View Full Version : SG during fermentation with a refractometer

10-19-2005, 04:28 PM
During fermentation, Promash will take the current refractometer reading and the original gravity of your wort, and tell you what your current SG is.

Does anyone know the formula it uses to do it? I want to put it into a spreadsheet.

10-19-2005, 04:43 PM
If you are asking how to calculate specific gravity from degrees plato:

SG=((Degrees plato)/(258.6-((degree plato/258.2)x227.1))+1

Source: MBAA T.W. 24(4):129 1987

10-19-2005, 05:14 PM
No, I already know that formula. What I'm asking is how to correct a refractometer reading that I do during fermentation, since the alcolhol screws up the refractometer.

I normally use ProMash's utility to do it, but like I say, I'd like to know the formula as well.

10-23-2005, 11:14 PM
Try sending an email to the guy who made promash.


10-24-2005, 01:52 PM
try here

11-18-2007, 11:43 AM
Resurrecting an old thread here, but i just came across an old New Brewer article (Vol. 17 NO.4 July/August 2000) and found a formula for calculating the gravity of fermenting beer using a refractometer reading.


It seems pretty simple, but i cannot get it to work.
I have a pale in fermentation right now, the Refractometer reading is 6.5Brix with an OG of 14P so:
9.945 - 8.26 = 1.685P

My hydrometer gives me a reading of 3.6P

Is there a math wiz out there that can show me the err of my ways?

11-18-2007, 08:21 PM
I think the issue is that the formula is a derived linear regression formula. In other words, it's more complicated than that. Promash and similar software I think can do the calculation. Otherwise, you might find this spreadsheet handy. You put in the original extract and it does the rest. Then as you're fermenting, you can look at the brix reading and figure out the rest by looking at the chart.

Here it is: http://www.bitboxsoftware.com/files/Gravity%20Chart.xls