What is apparent attenuation?

Apparent attenuation is the percentage of gravity points consumed during fermentation, measured with a hydrometer. It is called apparent because alcohol is lighter than water, so the hydrometer reads lower than the true sugar content, inflating the figure slightly compared with real attenuation.

What is the difference between apparent and real attenuation?

Apparent attenuation is what you read directly from gravity measurements. Real attenuation corrects for the lower density of ethanol in the finished beer and is roughly 0.82 times the apparent value. Real attenuation reflects the true proportion of sugar actually fermented.

What is a normal attenuation range?

Most ale and lager yeasts achieve 73-80% apparent attenuation. Malty styles may finish lower in the high 60s, while saison, Belgian, and highly attenuative strains can exceed 85%. Values much outside the expected range point to yeast health, mash temperature, or fermentability issues.

Why is my attenuation lower than the yeast spec sheet?

Under-attenuation usually comes from a high mash temperature that produces less-fermentable wort, under-pitching or unhealthy yeast, low fermentation temperature, or a stuck fermentation. Yeast lab figures assume a highly fermentable all-malt wort and ideal conditions.

Does this calculator estimate ABV too?

Yes. It applies the standard homebrew approximation, ABV is roughly (OG minus FG) times 131.25. This is accurate for typical beer strengths but drifts slightly for very strong worts, where a more advanced formula is preferable.

Apparent Attenuation Calculator

The Apparent Attenuation Calculator tells you how completely your yeast fermented the available sugars by comparing your original gravity (OG) and final gravity (FG). It is the single most useful number for judging yeast performance, diagnosing stuck fermentations, and comparing strains.

How it works

Attenuation is the fraction of fermentable sugar the yeast consumed, expressed as a percentage of the gravity points present at the start. Gravity points are just the decimal part of specific gravity multiplied by 1000, so 1.050 is 50 points and 1.012 is 12 points.

Apparent attenuation uses the points dropped during fermentation:

Apparent attenuation = (OG points - FG points) / OG points * 100

The word apparent matters. A hydrometer measures density, and ethanol is less dense than water, so the finished beer reads lighter than its true sugar content suggests. That makes the apparent figure slightly higher than the amount of sugar actually consumed. To get real attenuation, the calculator applies the standard correction:

Real attenuation = 0.8192 * apparent attenuation

It also estimates ABV using the common homebrew formula, ABV is roughly (OG - FG) * 131.25.

Worked example

A beer with OG 1.050 and FG 1.012:

OG points = 50, FG points = 12, so 38 points were consumed.
Apparent attenuation = 38 / 50 * 100 = 76%.
Real attenuation = 0.8192 * 76 = about 62.3%.
ABV is roughly (1.050 - 1.012) * 131.25 = about 5.0%.

A 76% result sits comfortably in the standard 73-80% range, indicating a healthy, well-attenuated fermentation.

Tips and interpretation

Use attenuation to compare your result against the yeast manufacturer’s stated range. If you fall short, the most common causes are a high mash temperature (which produces less-fermentable wort), under-pitching, unhealthy yeast, or too cool a fermentation. If you over-attenuate, you may have included simple sugars, mashed very low, or had wild yeast or enzymes at work. Always confirm fermentation is truly finished by taking two stable gravity readings two or three days apart before judging attenuation, since an early FG will understate the true figure.