Turn your hydrometer readings into ABV. Enter the original gravity (OG) taken before fermentation and the final gravity (FG) taken once fermentation is done and stable.
ABV (alcohol by volume) for homebrewing is calculated from two hydrometer or refractometer readings: original gravity (OG), taken before fermentation starts, and final gravity (FG), taken once fermentation is complete — the difference between the two reflects how much sugar was converted into alcohol.
The standard formula, ABV = (OG − FG) × 131.25, works well for typical beers and wines. For higher-gravity brews — big beers, meads, and some ciders above roughly 1.100 OG — this standard formula tends to under-read, so an advanced formula is available that corrects for that at higher starting gravities.
There's no reliable way to back-calculate a starting gravity after fermentation has already begun, which is why an accurate OG reading taken before pitching yeast is essential — if you missed taking one, this calculator can't retroactively estimate what it would have been.
The standard formula (ABV = (OG−FG) × 131.25) is accurate for most beers and wines. Above roughly 1.100 OG — big beers, meads, and some ciders — it under-reads, so the advanced formula corrects for that.
You need a hydrometer or refractometer reading taken before fermentation starts. There's no reliable way to back-calculate a starting gravity after the fact — note it down next time before pitching your yeast.
A stuck fermentation, a high mash temperature (which leaves more unfermentable sugars), or a yeast strain with low attenuation are the usual culprits — not a calculator issue.
