Refractometer Correction Calculator
Get accurate final gravity from your refractometer. Alcohol in fermented wort throws off Brix readings — this corrects for that.
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Why Refractometers Lie After Fermentation
A refractometer measures how light bends as it passes through a liquid — the refractive index. It is calibrated to convert this measurement into Brix, which represents the percentage of dissolved sugars. Before fermentation, this works perfectly.
The problem starts once yeast has done its work. Fermentation converts sugar into alcohol and CO2. Alcohol has a very different refractive index to sugar — it actually bends light less. So a refractometer reading on fermented beer will be significantly lower than the true remaining sugar content.
Without correction, a refractometer would make you think your beer has fermented further than it actually has, giving you a falsely low final gravity.
The Sean Terrill Formula
This calculator uses the Sean Terrill cubic polynomial, which is widely regarded as the most accurate refractometer correction available. It takes both the original Brix (before fermentation) and final Brix (after fermentation) to calculate the corrected specific gravity:
FG = 1.0000 - 0.0044993(OB) + 0.011774(FB) + 0.00027581(OB²) - 0.0012717(FB²) - 0.0000072800(OB³) + 0.000063293(FB³)Where OB and FB are the original and final Brix readings divided by the wort correction factor.
Wort Correction Factor
Beer wort is not pure sugar water. It contains proteins, minerals, and other compounds that affect the refractive index slightly. The wort correction factor (WCF) accounts for this. The standard value is 1.04. If you have calibrated your refractometer against a hydrometer, you can enter your own value.
Frequently Asked Questions
Why is my refractometer wrong after fermentation?
Refractometers measure the refractive index of a liquid and convert it to Brix (sugar content). After fermentation, alcohol is present in the sample. Alcohol refracts light differently than sugar, so the reading is lower than the actual sugar content. You need a correction formula to get the true final gravity.
What is a wort correction factor (WCF)?
Wort is not pure sugar water — it contains proteins, minerals, and other compounds that affect the refractive index. The wort correction factor (typically 1.04) adjusts for this. Most refractometers are calibrated with pure sucrose solutions, so the WCF corrects for the difference between sucrose and wort.
What is the Sean Terrill refractometer formula?
Sean Terrill developed a cubic polynomial formula that uses both the original and final Brix readings to calculate the corrected final gravity. It is widely regarded as the most accurate refractometer correction formula available and accounts for the non-linear effects of alcohol on refractive index.
Can I use a refractometer instead of a hydrometer?
Yes, but with a correction. A refractometer is accurate for pre-fermentation (OG) readings without correction. For post-fermentation (FG) readings, you must apply a correction formula because alcohol distorts the reading. Our calculator does this correction for you using the Sean Terrill formula.
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