Refractometer Converter
Select a mode — pre-fermentation or post-fermentation correction
ATC Brewing Refractometer: Automatic temperature compensation, dual Brix/SG scale. Takes a 2-drop sample — no test tube needed.As an Amazon Associate we earn from qualifying purchases.
View on Amazon →Brix to SG Quick Reference
Common Brix readings and their equivalent specific gravity and potential ABV values (pre-fermentation, assuming complete attenuation).
How to convert OG and FG readings to ABV — including the Balling and Terrill correction formulas.
| Brix (°Bx) | Specific Gravity | Potential ABV | Typical Use |
|---|---|---|---|
| 6 | 1.023 | 3.0% | Light beer |
| 10 | 1.040 | 5.2% | Standard ale / lager |
| 12 | 1.048 | 6.3% | Amber ale / craft beer |
| 14 | 1.057 | 7.4% | Strong beer / cider start |
| 16 | 1.065 | 8.5% | Wine / DIPA start |
| 18 | 1.074 | 9.7% | Medium wine / barleywine |
| 20 | 1.083 | 10.8% | High-gravity wine |
| 22 | 1.092 | 12.0% | Wine / sugar wash |
| 24 | 1.100 | 13.1% | Strong sugar wash |
| 26 | 1.109 | 14.3% | High-gravity sugar wash |
| 30 | 1.127 | 16.7% | Max practical fermentation |
How a Refractometer Works
A refractometer measures the bending (refraction) of light as it passes through a liquid sample. Dissolved sugars increase the refractive index, bending the light more — which shifts a shadow line across a graduated scale visible through the eyepiece. The reading is expressed in degrees Brix (°Bx).
For pre-fermentation wort, the reading is accurate and needs only a small correction factor (WCF, usually 1.02–1.06) to account for instrument calibration. For post-fermentation samples, the presence of ethanol distorts the reading significantly — ethanol refracts differently to sugar, causing an inflated Brix reading.
Compare refractometer and hydrometer readings — when to use each and how to cross-check results.
The Wort Correction Formula
The most widely adopted post-fermentation correction is the Sean Terrill cubic formula, derived from experimental data:
+ 0.00027581 × (OBrix/WCF)² − 0.0012717 × (FBrix/WCF)²
− 0.0000072800 × (OBrix/WCF)³ + 0.000063293 × (FBrix/WCF)³
Where OBrix = original Brix reading, FBrix = final apparent Brix reading, WCF = wort correction factor.
ABV is then calculated from OG and corrected FG using the standard formula:
The Brix to SG conversion used throughout this calculator is the ASBC polynomial (De Clerck, 1957), which is accurate to ±0.0001 SG across the 0–30 °Brix range:
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When to Use a Refractometer vs a Hydrometer
A refractometer requires only 2 to 3 drops of liquid, which makes it practical for testing wort during the mash or taking quick gravity readings without cooling a sample. A hydrometer requires a larger sample and needs to be at a reference temperature (usually 20°C) for an accurate reading. For pre-fermentation use, either instrument works and gives comparable results when correctly calibrated.
Once fermentation starts, the picture changes. Alcohol affects the refractive index of a liquid differently from sugar, which means a standard refractometer reading becomes inaccurate as ABV increases. The reading will be higher than the true specific gravity. This is not a calibration fault — it is a fundamental limitation of how the instrument works. The wort correction calculator on this page applies the Sean Terrill formula to back-calculate true FG and ABV from a pair of original and final refractometer readings.
If you are using a refractometer mid-fermentation to check progress, the corrected reading gives a reasonable estimate. For a definitive final gravity reading before distilling, a hydrometer in a temperature-corrected sample is more reliable. Use the refractometer for speed and convenience during the mash and early fermentation, and switch to the hydrometer for the final reading.
Calibrating and Using a Refractometer Correctly
All refractometers need calibrating before use. Place 2 to 3 drops of distilled water on the prism, close the cover, and look through the eyepiece. The boundary line between the blue and white fields should sit exactly at zero Brix. If it does not, turn the calibration screw until it does. Recalibrate at the start of each brewing or distilling session and whenever the ambient temperature changes significantly.
ATC (automatic temperature compensation) refractometers adjust for ambient temperature variation up to a point, typically 10 to 30°C. They do not compensate for sample temperature — the liquid you test should be at or near room temperature. Testing a hot wort sample directly off the heat will give a false low reading because heat affects how light bends through the prism.
Wort correction factor (WCF) varies between refractometer models. Most instruments use a WCF of 1.04 as a default, but the range across models is typically 1.02 to 1.06. Check your manual or test your refractometer against a known-gravity sample with a calibrated hydrometer to find the correct factor for your instrument. Using the wrong WCF produces systematic errors in all your FG and ABV calculations.