Solving for unknown F.G. - algebra

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85 Haro Designs

Well-Known Member
Let's get right to the chase......

If I know my O.G. and I know my approximated final ABV% - I should be able to solve for the unknown - which in this case is the F.G.

You see, I don't have a recipe for this batch but I was able to calculate the approximate final ABV by adding up the ingredients by fermentable sugar values.

Correct?

PS - this is a franken-brew made with the remaining stuff I had from my Mr. Beer setup.

cronxitawney

Well-Known Member
I am not sure that you can calculate ABV by adding the ingredient fermentable sugar values. Assuming your beer had finished fermenting, you can take the attenuation range of your yeast and your OG and make a guess at at your FG. This will certainly be a guess though, because the range can be big (Ex. 65%-75%)

Yooper

Ale's What Cures You!
Staff member
Mod
Well, if you can guess at your attenuation, you can make a guess at your fg. What kind of yeast did you use? Usually for Nottingham, for example, you can use 75% as a guideline. So, your fg would be 25% of your og.

But as cronxitawney said, some yeasts have a large range of attenuation, so you still would only have a best guess.

brewt00l

Well-Known Member
cronxitawney said:
Assuming your beer had finished fermenting, you can take the attenuation range of your yeast and your OG and make a guess at at your FG. This will certainly be a guess though, because the range can be big (Ex. 65%-75%)
yup...and you're still just taking a somewhat wild guess since it's all based on the assumption that you had a complete fermentation and hit somewhere that attenuation range.

A simple hyro measurement would obviously eliminate the need to calculate or guess at your FG. If you are an extract brewer, you can pretty easily calc your OG and that way, ABV can be found by just one measurement if that's all you have.

OP
8

85 Haro Designs

Well-Known Member
I'm trying to use the remainder of my Mr. Beer extracts and Mr. Beer doesn't provide you with SG readings on their recipes.

So, Here's the thing. My OG was 1.046 and I can GUESS my final ABV at 5.05% based on the ingredients used and the handy-dandy Mr. Beer chart that shows what each ingredient adds to the total ABV.

When I add them all up - I get 5.05% ABV - expected.

Follow me here. If I know my OG and I "know" my ABV% can't I solve for the unknown FG.

I'm going to take a FG reading but I'm trying to find out what that reading is supposed to be. It's like a viscious cycle I'm stuck in. Help, Help!

BTW - I used two packets of Mr. Beer's yeast, I know, I know - please commence flaming me, I deserve it!

FatMonsters

Well-Known Member
How did you know your OG was 1.046, did you measure this and if so how? If you did, use it to measure your FG.

The equation is (OG - FG) * 131 = ABV so yes you could back calculate given your numbers. But I'm going to go out on a limb here, as I did not own a Mr. Beer Kit, but the Handy-Dandy Chart was an approximation by Mr Beer to estimate your ABV.

ABV can only be estimated until you know your readings (ie - gravity or balling) since the readings depend on, as everyone has already talked about, the attenuation of yeast as well as unfermentables content.

Dude

Well-Known Member
Sorry to be a dick, but who gives a rip? Assume it is between a certain guideline, and DRINK it.

It isn't like the FDA is going to inspect it. It isn't like oyu are selling it. What's a few percentage points of ABV?

DRINK IT.

Donasay

Well-Known Member
You don't know your abv% without your FG, unless you send it off to the lab. Essentially by not taking the reading and adding up the fermentalbes you are guessing at the abv% and because you are guessing at your abv% you are mathematically figuring out your FG based on a guess.

It is possible to do the math, but why bother when you are guessing at one of the values which you use to calculate your other variable, the answer will be meaningless and not an accurate reflection of reality. As they say garbage in garbage out.

To paraphrase the statistician Tukey "an approximate answer to the correct question is worth a lot more than an exact answer to an approximate question"

david_42

Well-Known Member
Hurray for New Math!

If ABV = (OG-FG) * .131, then OG - ABV/.131 = FG. In this case, 46 - 5.05/.131 = 7.4

But, 85% attenuation isn't likely, unless half your fermentables are pure sugar.