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.
Alcohol can pull final gravity below 1.000, so apparent attenuation can exceed 100%. Real attenuation attempts to correct for alcohol, but the approximation used here has limits. An output above 100% real attenuation is outside the physical range and should prompt a check of the readings and model; it is not evidence that more than all the original extract fermented.
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 depends on wort composition as well as strain. A more fermentable mash and simple-sugar additions can raise apparent attenuation; dextrins and unfermentable additions can lower it. Results can fall outside a laboratory's typical range. See the mash-temperature guide for the limits of predicting the size of that change.
Is my fermentation stuck, or just done?
Take comparable readings several days apart. Matching readings show stability during that period, but a stalled fermentation can also remain stable and restart later. Check an unexpectedly high finish against the recipe, temperature, yeast condition, and a forced fermentation test where appropriate. Wyeast's guidance explains that test; our troubleshooting guide walks through the checks. Do not bottle an unexplained stall.
If gravity is still falling, fermentation is continuing. Keep the beer within the strain's recommended temperature range and allow time. Avoid routine open stirring or oxygen pickup. A published attenuation window is a planning range, not a packaging-safety threshold.
What raises or lowers attenuation?
Mash conditions, grain composition, simple-sugar additions, strain, and yeast health all affect the result. Extract brewers also depend on the maltster's wort fermentability. No single temperature change or specialty-malt quantity guarantees a specific attenuation shift.
An OG of 1.050 and FG of 1.012 give 76% apparent attenuation. Keep that FG fixed and change OG to 1.060: the result is 80%. This illustrates the formula, not a prediction for adding sugar. Fermenting added sugar also changes alcohol content and can lower FG.
Corrected October 10, 2026: Stable gravity and an attenuation range do not establish fermentation completion.