Calculating ABV

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level-one

Active Member
Hi, I'm a little confused on how to calculate ABV. I'm using the Brewfather app and it saying the final ABV of my beer is 5.6%. I've also put the same starting and ending gravity readings into two online calculators and they say 5.51%. Why are they different?

GilSwillBasementBrews

Supporting Member
HBT Supporter
I would assume just how they round the values of the gravity points of the different malts.

BrewnWKopperKat

.
Why are they different?
Perhaps each software uses a slightly different technique for estimating ABV.

IslandLizard

Progressive Brewing
Staff member
Mod
HBT Supporter
What they said^
And go with the higher value if alcohol is important to you.

Broken Crow

Ale's what cures 'ya
Forgive me if this is naive, (I don't use software) doesn't (FG – OG) X 131.25 = ABV% work for you?

GilSwillBasementBrews

Supporting Member
HBT Supporter
I think you meant (OG-FG)x131.25

Broken Crow

Ale's what cures 'ya
I think you meant (OG-FG)x131.25
Thanks! I keep getting things backwards ..think I'll go upstairs to my basement for a smoke.

Supporting Member
HBT Supporter
There was a good article in Zymurgy in the last couple years looking at ABV formulas. (ZYMURGY: JULY/AUGUST 2019 / How Much Alcohol is In Your Homebrew?) Looking at the article, the factor between formulas varies from 131 and 135. Based on analysis comparing measured ABV to calculated ABV, the author concludes that the factor 132.6 came very close to the measured ABV.

bracconiere

Jolly Alcoholic
HBT Supporter
And go with the higher value if alcohol is important to you.

that would depend on why it's important to you!

bracconiere

Jolly Alcoholic
HBT Supporter
(sorry just saw this was in begginers forum....)

but here it is simply....

10 mL × 0.790 g/cm^3 = 7.9 g of ethanol

density of sugar is 1.37178

but yeah i'd call the difference between 5.51% and 5.6% nominal....

just like i use the trick of multiplying or dividing by 0.8 ABV to get ABW....not spot on but close enough....

and i need to know how many grams i'm getting not Volume....

Last edited:

Toxxyc

New and loving it
Keep in mind that anything under 0.5% ABV isn't even strong enough to get you drunk, no matter how much you drink of it, so worrying about a difference between 5.51% ABV and 5.6% ABV on a homebrew level, for me, is just a waste of time. I typically work on the rounded value, so in that case I'd probably just say "5.6% ABV" to keep it easy.

BrewnWKopperKat

.
OG divided by 10 is pretty close to ABV (assuming 75% attenuation) for normal strength beers.

eta: i used 131.

patto1ro

Well-Known Member
This is what I use:

=((((OG-FG)*(125*1.05))+((OG-FG)*(1000/7.5)))/2)/1000

Can't for the life of me remember where I got it from. Some brewing text book or other.

BrewnWKopperKat

.
[1] =((((OG-FG)*(125*1.05))+((OG-FG)*(1000/7.5)))/2)/1000
• 125*1.05 = 131.25
• 1000/7.5 = 133.33

• ((OG-FG)*(125*1.05)) ==> (OG-FG) * 131.25
• ((OG-FG)*(1000/7.5)) ==> (OG-FG) * 133.33
• Average the different estimates, divide by 1000
[2] = (OG-FG) * 0.1323 should give the same result to about 2 decimal places.

ScrewyBrewer

ezRecipe - The Easy Way To Awesome Beer!
HBT Supporter
There are two widely used formulas today for calculating FG, as far as I know. A short easy-to-do one by Charlie Papazian and a complex one by Ray Daniels claimed to be more accurate for high gravity beers. The results of which affect either of the ABV calculations.

patto1ro

Well-Known Member
[1] =((((OG-FG)*(125*1.05))+((OG-FG)*(1000/7.5)))/2)/1000
• 125*1.05 = 131.25
• 1000/7.5 = 133.33

• ((OG-FG)*(125*1.05)) ==> (OG-FG) * 131.25
• ((OG-FG)*(1000/7.5)) ==> (OG-FG) * 133.33
• Average the different estimates, divide by 1000
[2] = (OG-FG) * 0.1323 should give the same result to about 2 decimal places.

View attachment 771759
When you put it like that, it doesn't make a significant difference which of the two formulae you use. As I have it programmed into my spreadsheets, it doesn't involve any extra effort using the more complex formula.

Gilbert Spinning Horse

Well-Known Member
I just put my og and fg into beersmith and, hey presto, it tells me the ABV.

BrewnWKopperKat

.
I just put my og and fg into beersmith and, hey presto, it [estimates ABV].
Many ways to enjoy the hobby.

And, like other places where software is used, it may be desireable to understand what the models and software is actually doing to for you.

Gilbert Spinning Horse

Well-Known Member
Every method mentioned is obviously an estimate, I thought that went without saying.

BrewnWKopperKat

.
One would thing so, but periodically people ask why two different brewing apps calculate different values brewing estimation tools provide different results.

Supporting Member
HBT Supporter
I just put my og and fg into beersmith and, hey presto, it tells me the ABV.

Yeah, this is what I generally do as well. The OG and FG readings I get with my hydrometer are only so accurate to start with. It is good to know if a beer is 4% vs 7% vs 12%, but I am not going to worry down to exact tenths.

The Brewer's Friend calculator (Alcohol By Volume ABV Calculator | Brewer's Friend) has a link to this article discussing the Standard and Alternate formulas you can pick. Sometimes I will use that to calculate a gravity for a high ABV beer since the Alternate formula is said to be more accurate for those.

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