In days of yore, refractometers were considered a pre-fermentation only tool. That’s because the alcohol present after fermentation begins throws off refractometer readings. That was until people starting figuring out that it was possible to reasonably estimate specific gravity after fermentation has begun. The first such calculations [I became aware of] were from MoreBeer. Check the documents tab on this product page to see their spreadsheet.
Since, additional work has been done. One such body of work belongs to Sean Terrill. Sean is a smart guy. Like nuclear engineer kind of smart. Sean did a good bit of work on the subject and came up with an even more accurate formula for estimating specific gravity after fermentation is begun.
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To do this you need: A pre-fermentation refractometer reading, a current refractometer reading and a spreadsheet or online calculator.
Sean has kindly made his findings available to the homebrewing community. You can use his easy to use refractometer calculator or his spreadsheet (available on the calculator page). He also makes it available for others to use with proper citation. For example, my Brewing Spreadsheets [Brewing Spreadsheet | Small Batch BIAB Spreadsheet | BIAB Brew Day Spreadsheet] all contain Sean’s formula. Thank you to Sean Terrill for your work on this subject and for making this available to homebrewers!
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