Fermentation Calculator

Gravity readings before and after brewing tell you exactly how much sugar your yeast converted to alcohol — the Fermentation Calculator turns those numbers into a complete alcohol analysis. Enter your Original Gravity (OG) and Final Gravity (FG) in either SG or Plato (°P), select your preferred calculation method, and optionally add your fermentation temperature to get your Alcohol By Volume (ABV). Secondary outputs include Apparent Attenuation, Gravity Drop, and Alcohol by Weight.

Gravity Unit *

Gravity reading before fermentation starts

Gravity reading after fermentation is complete

Calculation Method *

Choose calculation method for ABV

°F

Temperature during fermentation (optional)

Results

Alcohol By Volume (ABV)

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Apparent Attenuation

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Gravity Drop

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Alcohol by Weight

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Frequently Asked Questions

What is the difference between Original Gravity and Final Gravity?

Original Gravity (OG) is the specific gravity of your wort or must before fermentation begins, indicating the sugar content. Final Gravity (FG) is measured after fermentation is complete, showing how much sugar has been converted to alcohol.

Which calculation method should I use for ABV?

The standard formula (OG - FG) × 131.25 is most commonly used and accurate for most homebrewing situations. The alternative formula is more complex and accounts for additional factors but is typically unnecessary for home fermentation.

What is apparent attenuation and why does it matter?

Apparent attenuation measures how much of the available sugars were consumed during fermentation, expressed as a percentage. It helps evaluate yeast performance and fermentation completeness.

How accurate are fermentation calculators?

Fermentation calculators provide good estimates based on gravity readings, but actual alcohol content can vary due to factors like temperature, yeast strain, and measurement accuracy. Professional testing is needed for precise ABV values.

When should I take my final gravity reading?

Take your final gravity reading when fermentation appears complete - typically when airlock activity stops and gravity readings are stable over 2-3 consecutive days. This ensures accurate ABV calculations.

Can I use this calculator for wine and mead fermentation?

Yes, this calculator works for any fermented beverage where you have original and final gravity readings, including wine, mead, cider, and other fermented drinks.

What does alcohol by weight mean compared to ABV?

Alcohol by Volume (ABV) measures alcohol as a percentage of total liquid volume, while alcohol by weight measures alcohol as a percentage of total liquid weight. ABV is the standard measurement used on beverage labels.

Why is fermentation temperature important?

Fermentation temperature affects yeast activity, fermentation speed, and final alcohol content. Higher temperatures generally increase fermentation rate but may produce off-flavors, while lower temperatures slow fermentation but often improve flavor quality.