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.

Refractometer FG Correction Calculator

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.

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.

Tips

Calibrate your WCF once by reading a wort on both refractometer and hydrometer, then solve for the factor that makes them agree.
The correction assumes a normal beer fermentation; very high-gravity or unusual sugar profiles increase the error.
For the most reliable FG, confirm critical readings with a hydrometer.