How to figure FG using OG and attenuation

shaggynuts24 · 12-11-2007, 01:04 PM

How do you figure FG from the OG assuming you expect 75% attenuation?

Funkenjaeger · 12-11-2007, 01:30 PM

take the decimal part of the OG (ie - (OG-1)) and multiply by (1-attenuation), and then add 1.

example:
1.080 and 75% attenuation would be (1.080-1)*(1-0.75) + 1 = .080*.25 + 1 = 0.020 + 1 = 1.020

Chad · 12-11-2007, 02:02 PM

It's always easier when dealing with gravities to use "gravity points." That's the final two numbers of the hydrometer reading. A 1.052 OG would have 52 gravity points.

In simplest terms, subtract the attenuation percentage from the OG: 52 minus 75% = 13.

To find the attenuation percentage, just multiply the OG (in gravity points) times the expected percentage: 52 x .75 = 39, then just subtract the attenuated portion: 52 minus 39 = 13 or a final gravity of 1.013.

To find your apparent attenuation from your OG and FG numbers subtract the FG from the OG to get the amount attenuated, then divide that number by the OG. So if your OG was 1.052 and your Final Gravity was 1.013: 52 minus 13 = 39. 39 ÷ 52 = 75 or 75% apparent attenuation. A final gravity of 1.015 would be 71% attenuation (52 - 15 = 37; 37 ÷ 52 = 71)

Chad

Funkenjaeger · 12-11-2007, 02:08 PM

Quote:

Originally Posted by Chad

It's always easier when dealing with gravities to use "gravity points." That's the final two numbers of the hydrometer reading. A 1.052 OG would have 52 gravity points.

Technically in the more general case, you should be taking all 3 numbers after the decimal, for the cases where your OG is over 1.100 - however, I would assume that anyone who dares brew a beer that big would likely already be experienced enough to realize that

shaggynuts24 · 12-11-2007, 02:44 PM

i guess i was not experienced enough to know that

114 * .75 = 85.5

114 - 85.5 = 28.5

1.0285 should be my expected final gravity?

.114 * .25 + 1 = 1.0285

kk --- i get it ... thanks guys

Funkenjaeger · 12-11-2007, 03:45 PM

That'd be a rough guideline, and probably fine for your regular, run-of-the-mill, medium or low-gravity beers. However, attenuation is not fixed - it is going to depend on your wort to a degree, not just your yeast. If your wort has an unusually high or low level of unfermentables, your attenuation is likely to change as a result. I think it is probably also more up-in-the-air when you're talking about such a high-gravity beer, where the yeast is under a lot more stress.

shaggynuts24 · 12-11-2007, 04:15 PM

i am doing my best to keep the stress levels down (rousing and letting the CO2 escape)

unfermentable sugars will probably be my drawback