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Apparent Attenuation Calculator

How complete is your fermentation?

How it is calculated

Enter the original gravity (OG) and the final gravity (FG) as specific-gravity readings such as 1.060. Only the part above water (1.000) counts as sugar, so the tool computes AA% = (OG − FG) ÷ (OG − 1) × 100. That is the share of the original dissolved extract the yeast has used up. Both readings are unitless and the result is a percentage with 2 decimals. The tool accepts OG from just above 1.000 up to 1.200 and FG from 0.980 up to (but not including) OG; anything outside that range, including readings typed as 1060, shows no result. An FG below 1.000, as in very dry wines or meads, is allowed and can push the figure above 100 %. The result is the apparent value, not true attenuation, because alcohol lowers the hydrometer reading.

Worked example

A beer starts at OG = 1.060 and ends at FG = 1.014. The sugar above water is 1.060 − 1.000 = 0.060, and the gravity dropped by 1.060 − 1.014 = 0.046. Dividing, 0.046 ÷ 0.060 = 0.7667, so the apparent attenuation is 76.67%. Compare that with the range published for your yeast strain: if the label promises 73 to 77 %, this batch fermented as expected.

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Common mistakes

  • Dividing by OG instead of OG − 1. For 1.060 and 1.014 that gives about 4.3 %, which hides the real picture because water's 1.000 is not sugar.
  • Typing readings as 1060 and 1014 (the way some hydrometer scales are written). The tool treats these as out of range and shows no result, so enter 1.060 and 1.014.
  • Treating the figure as true attenuation. Apparent attenuation overstates how much sugar was fermented because alcohol makes the liquid lighter than it really is.
  • Entering FG equal to or higher than OG after a stuck or unstarted fermentation. There is nothing to calculate, so no result appears.
  • Measuring FG while fermentation is still moving. Take readings on several days; if the number keeps falling, the attenuation shown is not final.

About this calculator

Apparent attenuation is the brewing-world measure of how much sugar yeast has converted into alcohol and CO₂ during fermentation, expressed as a percentage drop in specific gravity: AA% = (OG − FG) / (OG − 1) × 100, where OG is your original gravity reading before fermentation and FG is the final gravity once it settles. A beer that starts at 1.050 and finishes at 1.010, for example, shows about 80% apparent attenuation.

Homebrewers and professional brewers check this number against the published attenuation range for their yeast strain (Wyeast and White Labs list typical ranges like 73-77% for many ale strains) to judge whether fermentation performed as expected or stalled early — a sign of underpitching, low temperature, or a stressed yeast culture. It’s called “apparent” rather than “real” attenuation because a hydrometer reading is skewed by the alcohol already present in the beer, which reads lighter than it truly is; true attenuation, used mainly in lab settings, corrects for that effect.

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