11, 2017 at 12:39

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