Friday, July 27, 2007

Gravity Math

Since the equations (really just a couple numbers) aren't ingrained in my head, this is a quick place to go to for recalling ABW and ABV mathematics using a beer's gravity readings.

Original Gravity (how much dissolved sugar is in the wort)
Final Gravity (the specific gravity after fermentation)

Alcohol by Weight (percentage weight of alcohol per volume of beer, approx. 20% less than ABV)
Calculate ABW
(subtract FG from OG and multiply by 105)
ABW example:
(1.054 - 1.013) x 105 = 4.31%

Alcohol by Volume (Alcohol content as a proportion of the total volume of a liquid)
Calculate ABV (multiply ABW by 1.25)
ABV example:
4.31 x 1.25 = 5.38%

But now after working it out here, I think I'll remember next time. If not...its right here.


David said...

Good to know this is on your site, for reference. But when are the optimal times to take the OG and FG readings?

Ted Danyluk said...

David, thanks for asking a great question.

The OG is taken after the boil is complete and before fermentation begins. Most easily your sample volume (for me about 1/4cup)can be drawn with a measuring cup at the end of the boil.

The FG is taken when fermentation is finished. A gravity sample can be drawn when racking to a secondary fermenter, and this will give you a good idea where it is "finishing." Secondary fermenting could take the gravity even lower so its good to take a reading right before adding priming sugar at bottling time. This will give you the beer's true FG.

The biggest influencing factor is temperature. The hydrometer gives an accurate reading at 60*F, and there are temperature corrections (written on the hydrometer instruction sheet) for every 10 degrees or so. So make sure the temperature of the sample is as close to 60*F, or 50,70,77,84,95,105 and simply add the correction points.

Another factor influencing the FG is alcohol and carbon dioxide. There is a way to record the volume of the sample, then boil off the alcohol & CO2, then record the volume again, and then do a little easy math, and this will give the most accurate final gravity reading. But most brewers, including myself, don't do this, and simply take the FG reading at the end of fermentation.