How this instrument works
Alcohol by volume estimates how much ethanol fermentation produced without a lab, by comparing the density of the liquid before and after fermentation. Sugar dissolved in wort or must makes it denser than plain water, so a hydrometer reads above 1.000 as original gravity (OG). As yeast consumes that sugar and produces alcohol — which is less dense than water — the reading falls; the final gravity (FG) captures where it settles once fermentation stops.
The gap between OG and FG is a direct proxy for how much sugar got converted, and the standard shortcut formula multiplies that gap by 131.25 to land on an approximate percentage ABV. It is an approximation rather than a lab assay: the constant assumes a roughly linear relationship between gravity drop and alcohol produced, which holds well for typical beer-strength brews but drifts at very high original gravities, where more precise cubic formulas are sometimes preferred.
Every major homebrewing reference — from club newsletters to Charlie Papazian's foundational homebrewing text — teaches this same OG-minus-FG-times-131.25 shortcut because it needs nothing beyond a hydrometer and two readings, taken before pitching yeast and again once gravity stops falling for several consecutive days.
- Take a hydrometer reading before pitching yeast and enter it as Original gravity (OG) — a typical ale sits around 1.040 to 1.060.
- Once fermentation has finished (gravity stable for 2-3 days), take a second reading and enter it as Final gravity (FG).
- Read Alcohol by volume (%) — the instrument multiplies the gravity drop by 131.25 automatically.
- Final gravity can never exceed original gravity — sugar only converts to alcohol in one direction, so the instrument rejects an FG reading higher than OG.
Worked example — a pale ale from OG 1.050 to FG 1.010
A homebrewer measures OG 1.050 before pitching yeast, ferments the batch, and measures FG 1.010 once gravity holds steady across three consecutive days. The gravity drop is 1.050 − 1.010 = 0.040. Multiplying by 131.25 gives 0.040 × 131.25 = 5.25% ABV — a typical, sessionable pale ale strength.
That 5.25% figure is an estimate accurate to within a few tenths of a percent for beer-strength brews. A commercial lab running a distillation assay on the same batch would likely report something close to this number, but not necessarily identical to the second decimal — the shortcut formula trades small precision losses for something anyone can do with a $10 hydrometer.
Questions
How accurate is the 131.25 ABV formula?
It is accurate to within a few tenths of a percentage point for typical beer-strength fermentations, roughly 1.030 to 1.070 original gravity. The formula assumes a linear relationship between gravity drop and alcohol produced, which is a good approximation at moderate strengths but understates ABV somewhat at very high original gravities, like barleywines or high-gravity meads, where brewers sometimes switch to a more complex cubic formula for better precision.
Why does my hydrometer reading need temperature correction?
Hydrometers are calibrated to read accurately at a specific reference temperature, commonly 60°F or 68°F depending on the instrument. A sample read straight from a warm fermenter will show a slightly lower gravity than it actually has, because warm liquid is less dense — so brewers either let the sample cool to the calibration temperature first or apply a small correction factor before entering OG or FG here.
What if I only have one gravity reading?
You need both — original gravity taken before fermentation starts and final gravity taken once it finishes — because ABV comes entirely from the difference between them. A single reading only tells you the gravity at that moment, not how much sugar has already converted to alcohol, so there is no way to back out an ABV figure from it alone.
Does this formula work for wine and cider, not just beer?
Yes — the OG-to-FG gravity-drop principle is identical for any sugar-based fermentation, including wine, cider, and mead, since it is measuring the same physical process: sugar converting to alcohol and carbon dioxide. Wines often start at a higher original gravity than beer and ferment closer to dryness, but the same (OG − FG) × 131.25 shortcut still applies.
Why is my final gravity higher than I expected?
A final gravity well above your recipe's target usually means fermentation stalled — from underpitched yeast, a fermentation temperature outside the yeast strain's comfort range, or insufficient aeration of the wort before pitching. Giving the batch more time, gently rousing the yeast, or raising the temperature a few degrees can sometimes restart a stuck fermentation and let gravity fall further.