Fermentation Calculator

What this calculator does

Fermentation is yeast turning sugar into alcohol and carbon dioxide, and gravity is how brewers and winemakers watch it happen: a sugary liquid is denser than water, an alcoholic one less so, and the fall between the two readings prices the transformation. This page computes the standard measures from those readings: alcohol by volume (two formulas, because the simple one drifts at high gravities), apparent attenuation (the share of the extract the yeast consumed, the working brewer’s health check on a fermentation), temperature correction for the hydrometer itself, and the Brix conversion for refractometer users, with the fermented-sample caveat printed where it belongs.

How to use it

• Degas and spin the sample. Carbon dioxide bubbles lift a hydrometer and fake a low reading; a degassed sample, spun to shed bubbles clinging to the stem, is the reading the formulas deserve.
• Correct warm readings, always. Wort straight from a mash tun or a fermenter in a warm room reads low; the correction section exists because a 90°F sample understates its gravity by about four points, which understates the alcohol with it.
• Read attenuation as the yeast’s report card. Most beer yeasts land in the 65 to 85 percent apparent range; a reading far below a strain’s habit, with gravity stable over days, is a finished-but-tired fermentation, and one still falling is simply not done.
• Never Brix-convert a fermented sample raw. Alcohol bends light differently than sugar water, so a refractometer reads high once fermentation starts; use the gravity side for fermented samples, or a proper refractometer correction, of which this converter is deliberately not one.

The formulas

• Standard ABV = (OG − FG) × 131.25.
• Advanced ABV = (76.08 × (OG − FG) / (1.775 − OG)) × (FG / 0.794), the widely used correction that tracks laboratory values better at higher gravities.
• Apparent attenuation = (OG − FG) / (OG − 1) × 100. It is called apparent because alcohol’s low density exaggerates the drop; real attenuation is lower.
• Hydrometer correction: the standard density polynomial for water, applied as the ratio between the sample temperature and the 60°F calibration temperature, multiplied by the reading.
• Brix to SG uses the standard sucrose conversion polynomial, valid for unfermented sugar solutions such as wort and must.

Worked examples

The house ale. OG 1.050, FG 1.010: the drop is 40 points. Standard ABV = 0.040 × 131.25 = 5.25%; the advanced formula gives 5.34%, the pair bracketing the honest answer. Apparent attenuation = 0.040 / 0.050 = 80.00%, a healthy figure for a typical ale yeast, and the kind of agreement between the three numbers that tells a brewer the fermentation behaved.

The warm sample. A hydrometer reading 1.050 in a sample at 90°F corrects to 1.05414 at the 60°F calibration: four points of gravity, and roughly half a percent of alcohol, that the warm liquid was hiding. Boundary case worth knowing: at exactly 60°F the correction is zero by construction, and below it, readings correct downward instead.

Reading your results

Read the two ABV figures as a range, not a disagreement: the truth for most beers sits between them, nearer the advanced figure as gravities climb. Read attenuation against the yeast, not against a universal bar: a high-attenuation saison strain at 80 percent is loafing, a traditional English strain at 80 percent has overperformed. And read any FG that rises between readings with suspicion about the measurement (temperature, bubbles, a bumped sample jar) before suspecting the beer of unfermenting, which liquids politely decline to do.

Limitations

• Estimates, not analysis. Gravity formulas model an average wort; actual alcohol depends on the true extract composition, and excise or label declarations in most places require laboratory methods (distillation or approved instruments).
• Valid ranges apply. The formulas assume typical brewing gravities (roughly 1.000 to 1.120). Honey-heavy musts, very high-gravity worts and mixed-sugar fermentations stray further from the model.
• Hydrometer calibration varies. This correction assumes a 60°F instrument; some are calibrated at 68°F (20°C), and applying the wrong calibration writes a small, systematic error into every reading until noticed.
• Fermentation is biology, not only arithmetic. The page prices finished readings; it cannot diagnose a stuck fermentation’s cause (temperature, nutrition, yeast health), which is a judgement for the brewer with the vessel in front of them.

Frequently asked questions

Which ABV formula should I use?

For ordinary beers and wines at ordinary gravities, either; they differ by about a tenth of a percent in the worked example. As original gravities rise past roughly 1.070, prefer the advanced figure, which was fitted against measured values to correct the standard formula’s growing understatement. For anything official (a label, a licence, a tax return), neither: those need the laboratory methods the limitations describe, and the honest home figure is the bracket the two formulas give together.

My gravity has not moved in two days. Is it done?

The working rule is a stable gravity across two to three days at a sensible temperature, read on degassed samples, means fermentation has finished or stopped, and the attenuation tells you which story is likelier: near the yeast’s expected range means finished; far below it means stuck, with causes (cold, exhausted yeast, a very high OG) to consider before bottling. Bottling on a guess is how bottles become fireworks, so the stable reading is the gate, not the calendar.

Why does my refractometer disagree with the hydrometer?

Because they measure different physics: the hydrometer reads density, the refractometer reads light bending, and alcohol bends light more than the sugar it replaced. In unfermented wort the two agree through the Brix conversion; after yeast pitch they diverge, the refractometer reading high. Brewers correct fermented refractometer readings with a wort correction factor and a proper formula; winemakers mostly use each instrument where it is honest, which is the practice this page follows.

What is a good attenuation?

Against the strain’s specification, which for most beer yeasts spans roughly 65 to 85 percent apparent, with wine and champagne yeasts effectively exhausting fermentable sugar altogether. The number’s use is comparative: a familiar recipe attenuating five points lower than usual is information about yeast health, temperature or mash profile. A single universal target would be meaningless, which is why the result prints the figure cleanly and leaves the judgement to the context only the brewer has.

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Method: standard and advanced gravity-to-alcohol formulas, apparent attenuation from gravity points, hydrometer correction by the standard water-density polynomial at a 60°F calibration, and the standard sucrose Brix conversion for unfermented liquids. Last reviewed: 15 October 2026.