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Redmoon Converters
🍺 Home Brewing

ABV Calculator (OG → FG + Refractometer)

Calculate beer ABV from original and final gravity using the simple formula or the more accurate Daniels/advanced formula. Refractometer correction (WCF) and attenuation %, with SG ↔ Brix ↔ Plato toggle.

Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on

Pre-fermentation reading.
Post-fermentation reading.
For refractometer FG correction (default 1.04).
Alcohol by volume
6.30 %
Attenuation 80.0% · Calories per 12 oz 200
MetricValue
Quick reference

Simple: ABV% = (OG − FG) × 131.25.

Advanced (Daniels): ABV% = (76.08 × (OG − FG) / (1.775 − OG)) × (FG / 0.794).

Brix → SG: SG = 1 + (Brix / (258.6 − (Brix/258.2)×227.1)).

Refractometer FG correction (Sean Terrill): FG = 1.0000 − 0.0044993×OGb + 0.011774×FGb + 0.00027581×OGb² − 0.0012717×FGb² − 0.00000728×OGb³ + 0.000063293×FGb³ — apply wort correction factor (default 1.04) to your Brix readings first.

Attenuation: (OG − FG) / (OG − 1). Calories per 12 oz: [(6.9 × ABW) + 4 × (real extract − 0.1)] × FG × 3.55, where ABW = 0.79 × ABV / FG and real extract = 0.1808 × OG°P + 0.8192 × FG°P.

Brix and Plato are both sucrose-mass scales; they differ by far less than a homebrew refractometer can resolve, so this tool treats them as interchangeable rather than implying a precision it does not have.

How it works

Two formulas are available. Simple is the standard homebrew shortcut, ABV% = (OG − FG) × 131.25, which turns 1.060 down to 1.012 into 6.30%. Advanced (Daniels) uses 76.08 × (OG − FG) / (1.775 − OG), multiplied by FG / 0.794, and reads 6.51% on the same numbers. Apparent attenuation is calculated separately as (OG − FG) / (OG − 1), giving 80.0% for that pair. Brix and Plato entries are converted to specific gravity first with the usual cubic polynomial.

The input unit and the formula choice matter most. The two equations agree closely on ordinary beers but diverge as gravity climbs — about 0.2 points apart at 1.060, but roughly 0.8 apart on a 1.090 tripel, where Daniels is the better bet. If you took your final reading with a refractometer, set the input unit to Brix and turn on the Sean Terrill correction, which divides both readings by the wort correction factor (default 1.04) before running a cubic that compensates for alcohol.

The refractometer correction only fires when the input unit is set to Brix — leave it on SG and the toggle silently does nothing. Plato is converted with exactly the same polynomial as Brix, which is close but not identical, since the two scales differ slightly at higher gravities. And every result assumes a normal beer fermentation: unfermentable additions, priming sugar, spirit fortification and heavy kettle caramelisation all move the true alcohol away from what gravity difference alone predicts.

Frequently asked questions

What is the formula for ABV from OG and FG?

The simple version is (OG − FG) × 131.25. An original gravity of 1.060 finishing at 1.012 gives a difference of 0.048, which multiplied by 131.25 comes to 6.30% ABV. The tool also converts that to approximate alcohol by weight at 79% of the ABV figure.

Why is my refractometer final gravity reading wrong?

Alcohol bends light differently from sugar, so a refractometer over-reads once fermentation has started. Set the input unit to Brix and switch Refractometer FG correction to On (Sean Terrill) — the correction is skipped entirely if the unit is left on SG.

What wort correction factor should I use?

The default is 1.04, which suits most refractometers on beer wort. The field accepts 0.5 to 1.5. To find yours, measure the same cooled sample with both a refractometer and a hydrometer and divide the Brix reading by the hydrometer-derived Brix.

What is apparent attenuation and what does 80% mean?

It is the fraction of the original gravity points the yeast consumed, calculated as (OG − FG) divided by (OG − 1). Going from 1.060 to 1.012 uses 48 of 60 points, which is 80% — a typical, healthy finish for a clean ale yeast.

When should I use the Advanced (Daniels) formula?

On strong beers. The two formulas track each other closely below about 1.070 but pull apart above it, because the simple multiplier assumes a linear relationship that breaks down at high gravity. On a 1.090 tripel the gap is close to 0.8 percentage points.

Two readings, and the error in both

ABV is derived from the drop between original and final gravity, which means an error in either reading is carried into the result — and because the calculation works on the difference between two similar numbers, a small absolute error becomes a larger proportional one.

Both readings need temperature correction unless the hydrometer is calibrated to the temperature you measured at, which it usually is not. A warm sample reads low.

The simple and alternate formulas offered diverge at higher gravities: the simple multiplier is a linear approximation fitted to ordinary beer strengths and overstates ABV on strong worts, where the alternate formula is the better estimate.

Refractometers after fermentation

A refractometer reading in Brix or Plato is accurate on unfermented wort and wrong once alcohol is present, because ethanol has a different refractive index from water and skews the reading. Using an uncorrected post-fermentation refractometer reading will substantially misstate final gravity.

Correction formulas exist and are built into the Brix and Plato options here, but they are empirical fits with their own error. For a figure that matters, a hydrometer reading on a degassed, temperature-corrected sample remains the more reliable measurement.

Where these numbers come from

The standard or formula behind this tool, so you can check it rather than take it on trust.

  • ABV is derived from the difference between two gravity readings, so an error in either is doubled in the result.

    Both readings need temperature correction unless the hydrometer is calibrated to the temperature you measured at. Different ABV formulas diverge at high gravity.

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