Attenuation Calculator
How much of your wort's sugar the yeast actually consumed, from the same two gravity readings that give you ABV.
How this was worked out
attenuation = (1.0500 − 1.0100) ÷ (1.0500 − 1) × 100 = 80.00%
This is apparent attenuation, the figure yeast labs publish. It reads higher than reality because alcohol is lighter than water, so it drags the hydrometer down alongside the sugar that actually went.
| Apparent attenuation | Final gravity | ABV |
|---|---|---|
| 60% | 1.020 | 3.94% |
| 65% | 1.018 | 4.27% |
| 70% | 1.015 | 4.59% |
| 75% | 1.012 | 4.92% |
| 80% | 1.010 | 5.25% |
| 85% | 1.008 | 5.58% |
| 90% | 1.005 | 5.91% |
Your beer is highlighted. For what a specific strain should reach, check that strain's own datasheet — labs publish an attenuation range per strain, and it varies far too much to generalise here.
What attenuation measures
Attenuation is the fraction of available gravity points that fermentation consumed. A wort at 1.056 has 56 points to work with; if it finishes at 1.012, 44 of those 56 went, which is 79% attenuation. It answers a different question from ABV — not how strong the beer is, but how completely it fermented.
That makes it the more useful diagnostic of the two. Strength depends heavily on how much sugar you started with, so a weak beer and a strong one can both be perfectly healthy fermentations. Attenuation is comparable across batches, which is why an unexpectedly low figure is usually the first sign something went wrong.
Worked example
A beer goes from 1.056 to 1.012.
It started with 56 gravity points and finished holding 12, so 44 were fermented.
attenuation = (1.056 − 1.012) ÷ (1.056 − 1) × 100 = 78.6%
Common mistake
Comparing your apparent attenuation against a strain's published figure without checking the mash. Yeast labs measure attenuation in a standard wort. Mash your own grain three degrees warmer and you create more unfermentable dextrin, which caps attenuation regardless of how good the yeast is. The strain is often not the variable that moved.
Keep going
- Attenuation says how much sugar went; the ABV calculator says what it turned into, from the same original and final gravity. ABV Calculator
- A fermentation that has genuinely finished is the precondition for bottling safely, and the priming calculator is the next step once it has. Priming sugar Calculator
Frequently Asked Questions
What is the difference between apparent and real attenuation?
Apparent attenuation is what you get from hydrometer readings, and it overstates reality. Alcohol is lighter than water, so as it accumulates it pulls the reading down alongside the sugar that genuinely fermented. Real attenuation strips that effect out and always comes in lower — typically around four fifths of the apparent figure. Yeast labs publish apparent, so that is what this calculates.
My attenuation is far lower than the strain should reach. Why?
The usual causes are a fermentation that has stalled rather than finished, underpitching or unhealthy yeast, a fermentation that got too cold, or a mash held hot enough to produce a lot of unfermentable sugar. Mash temperature is the one people overlook — it changes what the yeast is even capable of eating.
Can attenuation go above 100%?
Yes, and it is not an error. If the final gravity drops below 1.000 — which happens with very simple sugar bills, or with Brettanomyces breaking down dextrins ordinary yeast cannot touch — the beer is less dense than water and the formula returns more than 100%.
Does higher attenuation mean a better beer?
No, only a drier one. Attenuation is a measure of how much sugar was consumed, not of quality. A malty, full-bodied style is supposed to finish with residual sweetness, and driving it to maximum attenuation would strip out exactly the character it is meant to have.
How do I know fermentation is actually finished?
By stable gravity, not elapsed time. Take a reading, wait two or three days, take another. If they match, fermentation has stopped. A calendar tells you nothing — the same yeast can take four days or fourteen depending on temperature, pitch rate and wort composition.
A note on method. Apparent attenuation is a definition rather than a fitted formula — points consumed over points available — so unlike the ABV and bitterness pages there is no external constant here to cite. The table above is computed from your own original gravity, not from published per-strain figures; for what a particular strain should reach, its manufacturer's datasheet is the authority. This is a calculation tool for homebrewing and says nothing about consumption.
Related Calculators
- ABV Calculator — Gravity & Alcohol
- Brix ↔ gravity Calculator — Gravity & Alcohol
- Dilution & blending Calculator — Gravity & Alcohol
- IBU Calculator — Hops & Bitterness