Refractometer Correction Calculator
Once fermentation starts, alcohol makes a refractometer read high. Enter your original gravity and the raw refractometer reading to estimate the true specific gravity, ABV so far, and how far fermentation has gone.
Why refractometer readings need correcting
A refractometer measures how much light bends as it passes through a few drops of liquid and reports it as degrees Brix. Before fermentation the only thing dissolved is sugar, so Brix converts straight to gravity (see the Brix to gravity converter). Yeast turns that sugar into alcohol, and alcohol bends light too, so a raw reading taken mid- or post-fermentation looks much sweeter than the beer, mead, or cider really is.
How the correction works
A refractometer reads wort slightly higher than a pure sugar solution, so the calculator first divides each refractometer reading by your wort correction factor (default 1.04). It then applies Sean Terrill’s cubic regression, which estimates the true gravity from both the original Brix and the current Brix. If you enter the OG as specific gravity from a hydrometer, it needs no wort correction and is converted to Brix directly. ABV uses the formula for the beverage type you pick, as in the ABV calculator, which uses the same correction.
Worked example
A wort reads 14.0 °Bx on the refractometer. With a wort correction factor of 1.04, that is 13.5 °Bx, or 1.055 SG. After fermentation the refractometer reads 7.0 °Bx. Converting that reading without the alcohol correction would suggest 1.027 SG. The corrected gravity is 1.013 SG (3.2 °Bx), giving about 5.5% ABV for a beer.
Measuring your wort correction factor
- Take a sample of unfermented wort or must and cool it to your hydrometer’s calibration temperature.
- Read it with a hydrometer and with the refractometer.
- Convert the hydrometer reading to Brix with the converter, then divide the refractometer reading by that value.
- Repeat over a few batches and average the results.
Limitations
- The corrected gravity is an estimate based on a regression fitted mostly to beer worts.
- High-gravity meads and wines, and brews with a lot of unfermentable sugar, are less accurate. The calculator warns when the original gravity is above about 25 °Bx (1.106 SG).
- Calibrate the refractometer with distilled water and let the sample reach room temperature.
- Confirm final gravity with a hydrometer before bottling.
References
- Homebrewers Association forum — Improved refractometer correction calculator
- John Palmer — How to Brew
Frequently asked questions
Why is my refractometer reading wrong after fermentation?
Alcohol bends light more than water, so once fermentation starts a refractometer overstates the remaining sugar. The raw Brix reading has to be corrected using the original gravity to estimate the true gravity.
What is a wort correction factor?
A number that accounts for the difference between how your refractometer reads wort and how it reads a pure sugar solution. 1.04 is a common starting value, but measuring your own with paired hydrometer and refractometer readings of unfermented wort gives better results.
Can I use a hydrometer OG with a refractometer FG?
Yes. Switch the original gravity to SG and enter your hydrometer reading. The correction only needs the original gravity, however it was measured, plus the raw refractometer reading.
How accurate is refractometer correction?
Usually within a few gravity points of a hydrometer, but it can be off by more, especially for high-gravity meads and wines or when the wort correction factor is a guess. Confirm final gravity with a hydrometer before bottling.