Basics and units
Alcohol by volume with a refractometer
One drop is enough, but the final reading must be corrected: alcohol bends the light. Give the °Brix before and after fermentation, and the tool returns the true final gravity and the strength.
Result
Fill in the values and run the calculation, or try the example.
This is the calculation Krausi uses. Create an account to save it in a recipe or a brew.
Create my accountWhy correct the refractometer reading?
A refractometer lets you follow a fermentation with a drop of beer instead of the 100 mL of a trial jar: on a small batch, that is several bottles saved.
But the instrument measures how light is bent, and alcohol bends it differently from sugar. As soon as fermentation has started, the raw reading overestimates the gravity. Without correction, a beer finished at 1.009 still shows 6 °Brix, about 1.023: you wrongly believe fermentation is stuck.
The correction needs the original reading, taken before fermentation. Write it down on brew day.
How is the Brix reading corrected?
The corrected final gravity is a polynomial of the two readings, the initial reading Bi and the final reading Bf, in °Brix:
FG = 1.001843 − 0.002318474 × Bi − 0.000007775 × Bi² − 0.000000034 × Bi³
+ 0.00574 × Bf + 0.00003344 × Bf² + 0.000000086 × Bf³The original gravity is the direct conversion of Bi to SG. The alcohol content is then calculated as with a hydrometer: 131.25 × (OG − FG), plus the alcohol from the priming sugar if you enter it.
Worked example
Values entered
- Reading before fermentation, in °Bx: 12
- Reading at the end of fermentation, in °Bx: 6
Result from the tool
- Alcohol by volume, in % ABV
- 4.97 %
- Original gravity converted from °Bx, in SG
- 1.046
- Final gravity corrected for the effect of alcohol, in SG
- 1.009
What the tool does not do
- The tool assumes a refractometer calibrated with distilled water and a wort correction factor of 1.0.
- It reports an error if the corrected final gravity is above the original gravity: one of the two readings is wrong.
- Accuracy remains lower than a hydrometer's: about ± 0.002 of gravity.
Frequently asked questions
Why does my refractometer still show 6 Brix at the end of fermentation?
Because alcohol skews the reading. With an initial reading of 12 °Brix, 6 °Brix at the end of fermentation corresponds to a true gravity of 1.009: fermentation is indeed finished.
Can I use a refractometer for the final gravity?
Yes, provided you correct the reading with the original value. For a decisive measurement, before bottling for example, confirm with a hydrometer.
And with the assistant?
Ask Krausi the same question in plain language: he picks this tool, does the calculation and files the result in your project.
Refractometer: 12 Brix at the start, 6 Brix this morning.
Corrected for alcohol, your gravity is 1.009 for an original gravity of 1.046, which is 4.97% ABV. I've logged the reading in the brew: fermentation is finished or nearly so.