Free Distilling Tools

Refractometer Calculator

Convert Brix readings to ABV, SG, and Plato. Correct post-fermentation refractometer readings for alcohol presence.

Refractometer Converter

Select a mode — pre-fermentation or post-fermentation correction

Brix scale position
—
0 °Bx51015202530 °Bx
°Bx
Enter your refractometer reading in °Brix
×
Typical range 1.02–1.06 (check your refractometer manual)
Specific Gravity
—
Potential ABV
—%
Degrees Plato
—°P
Potential ABV assumes complete fermentation to 0 °Brix. Actual ABV will be lower.
Refractometers read falsely high once alcohol is present. Enter both your original and final refractometer readings to calculate true FG and ABV.
°Bx
Pre-fermentation reading
°Bx
Post-fermentation refractometer reading
×
From refractometer manual
True Final Gravity
—
Actual ABV
—%
Original Gravity
—
Uses the Sean Terrill cubic correction formula, widely accepted as the most accurate method for home fermenters.

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Brix to SG Quick Reference

Common Brix readings and their equivalent specific gravity and potential ABV values (pre-fermentation, assuming complete attenuation).

Brix (°Bx)Specific GravityPotential ABVTypical Use
61.0233.0%Light beer
101.0405.2%Standard ale / lager
121.0486.3%Amber ale / craft beer
141.0577.4%Strong beer / cider start
161.0658.5%Wine / DIPA start
181.0749.7%Medium wine / barleywine
201.08310.8%High-gravity wine
221.09212.0%Wine / sugar wash
241.10013.1%Strong sugar wash
261.10914.3%High-gravity sugar wash
301.12716.7%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.

The Wort Correction Formula

The most widely adopted post-fermentation correction is the Sean Terrill cubic formula, derived from experimental data:

FGtrue = 1.0000 − 0.0044993 × (OBrix/WCF) + 0.011774 × (FBrix/WCF)
+ 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:

ABV% = (OG − FG) × 131.25

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:

SG = 1.000019 + 0.003865613×B + 0.00001296425×B² + 0.00000005701128×B³

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Frequently Asked Questions

Refractometers are calibrated for sugar solutions, not alcohol-water mixtures. Once alcohol is present it refracts light differently, causing the refractometer to read higher than the true specific gravity. Use the wort correction mode above to get an accurate final gravity.

The most accurate method uses the ASBC polynomial: SG = 1.000019 + 0.003865613×B + 0.00001296425×B² + 0.00000005701128×B³. For a quick approximation: SG ≈ 1 + (Brix ÷ 258.6). This calculator uses the full polynomial for all conversions.

Yes — for original gravity (pre-fermentation) a refractometer is ideal, requiring only 2–3 drops of sample. For final gravity after fermentation you must apply an alcohol correction, or use a hydrometer directly which reads accurately with no correction needed.

Brix and Plato both express dissolved sugar concentration as grams of sucrose per 100 g of solution — they are nearly identical scales (1 °Brix ≈ 1 °Plato). Specific gravity expresses liquid density relative to water at 4 °C. A wort at 12 °Brix is approximately 1.048 SG and 11.7 °Plato.

A wort correction factor (WCF) accounts for the fact that real wort or wash contains compounds other than pure sucrose. Most refractometers are factory calibrated for sucrose solutions, so a WCF of 1.02–1.06 is applied to get accurate SG readings from wort or sugar wash. Check your instrument's manual — a typical value is 1.04.

Knowledge Base

Distilling Guides & Reference Articles

In-depth guides written for home distillers and craft producers — from reading a hydrometer to making clean spirit cuts.

Technique
Measurement
Fermentation
Craft & Aging
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