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Sugar (in fermentation) - odd FG/ABV results? (using Build 38)


Master Brewer
Mar 26, 2011
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Trying to understand the following difference in version 2 compared to version 1.4.  I'm using build 38, but I think this may have existed in build 37, too.

The issue is with any sugars added from the "grains" list in the fermentation step.  I noticed some "aggressive" numbers for final gravity and ABV, and was able to narrow it down to the sugar.

To isolate, if I open a new recipe, 5 gallons, and just add 3 lbs of table sugar -- no other ingredients -- I see:
v1.4:  OG- 1.028  FG- 1.007  ABV 2.64%    seems reasonable
v2:    OG- 1.028  FG- 0.994  ABV 4.4%      seems...impossible?

Color is 1.1 SRM in both versions.  The color and OG seem correct, but I'm questioning the validity (and logic behind) those FG/ABV numbers. 

Any insights?

  Yes - the FG calculation was changed in version 2.0 for sugar.  Sugar is treated separately for estimating FG.  The rationale is that sugar is almost 100% fermentable, and does significantly lower the FG.

  The old version simply applied the yeast attenuation to the sugar, which generally resulted in 72% attenuation for an average yeast with table sugar (for example).

  The new version (more correctly) takes into account the fact that the sugar will ferment to a much higher degree (nearly 100%), reducing the final gravity.

  In cases like the one you cite above - where you have pure sugar - it is actually very easy to achieve final gravities below 1.000. Alcohol is less dense than water, so sugar (which is almost fully fermentable) creates enough alcohol to reduce your FG reading below 1.000.