Refractometer alcohol correction
Instrument correction (WCF) and alcohol correction using the Novotný model.
A refractometer needs two corrections: one for the instrument and one for alcohol. The first accounts for the fact that wort is not a sucrose solution; the second, for the fact that alcohol refracts light differently from water. Without them a reading on fermenting wort is several degrees Brix too high.
Calculator
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Instrument correction (WCF)
A refractometer is calibrated against a sucrose solution, while wort consists of maltose, maltotriose and dextrins, which refract light slightly differently. As a result the instrument systematically overreads. The wort correction factor (WCF) is the number by which the raw reading is divided.
A typical value is 1.04, but every instrument has its own: observed values range from roughly 1.02 to 1.06. The factor is determined once, from a control measurement: measure the same wort with the refractometer and with a trusted hydrometer at 20 °C, convert the hydrometer reading to Brix, and divide the raw refractometer reading by the result.
WCF = °Bx(raw refractometer reading) / °Bx(hydrometer reading converted)
Alcohol correction
Alcohol has a higher refractive index than water, so in fermenting wort the refractometer “sees” more extract than actually remains. The stronger the beer becomes, the greater the overreading — at final gravity the error reaches 4–6 °Bx, which makes measuring with a refractometer without a correction pointless.
The Novotný model is used here in the edition by A. Ascher (“Alkoholmessung mit dem Refraktometer”, 2021): gravity is computed from the original and current refractometer readings. The model works throughout fermentation. The Terrill formula, popular for final gravity, produces unphysical results at high original gravity — for example, about 1.375 instead of 1.101 at 24 °Bx — so it is not used in the journal or the calculator.
Why the original gravity is needed
An alcohol correction is impossible without the original gravity: to subtract the effect of alcohol you need to know how much has accumulated. That is why the calculator requires two values: the reading on the original wort and the current reading.
If the original gravity was not recorded, the correction cannot be recovered by any trick. For future brews, record the refractometer reading before pitching — the journal will need it to draw the fermentation graph.
How to work with the instrument
- Take the reading and write it down as it is, without rounding to whole numbers.
- Divide it by your WCF to get the corrected Brix.
- Enter the original and current corrected Brix to get the true gravity and alcohol.
- Check the result against a hydrometer at least occasionally: a discrepancy of more than 2 points means the WCF is inaccurate.