Refractometer Correction Calculator
A refractometer correction converts a Brix reading taken during or after fermentation into true specific gravity by accounting for the refractive index of ethanol. Without correction, readings run several points high because alcohol bends light more than its concentration suggests. The Terrill cubic model fits the correction from the original gravity and the current Brix reading.
A refractometer reads high once there is alcohol in the sample. Enter your original gravity and the current Brix reading to get the real gravity.
Why refractometers go wrong after pitching
A refractometer measures how much a sample bends light, and calibrates that against sugar concentration. Ethanol bends light differently from water, and considerably more than its concentration would suggest. Once fermentation starts, the instrument is reading a mixture it was never calibrated for, and it reads high — often by four or five gravity points at terminal gravity.
This calculator applies the Terrill cubic model, which fits the correction against original gravity and the current reading. It is what Brewgravity uses internally when you record a refractometer reading against a session.
Wort correction factor
Refractometers are calibrated for sucrose, and wort is not sucrose. Most need a small correction — typically between 1.02 and 1.06, with 1.04 the usual default. To find yours, take a reading of a cooled sample and compare it against a hydrometer reading of the same sample: the correction factor is the Brix reading divided by the hydrometer’s Plato equivalent.
Before fermentation
Before any alcohol is present the conversion is straightforward, and this is where a refractometer earns its keep: two drops of hot wort, cooled on the glass in seconds, versus a full hydrometer sample you have to cool and then throw away. Use the Brix to gravity converter for pre-fermentation readings.
What still needs a hydrometer
The corrected figure is an estimate with a real error bar — typically a point or two. For a final gravity you intend to bottle against, take a hydrometer reading. The refractometer is for tracking the fermentation, not for deciding it is finished.
Refractometer Correction Calculator — questions
Why is my refractometer reading higher than my hydrometer?
Alcohol. The refractometer over-reads once fermentation has started, and the gap widens as the beer attenuates. The correction on this page converts the reading back to true gravity.
Can I use a refractometer for final gravity?
For tracking, yes. For the reading you bottle against, use a hydrometer — the corrected value carries a point or two of uncertainty and priming calculations assume the fermentation really has finished.
What is Brix?
Grams of sucrose per 100 grams of solution. It is very close to degrees Plato, which brewers use for the same thing; 1 °Bx is roughly 1.004 specific gravity in the range wort occupies.
Log your gravity readings in the recipe itself
Brewgravity's fermentation log takes refractometer or hydrometer readings and tracks attenuation against the predicted final gravity — no separate correction step.
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