Brewing ABV Calculator

How It Works

Alcohol by volume (ABV) is calculated from the drop between Original Gravity (OG) and Final Gravity (FG). Before fermentation, dissolved sugars make the wort denser than water. As yeast converts sugars to ethanol and CO₂, that density falls. The gap between the two hydrometer readings is what drives the alcohol number.

Two formulas cover the practical range:

• Standard: ABV = (OG − FG) × 131.25
• Alternate: ABV = 76.08 × (OG − FG) / (1.775 − OG) × (FG / 0.794)

The standard formula assumes a roughly linear relationship between the drop in specific gravity and ethanol yield. That assumption holds well for beers with an OG below about 1.070, which is where John Palmer's How to Brew, 4th ed., Ch. 12 (Priming, Bottling, and Force Carbonation) treats it as the working default for typical ales and lagers.

The linear assumption breaks down on high-gravity worts. Ethanol-water mixtures do not follow a linear density curve at high alcohol concentration — the more sugar dissolved in the starting wort, the more the simple factor drifts low. The alternate formula, documented in the BeerSmith gravity formula reference, is the corrected form: it accounts for the changing density of the ethanol-water mixture and returns more realistic numbers on barleywines, imperial stouts, meads, and any wort starting above ~1.080.

How to Use This Calculator

1. Enter your Original Gravity (OG) — the hydrometer or refractometer reading taken before pitching yeast.
2. Correct for temperature if your hydrometer is calibrated at 20°C (68°F) and your wort is not — hot wort reads low.
3. Enter your Final Gravity (FG) — the reading after fermentation is stable for 2-3 consecutive days.
4. Choose a formula — standard works for most beers below 1.070 OG; alternate is more accurate for barleywines, meads, and high-ABV worts.
5. Read your ABV, apparent attenuation, calorie estimate, and Brix conversion. Everything updates in real time.
6. If refractometer readings are used post-fermentation, apply a Wagner or Sean Terrill correction — the calculator surfaces the toggle.

Real-World Use Cases

Standard 5-gallon IPA

A homebrewer measures a 5-gallon batch with OG 1.062 and FG 1.014 to compute ABV for bottle labeling. The standard formula returns (1.062 − 1.014) × 131.25 ≈ 6.3% ABV, in line with the numbers Palmer's How to Brew uses for a typical American IPA. Three consecutive daily FG readings at 1.014 confirm the yeast has finished; if the number were still dropping, the beer is not ready to bottle and any carbonation math is premature.

High-gravity mead

A mead maker with a honey wine at OG 1.120 and FG 1.010 needs to pick a formula. The standard formula reports ~14.4% ABV. The alternate formula, corrected for the non-linear ethanol density curve, reports closer to ~15.3% — a 0.9-point spread that matters for a beverage sold under label rules. For anything above 1.080 OG, the alternate is the honest choice, and BeerSmith's gravity formula documentation calls out the same threshold.

Cider fermentation check

A cider maker has SG stable at 1.000 for three consecutive days and wants to bottle. Stable SG at or just below 1.000 indicates complete attenuation of the natural apple sugars. A neighbouring batch stuck at SG 1.020 for the same three days points to a different problem: either yeast exhaustion (nitrogen deficiency is common in cider) or temperature stress. Both need addressing before a still airlock is treated as a green light.

Tips & Safety Notes

• Temperature-correct your hydrometer. Most are calibrated at 20°C (68°F); hot wort reads low, cool wort reads slightly high.
• Wait for a stable FG. Two readings 2-3 days apart at the same number is the standard sign fermentation is complete.
• Record both readings. Write OG on the fermenter with a dry-erase marker — it is easy to forget by the time FG comes around.
• Refractometer users need a correction. OG readings are direct, but any post-pitch reading in the presence of alcohol needs a Wagner or Sean Terrill correction to be trustworthy.
• Carbonation adds ~0.2% ABV. Bottle-conditioned beers ferment a small amount of priming sugar in the bottle, which lifts the final ABV slightly.

Limitations & What This Tool Cannot Do

• No sweetness modeling. Two beers with the same FG can taste dramatically different depending on residual sugar type (glucose vs unfermentable dextrins) and yeast attenuation profile. The calculator reports ABV only, not perceived sweetness or body.
• Simple formula drift. The standard SG × 131.25 formula drifts on OG above 1.080 — the underlying assumption of linear density-to-alcohol conversion breaks down at high sugar concentration. Use the alternate formula for barleywines, meads, and any wort above 1.080 OG.
• Refractometer correction is a curve, not a constant. Post-fermentation refractometer readings need a Wagner or Sean Terrill correction. Both work for typical worts but drift at very high or very low OG; if precise ABV matters, cross-check with a hydrometer sample.

FAQ

What is the difference between the simple (SG × 131.25) and the high-gravity (alternate) ABV formula?

The simple formula assumes a linear relationship between gravity drop and ethanol yield. That works well below OG 1.070 but drifts low above ~1.080 because ethanol-water mixtures do not follow a linear density curve at high alcohol concentration. The alternate formula corrects for the non-linear density change and is the honest choice for barleywines, meads, and any wort above 1.080 OG.

Do I need to correct my hydrometer reading for temperature?

Most hydrometers are calibrated at 20°C (68°F). Wort warmer than that reads artificially low; wort cooler reads slightly high. If you are not sampling at the calibration temperature, either use a published correction table or cool the sample before reading. Errors of 0.002-0.005 SG are common on hot post-boil samples, which is enough to shift the final ABV number by 0.3% or more.

Can I use a refractometer (Brix) reading after fermentation has started?

Yes, but not without correction. Pre-fermentation Brix converts directly to gravity through the standard sugar-to-SG relationship. Once ethanol enters the sample, alcohol shifts the refractive index and every reading drifts. The Wagner correction and the Sean Terrill correction are the two widely used curve fits; the calculator surfaces the toggle so you can pick either.

Why does my calculated ABV differ from what the yeast package advertises?

The percentage on a yeast package is the strain's tolerance ceiling — the highest ABV the yeast can survive before shutting down — not the ABV any given recipe will hit. Actual ABV depends on the sugar content of your wort, fermentation temperature, and nutrient availability. A US-05 packet rated to 11% will still finish a 5% pale ale at 5%.

What is the practical ABV upper limit for brewer's yeast?

Most ale yeast tops out around 8-10% ABV. Specialty champagne and turbo strains push 14-18%, and distiller's strains and Kveik variants can push further. Above the tolerance limit fermentation stalls even if sugar remains — the yeast is inhibited by its own ethanol before it can consume the rest of the wort.

How do I know if my fermentation is stuck?

Three signals together: FG stable for 2-3 days at a value well above expected, no visible airlock activity, and a taste sample that comes out noticeably sweet. Common causes are a temperature drop, an underpitched slurry, or low nutrients. The usual fixes are to warm the fermenter, re-pitch fresh yeast, or add yeast nutrient — in that order, since a warm restart on the existing yeast is the cheapest option.

Related Tools

• Sourdough Hydration Calculator — Baker's math for a different ferment
• Coffee Extraction Calculator — Same family of TDS/gravity analytics
• Charcuterie Cure Calculator — Fermentation and food-science-adjacent

References

• John Palmer, How to Brew, 4th ed., Ch. 12 (Priming, Bottling, and Force Carbonation)
• Sean Terrill refractometer correction methodology (seanterrill.com)
• BeerSmith gravity formula documentation

Reviewed by the Craft Calc Lab team on July 29, 2026.