Input
How to use this calculator
For a pre-fermentation reading, enter the wort Brix and wort correction factor. During fermentation, enter the original Brix and current Brix so the alcohol correction can be applied. Take a clear sample and avoid bubbles or suspended solids on the prism.
Do not treat a post-fermentation Brix value as a direct SG conversion. Alcohol changes the optical reading, which is why this tool has separate before- and during-fermentation modes.
How the calculation works
Before fermentation, the implementation converts Brix through the project gravity conversion and applies the selected wort correction factor. During fermentation it first estimates original SG, then uses the implemented alcohol-aware regression to estimate corrected SG, ABV and apparent attenuation.
The regression is an estimate tied to the inputs and correction factor. It is not a substitute for laboratory analysis, and unusual wort composition or an incorrect original reading can produce misleading results.
Example calculation
With original wort at 12 Brix, the before-fermentation mode returns an estimated SG. If the current reading later falls to a lower Brix, the during-fermentation mode estimates corrected SG, ABV and apparent attenuation from both readings.
Understanding the result
Refractometers are convenient because a tiny sample is enough, but fermentation changes the relationship between refractive index and density. The corrected result is useful for tracking a trend, not for pretending the instrument directly reads finished-beer SG.
For best repeatability, use the same instrument, correction factor and sampling practice across the batch. A hydrometer reading can be a valuable cross-check.
Common mistakes
- Using direct Brix-to-SG conversion after fermentation begins.
- Forgetting the original Brix reading.
- Changing wort correction factor between measurements.
- Taking a sample with foam or suspended yeast on the prism.
Frequently asked questions
Can I convert fermented Brix directly to SG?
No. Alcohol changes the optical reading, so a fermentation correction using original and current Brix is required.
What is the wort correction factor?
It is an adjustment for the composition of the unfermented wort; the correct value depends on your wort and instrument practice.
Is refractometer ABV exact?
No. It is an estimate based on optical readings and a correction regression.
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