apparent attenuation

I all!

I read on "brew like a monk" the leffe blond specification:

Og: 1,064, Alcohol by volume: 6.6%, Apparent degree of Att. : 81%

But how is it possible? Usally I calculate the alcohol by volume in this way:

1, 064 - 1,013 (81% AA) --> 51 / 7.5 --> 6.8% that is different from 6.6%.

Is my formula wrong or is because I use the apparent attenuation and not the real?

Thanks!

Enrico

Pretty much all math in brewing is based on apparent attenuation, though it is relatively easy to convert between apparent and actual.

The differences you are getting are just a result of when and where you are rounding your values. For example, 81% attenuation of 1.064 is not 1.013, but 1.01216. Your formulas are fine (though sometimes people use (OG-FG)/7.53), but they're just estimates, and even estimates are only as good as your accuracy/precision of measurement.