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Refractometers are great on brewday. You can take a drop or two of wort and get a half decent gravity reading. They’re quite finicky after fermentation has begun though, but there are few methods to get a slightly more accurate result. Sean Terril had some calculations for this, and I’ve put several different methods here: New Cubic, Old Cubic and New Linear. You don’t really need to worry about the names, just start with New Cubic and compare it to the hydrometer readings. If Old Cubic or New Linear are more accurate for you, use those.

To Use

  1. Enter your initial refractometer readings
  2. Enter either the refractometer or hydrometer FG reading. If you enter a hydrometer reading, it’ll use that instead of the refractometer one.
  3. If you have a sugary (e.g. table sugar) wort - say 10% or more, change the wort correction figure. I’d drop it a point per 10% of sugar till you get to zero, so 10% = 1.03, 20% = 1.02, 40% = 1.00, 50% = 1.00.
  4. Look at the results in the greyed out boxes.


  • Specific Gravity: calculated SG based on your refractometer reading
  • ABV%: calculated alcohol by volume percentage
  • Apparent Attenuation %: density of the finished vs initial wort. This doesn’t take alcohol into account when comparing the attenuation of the unfermented vs fermented wort.
  • Attenuation %: actual attenuation with alcohol density being taken into consideration. This should be the measure of how much sugar in your wort got fermented.
  • New Cubic: FG using the New Cubic calculation method.
  • Old Cubic: FG using the Old Cubic calculation method.
  • New Linear: FG using the New Linear calculation method.

©2024 James Torr