Apparent attenuation is what most brewers mean when they say "attenuation" — it comes straight from your two gravity readings, (OG - FG) / (OG - 1) x 100, no correction applied. Real attenuation corrects for something apparent attenuation ignores entirely: dissolved alcohol is less dense than water, so a hydrometer reading a fermented beer reads lower than the sugar content alone would produce, making the beer look more fermented than it really is. Whichever pair you check, expect real attenuation to land under apparent attenuation every time — the direction of the gap never flips, only its size does.
Four pairs, both figures
| OG | FG | Apparent attenuation | Real attenuation | Gap |
|---|---|---|---|---|
| 1.050 | 1.012 | 76.0% | 61.6% | 14.4 pts |
| 1.060 | 1.015 | 75.0% | 60.7% | 14.3 pts |
| 1.070 | 1.014 | 80.0% | 64.8% | 15.2 pts |
| 1.090 | 1.020 | 77.8% | 62.6% | 15.2 pts |
Run any of these through the ABV calculator, which reports both figures on every result — real attenuation isn't a separate tool, it's computed from the same OG/FG input using the Balling real-extract formula.
Why the gap is roughly 14-16 points, and not some other size
The gap between the two figures isn't fixed, but it clusters in a fairly narrow band across normal beer-strength fermentations — it's driven by how much alcohol is actually present, since alcohol is what causes the discrepancy in the first place. A beer that fermented further (more alcohol produced) shows a wider gap between apparent and real attenuation than a lightly-fermented one, which is part of why the gap in the table above grows slightly at higher OG rather than staying perfectly constant.
Which number should you actually use
Apparent attenuation is what to compare against a yeast manufacturer's published attenuation range — see what attenuation should I expect from my yeast — because that's the figure manufacturers themselves report and the figure every piece of brewing software defaults to showing. Real attenuation is the more physically accurate description of how much of the wort's original extract the yeast actually consumed, and matters more if you're doing precise calorie or residual-extract calculations, since how many calories are in a homebrew is built on the real-extract figure, not the apparent one.
Neither number is "wrong" — they're answering slightly different questions from the same two readings, and a beer described as "76% attenuated" without specifying which kind is almost always referring to the apparent figure by default, since that's the number that comes directly off a hydrometer with no correction step at all.