Attenuation Calculator

What this calculator does

Know whether the batch actually finished — or just reads that way.

Attenuation is the single fastest read on whether a fermentation behaved. But there are two attenuation numbers — apparent (what your hydrometer or refractometer reads) and real (the actual sugar the yeast consumed) — and they are not the same number. Confusing them is the most common reason brewers think a healthy batch stalled, or a stalled batch looks fine. This tool takes your OG and FG and returns both, plus the real extract still in the beer and how many gravity points the yeast dropped — the numbers that tell you what actually happened in the fermenter.

Everything runs client-side. Nothing is stored, nothing is sent — change any input and the result updates live.

The attenuation formula, explained

Apparent vs real — and why the gap exists.

Apparent attenuation: what the hydrometer sees

A hydrometer measures the density of the whole liquid — sugar, alcohol, and everything else dissolved in it. Alcohol is less dense than water, so as fermentation produces ethanol, the reading drops faster than the sugar loss alone would explain. That extra drop is real, but it's not sugar — it's alcohol lightening the liquid. Apparent attenuation (ADF%) is this raw, uncorrected number:

Real attenuation: what the yeast actually ate

To get the real number you have to subtract the alcohol's effect back out. That's what real extract (RE) does — it's a weighted blend of original and apparent extract that estimates what the gravity would read if the alcohol produced weren't there diluting the density:

Because RE always lands between AE and OE, real attenuation is always lower than apparent — never the other way around. As a rule of thumb: RDF ≈ ADF × 0.81. A strain quoted at 75% apparent is really eating through roughly 61% of the actual extract — and that's normal, not a stall.

The part no calculator farm writes

When the standard number is wrong.

Attenuation isn't a fixed strain property — it's the outcome of a strain interacting with a specific wort and process. Here's where reading it wrong costs you a batch's worth of confidence.

Apparent vs real is the big one

Strain spec sheets quote apparent attenuation — typically 75–82% for a standard ale strain. The real number, the actual sugar consumed, runs roughly 0.81× that — closer to 62–67%. Brewers who don't know this gap compare their apparent reading against a number they half-remember as "real," or vice versa, and conclude the batch stalled or blew past spec when it did neither. Rule of thumb: RDF ≈ ADF × 0.81. If your apparent number lines up with the strain's published apparent range, the batch is on track — full stop.

Mash temperature moves fermentability more than the strain does

A strain rated "75% attenuating" is not a fixed outcome — it's a range that your mash schedule picks a point inside of. Mash low, around 148°F, and β-amylase has time to chew wort into mostly fermentable sugar; that same strain can finish near 82%. Mash high, around 158°F, and α-amylase leaves more unfermentable dextrin behind; the same yeast lands closer to 68%. Same tube of yeast, same recipe otherwise — a 10°F mash decision moved the outcome more than the strain choice did.

The spec sheet is one wort, not your wort

A strain's published attenuation range comes from a defined lab wort under controlled conditions. Your grist bill, adjuncts, and any sugar additions (which are close to 100% fermentable) all shift the real number your batch will hit — sometimes well outside the printed range, with no yeast health problem involved at all.

An uncorrected refractometer reading is a false alarm waiting to happen

Refractometers measure refractive index, and alcohol bends light differently than sugar does — so a refractometer read during or after fermentation, without an alcohol correction applied, reads artificially high. That inflated FG understates attenuation and can trigger a false "stuck fermentation" call on a batch that's actually finished. If a refractometer FG looks off, confirm it with a hydrometer before you pitch more yeast or start troubleshooting a fermentation that isn't actually broken.

Notes from the lab