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🔬 Refractometer Correction Calculator

Correct Brix readings for alcohol presence during fermentation

Refractometers are calibrated for sugar solutions without alcohol. Once fermentation begins, alcohol affects the reading. Use this calculator to convert refractometer readings to true gravity and calculate ABV.

Reading taken before fermentation starts
Reading taken during or after fermentation
Calibrate your refractometer with known gravity sample

📚 Why Refractometers Need Correction

Refractometers measure the refractive index of a solution. They're calibrated for pure sugar solutions, but alcohol has a different refractive index than sugar. Once fermentation begins and alcohol is present, the refractometer reading becomes inaccurate.

When to Use This Calculator:

  • During fermentation: Track fermentation progress with small samples
  • Final gravity: Get accurate FG without using large hydrometer samples
  • Quick checks: Only need a few drops instead of a full hydrometer tube
  • Hot wort: Can take readings at any temperature (with temp correction)

Correction Formula:

This calculator uses the Terrill Linear equation, one of the most accurate methods:

FG = 1.0000 - 0.00085683 × OB + 0.0034941 × CB

Where OB = Original Brix, CB = Current Brix

Wort Correction Factor:

Different refractometers can have slight calibration variations. The wort correction factor (WCF) accounts for this:

  • Default WCF: 1.04 (works for most refractometers)
  • To find your WCF: Measure a known gravity sample with both refractometer and hydrometer
  • Calculate WCF: Hydrometer SG ÷ Refractometer SG
  • Example: Hydrometer reads 1.050, refractometer reads 1.048 → WCF = 1.050 ÷ 1.048 = 1.002

Brix to Specific Gravity Conversion:

SG = 1 + (Brix ÷ (258.6 - ((Brix ÷ 258.2) × 227.1)))

This Plato-based conversion is accurate for brewing worts.

Tips for Accurate Readings:

  • Calibrate refractometer with distilled water before use (should read 0°Bx)
  • Ensure sample is at room temperature or use temperature compensation
  • Clean and dry the prism between readings
  • Use only 2-3 drops of wort/beer for reading
  • Take multiple readings and average for best accuracy
  • Record your original pre-fermentation reading - it's essential for corrections

Advantages of Refractometers:

  • Small sample size: Only need a few drops (vs 250ml for hydrometer)
  • Temperature flexibility: Most have automatic temperature compensation
  • Quick readings: Get results in seconds
  • Easy to use: No need for tall sample tubes
  • Portable: Perfect for brew day and checking fermentation

Limitations:

  • Must correct for alcohol presence during/after fermentation
  • Requires accurate original gravity reading
  • Can be affected by suspended particles (filter sample if needed)
  • More expensive than hydrometers initially