Why can't I read FG directly off a refractometer?

A refractometer measures refractive index, which alcohol also affects strongly. Once fermentation produces ethanol, the raw Brix reading no longer maps to gravity, so a correction formula is needed to back out the alcohol's contribution and recover true final gravity.

What is the wort correction factor (WCF)?

Refractometers are calibrated against sucrose solutions, but wort sugars refract slightly differently. The WCF divides your readings to compensate; for most worts it falls between 1.02 and 1.06, with 1.04 a common default. Calibrate yours against a hydrometer for best accuracy.

Which correction formula does this use?

It uses the Sean Terrill cubic correction, a widely-adopted empirical fit that takes corrected original and final Brix and returns final gravity. It is generally accurate to within a few gravity points for normal-strength beers.

How is ABV estimated from the corrected gravity?

Once original gravity and corrected final gravity are known, ABV is estimated with the standard formula ABV ≈ (OG − FG) × 131.25. This is the same approximation used by most homebrew calculators and is accurate for typical beer strengths.

Is a hydrometer more accurate for final gravity?

A hydrometer reads final gravity directly with no alcohol correction needed, so it is the gold standard for FG. Refractometers are convenient because they need only a drop or two of sample; the correction formula makes them usable, but cross-check important readings with a hydrometer.

What is the Refractometer FG Correction Calculator?

Apply the Sean Terrill correction to convert post-fermentation refractometer Brix readings into true final gravity, accounting for alcohol's effect on refractive index. Also estimates ABV. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Refractometer FG Correction Calculator

Name: Refractometer FG Correction Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

The Refractometer FG Correction Calculator turns post-fermentation Brix readings into true final gravity. Alcohol bends light strongly, so a refractometer reads incorrectly once fermentation begins. This tool applies the well-known Sean Terrill correction to recover real FG and estimate ABV.

Why a refractometer reads wrong after fermentation

A refractometer measures how sharply a liquid bends (refracts) a beam of light, expressed as a Brix reading scaled to sucrose solutions. Before fermentation, wort contains sugars with a known refractive index — so Brix maps reliably to gravity with just a small wort correction factor. After fermentation, the wort also contains ethanol, which refracts light at a substantially different angle than sugar. The raw post-fermentation Brix reading is therefore a blend of the residual sugar signal and the alcohol signal, and it consistently reads higher than the true final gravity.

A raw post-fermentation Brix of 6.5 might imply a gravity of 1.026 on the standard Brix-to-SG conversion, when the actual FG is closer to 1.010. Without the correction, the batch appears much less attenuated than it actually is.

How it works

First, both Brix readings are divided by the wort correction factor (WCF), because refractometers are calibrated on sucrose and wort sugars refract a little differently. With corrected original Brix OB and corrected final Brix FB, the Sean Terrill cubic gives final gravity directly:

FG = 1.0000
   − 0.0044993 × OB
   + 0.0117741 × FB
   + 0.0007520 × OB²
   − 0.0026820 × FB²
   − 0.00000587 × OB³
   + 0.00001880 × FB³

Original gravity is derived from the corrected original Brix using the standard Brix-to-SG conversion:

OG = 1 + (OB ÷ (258.6 − (OB ÷ 258.2) × 227.1))

Finally, ABV is estimated with the common approximation:

ABV ≈ (OG − FG) × 131.25

Worked example

You read 12.0 Brix on the unfermented wort and, after fermentation, 6.5 Brix on the same refractometer with a WCF of 1.04. Dividing gives corrected readings of about 11.54 and 6.25 Brix. The Terrill formula returns a final gravity near 1.010, an original gravity near 1.048, and an estimated ABV of roughly 5.0% — whereas the raw 6.5 Brix would have implied a falsely high finishing gravity around 1.026, making the beer appear 2.5% ABV when it is actually 5%.

Finding and calibrating your wort correction factor

The WCF is specific to your refractometer and wort. To find it:

Pull a sample of unfermented wort before pitching.
Measure it on your refractometer and note the raw Brix.
Measure the same sample with a hydrometer (temperature-corrected) and convert to Brix using °Bx = (SG − 1) × 250.
Divide the hydrometer Brix by the refractometer Brix. The result is your WCF.

A typical WCF falls between 1.02 and 1.06. A value near 1.00 means your refractometer is well-calibrated for wort; a higher value means it over-reads slightly relative to the true sugar content.

Tips

Calibrate your WCF once per refractometer using an unfermented sample, not by adjusting to hit a target FG.
The correction assumes a typical beer fermentation profile; high-adjunct, fruit, or very high-gravity fermentations can produce larger errors because the sugar and ethanol profiles differ from the formula’s assumptions.
For the most reliable FG on a critical batch, confirm with a hydrometer — the correction is an approximation, accurate to a few gravity points for normal beers.