Refractometer or Hydrometer: Which to Trust When
Refractometers and hydrometers both measure sugar in wort, on different scales and by different physics. Knowing which to trust when is worth more than owning either.
Two instruments, two scales
A hydrometer floats in a sample and reads density — specific gravity, the wort's weight relative to water. A refractometer bends light through a few drops and reads °Brix, the percentage of sugar by mass.
The Brix converter moves between them. Some working figures: 10 °Bx is 1.040, 12 °Bx is 1.048, 14 °Bx is 1.057, and 20 °Bx is 1.083. Going the other way, an SG of 1.050 is about 12.4 °Brix.
A rough rule, and where it breaks
Brewers often use "gravity points ≈ Brix × 4" as mental arithmetic. At 10 °Bx that predicts 1.040 and the conversion agrees exactly.
At 20 °Bx it predicts 1.080 against an actual 1.083, and the gap widens from there. The rule is fine for a pre-boil sanity check and not for recording an original gravity.
The refractometer's one big limitation
Alcohol refracts light differently from sugar. The moment fermentation starts producing alcohol, a refractometer's Brix reading stops converting cleanly to gravity, and the error grows as the beer ferments.
So the converter is for unfermented wort only — pre-boil checks, and original gravity before pitching. Taking a final gravity with a refractometer and running it through a plain conversion produces a reading that is wrong in a direction that flatters the beer, which then makes the ABV calculation wrong too. Alcohol-corrected formulas exist for this; a plain conversion is not one.
What each is actually good for
The refractometer's advantage is sample size and speed: a few drops, read instantly, cooled by the instrument itself. That makes it ideal for checking pre-boil gravity while there is still time to adjust, and for spot checks during a sparge.
The hydrometer's advantage is that it is the reference. It needs a large sample, it is sensitive to temperature, and it works throughout fermentation. Original and final gravity belong to the hydrometer.
Temperature affects both
Hydrometers are calibrated at a set temperature, commonly 20 °C. Read a hot sample and the figure comes out low; correct it or let the sample cool.
Most refractometers have automatic temperature compensation, which handles the instrument but not a scalding sample — give the drops a few seconds on the glass before reading.
Calibrate before the season
A hydrometer should read 1.000 in distilled water at its calibration temperature; a refractometer should read 0 °Brix. Both drift, and both are cheap enough that people rarely check.
If one is off by a fixed amount, note the offset and apply it every time rather than replacing the instrument. A known-biased instrument used consistently is more useful than an unknown one.
Keep them consistent within a batch
The conversion formulas assume a sucrose-like solution, and real wort contains maltose and dextrins that behave slightly differently. Small disagreements between the two instruments on the same sample are normal.
Pick one instrument for the figures you record and stick with it. A batch measured half one way and half the other produces an attenuation number that means nothing.