Using a refractometer for brewing and winemaking
A refractometer needs only a few drops, which makes it handy for checking gravity without wasting beer, wine or mead. It reports sugar content in degrees Brix (°Bx), which is close to degrees Plato and converts directly to specific gravity.
Before fermentation, the conversion is simple: divide by your correction factor and convert Brix to gravity. Set both Brix fields to the same reading to get your original gravity.
After fermentation starts, raw readings become wrong. Alcohol has a higher refractive index than water, so it makes the sample look sweeter than it is. A beer that started at 16 °Bx and now reads 7.6 °Bx looks like 1.029 on a straight conversion, but its true gravity is about 1.012. To correct for this, the calculator uses Sean Terrill's 2011 cubic equation, which estimates true gravity from your original and current Brix:
FG = 1 − 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 current Brix, each divided by the correction factor.
Getting accurate readings
- Calibrate the refractometer with distilled water to 0 °Bx before each session.
- Let the sample reach room temperature, or use a refractometer with automatic temperature compensation (ATC).
- Find your own correction factor once (see the FAQ below) and keep using it.
- The correction is least reliable in the first day or two of fermentation and for very high gravity meads and dessert wines. Confirm the final reading with a hydrometer before bottling.
Once you have true original and final gravity, you can also use the ABV calculator or the wine ABV calculator.
Method and sources
- Before fermentation: the Brix reading is divided by your wort correction factor (1.04 by default) and converted to gravity.
- After fermentation starts: alcohol bends light differently from sugar, so a raw Brix reading overstates the sugar left. The calculator uses Sean Terrill's cubic equation, fitted to paired refractometer and hydrometer readings.
- Limits: Terrill's data came from beers with starting gravities of 1.036 to 1.106. Results for strong wines and meads are extrapolated, so confirm a final reading with a hydrometer before bottling.
Every gravity-based ABV is an estimate. A lab measures alcohol directly, and its result can differ slightly from any hydrometer formula.
Sources
Frequently asked questions
Why is my refractometer reading wrong after fermentation?
Alcohol bends light more than water does, so once fermentation starts a refractometer reads higher than the true sugar content. Enter your original and current Brix here and the calculator removes that error.
What is a wort correction factor?
Refractometers are calibrated for sucrose in water, and wort or must reads slightly differently. Dividing by a correction factor, typically 1.02 to 1.06 (1.04 is a common default), lines your refractometer up with a hydrometer.
How do I find my own correction factor?
Take a hydrometer reading and a refractometer reading of the same unfermented sample. Convert the hydrometer reading to Brix, then divide the refractometer reading by it. The result is your correction factor.