The two formulas
Almost every calculator uses the short form: subtract final gravity from original gravity and multiply by 131.25. It is close enough for ordinary beer and wrong for strong beer.
The constant 131.25 is an approximation that assumes a fixed relationship between sugar consumed and alcohol produced. That relationship drifts as gravity climbs, because more of the extract is fermentable and the resulting solution is less dense.
This second form accounts for the changing density and tracks reality better above roughly 7% ABV.
How far apart do they get?
At ordinary strength the gap is negligible. A beer going 1.050 to 1.010 reads 5.25% on the standard formula and 5.34% on the alternative — nine hundredths of a percent, invisible in practice.
A barleywine going 1.090 to 1.020 reads 9.19% on the standard formula and 9.99% on the alternative. That is a difference of eight tenths of a percent, and it is the difference between a label that is right and a label that is not.
Which one to use
- Under about 7% ABV: either is fine. Use the standard formula and stop thinking about it.
- Above 7%: use the alternative. The standard formula reads low, and the error grows with strength.
- If you are reporting ABV to anyone else, say which formula you used.
Apparent attenuation
The calculator also reports apparent attenuation, which is how much of the extract the yeast removed, expressed as a percentage. It is computed in degrees Plato rather than specific gravity, because Plato is a weight-percent scale and the arithmetic is meaningful.
It is called apparent because alcohol is lighter than water, so the hydrometer reads lower than the true remaining extract. Real attenuation is always a smaller number. Apparent attenuation is the one quoted on yeast packets, so it is the one to compare against.