How to calculate OG if you only have FG

[andy77]

I just figured this out and wanted to share.

Like many, I use the morebeer spreadsheet to calculate OG and FG using only refractometer measurements. http://morebeer.com/public/beer/refractbeer.xls The spreadsheet depends on knowing the refractometer reading for both OG and FG, and makes the calculation for you.

In this case you've got three values: the refract FG value, the hydrometer FG value, and an unknown OG. All three values are part of the same equation, so you know two of the three variables, and can figure out the third.

To calculate the OG based on the final gravity measurement:

-Take both reftactometer and hydrometer FG measurements.
-Open refractometer calculation spreadsheet.
-Enter the refractometer value in the FG cell.
-Adjust the OG value until the converted final gravity that reads in gravity points matches your hydrometer reading.
-There's your OG.

Cool, huh?



Not sure when I'd use it, but I suppose this could come in handy in case of emergency...or drunkenness.

[ajdelange]

Not sure when I'd use it, but I suppose this could come in handy in case of emergency...or drunkenness.

This concept is actually used fairly frequently by large scale brewers. Think about whether you really know your OG. You measure it in the fermenter and then dump in a starter. What are the effects of the starters sugars and alcohol on the OG of your beer? During fermentation water evaporates? What is the effect of that on the effective OG? While most homebrewers take the OG and the FG and then use the Balling equation to calculate the alcohol content professional brewers tend to turn the problem around and use the FG and alcohol content to calculate the OG. Alcohol content is a PITA to measure because it usually requires distillation or gas chromatography but if you calibrate for the type of beer you are analyzing (by distillation or GC) you can get a good estimate of alcohol content from FG and a refractometer reading. Modern beer analysis instrumentation e.g. Anton Paar's combination of density meter and alcohol content meter (based on NIR absorption) can give FG and alcohol and then calculate OG from that.