A refractometer is calibrated against sucrose, but wort's mix of maltose,
dextrins and protein refracts more per unit of extract, so a raw reading runs
high by ~4 %. 1.04 is the conventional default. Brewers who calibrate their
own instrument against a hydrometer typically get between 1.02 and 1.06.
Default wort correction factor.
A refractometer is calibrated against sucrose, but wort's mix of maltose, dextrins and protein refracts more per unit of extract, so a raw reading runs high by ~4 %. 1.04 is the conventional default. Brewers who calibrate their own instrument against a hydrometer typically get between 1.02 and 1.06.