Beer ABV from Gravity Calculator

How it works

This beer gravity calculator goes one step further than a general-purpose ABV tool by also reporting apparent attenuation — the metric serious homebrewers watch to judge whether a fermentation actually finished properly, not just how strong the beer turned out. Original gravity (OG) measures the density of your wort before pitching yeast, when it’s loaded with unfermented sugar. Final gravity (FG) measures the same wort once fermentation has stopped, after the yeast has eaten as much sugar as it’s going to. Both readings come straight off a hydrometer.

ABV uses the standard, widely-used homebrew approximation:

ABV % = (Original Gravity − Final Gravity) × 131.25

Apparent attenuation asks a different question: not “how strong is this beer” but “how much of the available sugar did the yeast actually convert.” It’s expressed as a percentage of the total fermentable sugar consumed:

Apparent attenuation % = ((Original Gravity − Final Gravity) ÷ (Original Gravity − 1)) × 100

For most ale yeast strains, apparent attenuation typically lands somewhere in the 65-80% range — a useful rough sanity-check band rather than a hard rule for any particular strain. A reading well below that range, alongside a gravity that’s stopped dropping day to day, is the classic sign of a stuck fermentation. A reading at or above it usually means the batch fermented out cleanly. When you calculate beer gravity at both ends of fermentation and run both formulas, you get a fuller picture than ABV alone: two beers can finish at the same strength while telling very different stories about how their fermentation actually went. Once you know the ABV, you can also cross-reference it against Australia’s official standard drinks definition — 10 grams of pure alcohol per standard drink — set out in the Australian Government Department of Health’s standard drinks guide, for context on how a finished batch compares to a labelled commercial beer.

Worked example

Priya, a homebrewer in Adelaide, brews a pale ale and takes an original gravity reading of 1.052 before pitching a packet of US-05 ale yeast. Two weeks later, once the airlock has stopped bubbling and the gravity hasn’t moved in three consecutive days, she takes a final gravity reading of 1.012. Her beer gravity calculator gives her (1.052 − 1.012) × 131.25 = 5.25% ABV, and an apparent attenuation of (0.040 ÷ 0.052) × 100 = 76.9% — comfortably inside the typical 65-80% range for ale yeast, telling her the fermentation ran cleanly.

A month later Priya brews the same recipe again, but her fermentation fridge fails and the wort sits several degrees cooler than intended. This time the gravity stalls early at 1.022. Running the same formulas raises a flag:

Batch	OG	FG	ABV	Apparent attenuation
First batch (healthy)	1.052	1.012	5.25%	76.9%
Second batch (cold ferment)	1.052	1.022	3.9%	57.7%

The ABV alone might have looked like just a weaker beer, but the attenuation figure — well below the typical 65-80% band — tells Priya the fermentation likely stuck rather than simply finished lighter, so she knows to warm the fermenter and give the yeast a gentle rouse before bottling.