Apparent Attenuation Calculator

Work out apparent attenuation instantly with clear inputs, formula shown and shareable results.

About Apparent Attenuation Calculator

Apparent attenuation is the percentage drop in gravity points, but it overstates true fermentation because ethanol is less dense than water and depresses the hydrometer reading. Real attenuation is about 0.81 times apparent. Unfermentable dextrins set a floor below which gravity cannot fall.

Formulas

Apparent attenuation

AA % = (OG points - FG points) x 100 / OG points

Real attenuation

RA % = apparent attenuation x 0.81

Worked example

With the default values below, Apparent Attenuation Calculator returns Apparent Attenuation of 78.182%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Original Gravityaccepts 0.9–1.2: 1.055
Final Gravityaccepts 0.9–1.2: 1.012
Yeast Expected Attenuationaccepts 1–100 %: 78 %
Unfermentable Extractaccepts 0–60 %: 20 %

Results

Apparent Attenuation: 78.182%
Real Attenuation: 63.327%
Expected Final Gravity: 1.01210
Gravity At Limit Of Attenuation: 1.01100

Step by step

1Values used
2Apparent attenuation
3Real attenuation
4Apparent Attenuation= 78.182
5Real Attenuation= 63.327
6Expected Final Gravity= 1.01210
7Gravity At Limit Of Attenuation= 1.01100

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is attenuation lower than expected sometimes?

High mash temperature produces more unfermentable dextrins, and under-pitching, low oxygen, poor nutrient status or low fermentation temperature all cause the yeast to stall early.

Can attenuation exceed the yeast specification?

Yes, with a very fermentable wort from a low-temperature mash or with added simple sugars. It can also signal a diastaticus or wild yeast contamination, which is a package safety risk.