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Refractometer tool for partially fermented wort


May 29, 2020
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I'm trying to use the Refractometer tool in BeerSmith 2 to measure the gravity of wort while it's fermenting.

Over the past few brews, I've sampled several batches of unfermented wort, and got the results below. They seem reasonable enough.

Sample TempRefractometer (Brix WRI)Hydrometer (SG)

Using the last row in that table, I calibrated BeerSmith's hydrometer tool:

  • Calculation = unfermented wort gravity
  • Distilled water reading: 0.2 Brix
  • Refractometer reading: 11.4 Brix
  • Hydrometer reading: 1.046
  • => Brix correction factor: 0.983

That seems fine, and when I use the Unfermented Wort Gravity option in BeerSmith, it outputs a Corrected Gravity that's pretty close to my hydrometer readings. Okay, cool.

The problem is, when I use the Fermenting Wort Gravity option, the calculations are way off. I punched in a refractometer reading for a batch that's been brewing for 4 days:

  • Calculation = fermenting wort gravity
  • Refractometer reading: 7 Brix
  • Original gravity: 1.046 SG
  • => Corrected gravity: 1.015 SG

BeerSmith tells me the SG is 1.015, but a hydrometer reading on the same wort gives 1.024, so BeerSmith's number is way too low.

What could I be doing wrong here? What potential sources of error should I look at?


Grandmaster Brewer
Mar 16, 2013
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New Hampshire, US
The first thing I would recommend is to set your refractometer to 0 Brix with distilled water at room temperature (20C/68F).  There is a calibration screw on the top of the refractometer which you can use to set the base reading with water.  Before you do that, clean off the face of the prism and the cover slip very well.  Many times I have seen a dried sugar film left on the surface give higher than typical readings.  Most decent refractometers (an many cheap ones) really do not drift much once set.  It is the film on the surface which throws off readings. 

Once you have that completed, try redoing your refractometer calibration.  Generally, the correction factor should be around 1.04, give or take a few hundredths. 

My suspicion is that with the high 'zero' point you are compressing the scale after conversion which is throwing off your reading on fermenting wort.

When I was using my refractometer on fermenting wort, it was only to note the daily change and when I reached a consistent end point. I then checked final gravity with my hydrometer.  I seldom bothered to convert the Brix readings to specific gravity, mostly because that was not my goal.