How do you calculate attenuation?
Attenuation is the share of the original gravity that fermentation removed. Take the gravity points before and after and divide:
Apparent attenuation = (OG − FG) ÷ (OG − 1) × 100
A beer that starts at 1.050 and finishes at 1.010 dropped 40 of its 50 points — 80% apparent attenuation. That is the number yeast labs quote, the number on the packet, and the one to compare against; when a brewer says "attenuation" without a qualifier, this is what they mean.
Apparent vs real attenuation — why are they different?
The hydrometer measures density, and the alcohol now in the beer is lighter than water. So the FG reads lower than the sugar left behind would justify, and the "apparent" drop overstates how much sugar the yeast really ate. Real attenuation corrects for it (Balling's approximation: real extract ≈ 0.1808 × OE + 0.8192 × AE, in °Plato) and always comes out lower — 65% for the same beer, with about 4.3 °P of real extract still in the glass. Across ordinary strengths the gap is a steady 15 points or so:
| OG | FG | Apparent | Real |
|---|---|---|---|
| 1.040 | 1.008 | 80% | 65% |
| 1.050 | 1.010 | 80% | 65% |
| 1.060 | 1.015 | 75% | 61% |
| 1.070 | 1.016 | 77% | 62% |
| 1.080 | 1.020 | 75% | 60% |
| 1.100 | 1.024 | 76% | 61% |
ASBC Plato polynomial and Balling's real-extract approximation, from this page's engine.
Ferment a 1.040 wort until literally nothing fermentable is left and the hydrometer reads about 0.991 — below water, because the beer is now part alcohol. That's 123% apparent attenuation for a beer that is 101% really attenuated. Real attenuation is the honest physical number; apparent is the useful one, because it's what everyone measures and quotes.
What final gravity should my beer finish at?
Apply the yeast's published range to your OG and you have the FG window it should land in — the calculator does this for a couple of dozen common strains, or for any range you type in. For everything else, read across:
| OG ↓ · attenuation → | 65% | 70% | 75% | 80% | 85% |
|---|---|---|---|---|---|
| 1.040 | 1.014 | 1.012 | 1.010 | 1.008 | 1.006 |
| 1.045 | 1.016 | 1.014 | 1.011 | 1.009 | 1.007 |
| 1.050 | 1.018 | 1.015 | 1.012 | 1.010 | 1.008 |
| 1.055 | 1.019 | 1.016 | 1.014 | 1.011 | 1.008 |
| 1.060 | 1.021 | 1.018 | 1.015 | 1.012 | 1.009 |
| 1.065 | 1.023 | 1.020 | 1.016 | 1.013 | 1.010 |
| 1.070 | 1.024 | 1.021 | 1.018 | 1.014 | 1.010 |
| 1.075 | 1.026 | 1.022 | 1.019 | 1.015 | 1.011 |
| 1.080 | 1.028 | 1.024 | 1.020 | 1.016 | 1.012 |
| 1.090 | 1.032 | 1.027 | 1.022 | 1.018 | 1.014 |
| 1.100 | 1.035 | 1.030 | 1.025 | 1.020 | 1.015 |
FG = 1 + (OG − 1) × (1 − attenuation). Apparent attenuation, as labs quote it. Most ale strains publish 70–80%; the wort's fermentability decides where in the range you land.
Attenuation is a property of the wort as much as the yeast. The same strain that stops at 70% in a beer mashed at 156°F with a pound of crystal malt will run to 80% in one mashed at 148°F with sugar in the kettle — the yeast eats whatever is fermentable, and the mash decides how much that is. Read the range as "on a normal wort, somewhere in here"; treat the low end as your safety margin, not the high end as a promise.
Is my fermentation stuck, or just done?
Almost every "stuck fermentation" post is a beer that has finished a few points higher than the recipe hoped. The test is simple: take two readings three days apart. If they match, fermentation is over, whatever the number is — and the question becomes why it stopped there. Compare against the yeast's range: within it, or up to a couple of points above the top, is a finished beer with a fuller body than planned (a warm mash, an extract with more dextrins, more crystal malt). Five or more points above the range with a healthy pitch is the exception worth investigating — a mash that never converted, a fermenter that got cold, or yeast that gave up early.
If the readings are still moving, it isn't stuck — it's slow. Warm the fermenter a few degrees, rouse the yeast gently, and wait. Only when a beer is genuinely stalled well above its window is a fresh pitch of a neutral, high-attenuating strain worth the trouble, and even then it often can't eat what a hot mash left behind.
What raises or lowers attenuation?
In rough order of how much they matter: mash temperature (lower is more fermentable — the difference between 148°F and 156°F is several points of attenuation), simple sugars in the recipe (fully fermentable, so they push the number up), crystal and dextrin malts (partly unfermentable, so they pull it down), yeast strain, and then yeast health — pitch rate, oxygen, nutrients, and a fermentation that didn't get cold halfway. Extract brewers inherit their fermentability from the maltster, which is why extract beers so often finish at 1.016 no matter what the packet promised.
Take a 1.050 beer that finishes at 1.012 — 76% apparent. Add a pound of table sugar to the 5-gallon batch: OG rises about ten points to 1.060, and because every one of them ferments out, the FG stays put — 1.012 — and the attenuation reads 80%. Same yeast, same mash; the recipe moved the number, and the beer got drier and stronger, not thinner.