Using a Refractometer for Brewing: Brix to SG Conversion

A refractometer is a brewer’s best friend — it lets you take gravity readings with just a single drop of wort, no hydrometer sample needed. But there’s a catch: alcohol changes the reading, so you need math to get accurate FG and ABV.

How Refractometers Work

A refractometer measures the refractive index of a liquid. In brewing, this translates to °Brix (grams of sugar per 100g of solution). Pure water reads 0°Brix; a typical wort might read 12-25°Brix.

The Wort Correction Factor

Refractometers are calibrated for sucrose solutions, but wort contains other dissolved solids. Most brewers use a wort correction factor of 1.04:

Plato = Brix / 1.04

Then convert Plato to SG using the standard formula.

Estimating FG During Fermentation

Once fermentation starts, alcohol in the wort skews the refractometer reading higher. The Sean Terrill cubic equation is the most accurate home method for correcting this:

The equation accounts for the non-linear relationship between °Brix readings during fermentation and actual specific gravity. Our Refractometer Calculator implements this full equation.

Quick Reference

Reading	Correction Method	Accuracy
Pre-fermentation OG	Simple Brix→SG conversion	High
During fermentation	Terrill cubic equation	Good (±0.002)
Final FG verification	Hydrometer confirmation	Best

Important Caveats

Always verify final ABV with a hydrometer — refractometer equations are estimates
The Wort Correction Factor varies — measure yours by comparing refractometer and hydrometer readings on the same sample
High-alcohol brews (>10% ABV) have larger estimation errors
Temperature matters — most refractometers are ATC (auto-temperature compensating), but extreme temps can still affect readings

Using the Free Calculator

The Refractometer Calculator supports:

OG-only conversion (°Brix → SG → Plato)
OG + FG conversion with ABV estimation
Wort Correction Factor adjustment
Full Terrill cubic equation for FG estimation