Original Gravity, Final Gravity and ABV in Homebrewing

Why original and final gravity matter

Original gravity (OG) is the specific gravity of wort before fermentation. Final gravity (FG) is the stable reading reached near the end. Apparent attenuation measures the observed gravity reduction; because alcohol lowers the liquid’s density, it is not the same as the true mass of extract consumed.

Together, these readings help you follow fermentation, estimate alcohol by volume (ABV), and understand whether a beer is likely to finish dry or retain more body. The grain bill, other fermentables, mash profile and yeast strain all influence the result.

OG, FG, ABV and attenuation at a glance

OG describes the wort before fermentation. FG describes the beer near the stable endpoint. ABV estimates the final percentage of alcohol by volume. Apparent attenuation expresses the observed gravity drop relative to the original gravity points; it is an apparent density-based value, not real attenuation.

Think of OG as the starting concentration of dissolved extract and FG as the reading left at the end. A larger gravity drop usually means more alcohol and a drier finish, but the expected FG still depends on the recipe and yeast.

How to calculate ABV from OG and FG

A common homebrewing estimate is: ABV ≈ (OG − FG) × 131.25. It converts the change in specific gravity into an estimated percentage of alcohol by volume. It is a practical approximation, not a laboratory measurement.

In the homebrewing glossary: Original gravity.

How to calculate apparent attenuation

Many ale yeast strains publish an apparent attenuation range around 70–85%, although the actual result depends on the strain, wort composition, mash profile and fermentation conditions. Higher attenuation often produces a drier beer; lower attenuation can leave more body and sweetness.

Calculate apparent attenuation with: ((OG − FG) ÷ (OG − 1)) × 100. A beer that moves from 1.060 to 1.012 reaches 80% apparent attenuation. Compare that figure with the yeast manufacturer’s range, your recipe and previous batches.

How to interpret your readings

The same FG can taste different from one beer to another. A reading near 1.010 may seem dry in a modest, crisp beer yet fuller in a stronger or more dextrinous recipe. Original gravity, alcohol, dextrins, acidity, carbonation, bitterness and serving conditions all affect the impression.

These numbers are reference points, not pass-or-fail limits. Beer style, yeast strain, fermentation temperature, wort composition and mash profile can all move the expected FG. One isolated reading cannot prove that fermentation is complete.

In the homebrewing glossary: Apparent attenuation.

Use a hydrometer and refractometer correctly

For a hydrometer reading, collect a representative sample in a clean, sanitized trial jar and cool it near the instrument’s calibration temperature. Add enough liquid for the hydrometer to float freely, remove clinging bubbles, read at eye level using the instrument’s specified meniscus, then apply a temperature correction when needed.

A refractometer needs only a small sample and is convenient before fermentation. Calibrate it as directed. Once alcohol is present, enter the original and current refractometer readings into an alcohol-correction model; the raw post-fermentation value is not final gravity.

Avoid common gravity-reading errors

Check your hydrometer’s calibration temperature and correct readings taken from warmer or colder samples. Do not mix specific gravity, degrees Plato and gravity points without converting them. Take a representative sample with clean, sanitized equipment and never return it to the fermenter.

A refractometer reading also needs an alcohol correction once fermentation has started. Alcohol changes how the instrument bends light, so an uncorrected post-fermentation value is not a reliable FG.

Confirm fermentation before packaging

A stable FG across repeat readings taken at least 48 hours apart, after the expected fermentation timeline, is more useful than a target date or a quiet airlock. If gravity is still falling, fermentation is still active. Strong, slow, cold-fermented or dry-hopped beer may need a longer check.

Do not rely on airlock activity alone. A leaking fermenter may show few bubbles while fermentation proceeds normally, and a quiet airlock does not guarantee that the beer is ready to bottle.

Read next: Homebrew Fermentation: Time, Temperature and Signs