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Author Topic: ABV calculator based on volume?  (Read 2509 times)

Offline denny

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Re: ABV calculator based on volume?
« Reply #15 on: August 25, 2019, 12:56:59 pm »
   One easy way is to multiply the OG by the gallons of wort/beer and divide by the number of gallons of diluted wort/beer. For example 5 G of 1.065 wort diluted with 1 G of H2O would be 65x5=325 gravity points divided by 6G = 54 or 1.054. Measure the FG after dilution and calculate your ABV from the adjusted OG [1.054]. Hope that didn't muddy the water.

Not muddy, I understand that.  But I didnt actually add water to the beer/wort, not sure if I should recalc or not?  Its just that I ended up with 6 gallons because I started with 8g of RO water at mash in, which ended up at 5.75g post boil vol, and just before bottling I added a 2000ml starter (for reyeasting/bottle conditioning) which made the whole volume just a tad under 6gallons. 

If I recalc as above, for 6 gallons, then my abv drops from 10.5% (OG1.086, FG1.006)  to 7.8%.  Which is the truer abv?

Whichever one comes out to (OG-FG)*.132, which makes it 10.56%.  Volume plays no part in ABV.  You seem to be getting it confused with efficiency.
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Offline jverduin

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Re: ABV calculator based on volume?
« Reply #16 on: August 25, 2019, 01:07:13 pm »
   One easy way is to multiply the OG by the gallons of wort/beer and divide by the number of gallons of diluted wort/beer. For example 5 G of 1.065 wort diluted with 1 G of H2O would be 65x5=325 gravity points divided by 6G = 54 or 1.054. Measure the FG after dilution and calculate your ABV from the adjusted OG [1.054]. Hope that didn't muddy the water.

Not muddy, I understand that.  But I didnt actually add water to the beer/wort, not sure if I should recalc or not?  Its just that I ended up with 6 gallons because I started with 8g of RO water at mash in, which ended up at 5.75g post boil vol, and just before bottling I added a 2000ml starter (for reyeasting/bottle conditioning) which made the whole volume just a tad under 6gallons. 

If I recalc as above, for 6 gallons, then my abv drops from 10.5% (OG1.086, FG1.006)  to 7.8%.  Which is the truer abv?

You can use a basic template for this calculation and just change numbers on future batches. If your added starter was exactly the same original gravity as your beer, you could just assume no change in ABV, but if it was different, you could do this (it is going to be an estimate because who knows exactly how much the starter ferments out - but it is the smaller volume, so estimate is good enough):

New original gravity = (OG of original beer 86 x volume or 5.75 gallons) + (OG of starter - let's assume 1.050 or just 50 x volume of starter 0.5 gallons) / New volume or 6.25

(86 x 5.75) + (50 x .5 = 25) = 494.5 + 25 = 519.5 and then divide that by your new volume which is 6.25 (assuming you added the full starter)
You can plug this revised formula from above directly into Google for it to calc: ((86 x 5.75) + (50 x .5)) / 6.25


OG = 1.083 - this represents the new OG of your beer + starter - again, assuming you pitched a 1.050 starter - if it was lower, replace the 50 in the formula above

New final gravity = (FG original x 5.75) + (FG of starter - guessing 1.008 x 0.5) / 6.25
((6 x 5.75) + (8 x 0.5)) / 6.25
New FG = 1.006

OG 1.083 and FG 1.006 = new ABV of: 10.11% ABV (according to Brewer's Friend)

...and for what it is worth, that is a really, really low final gravity for a beer starting above 1.080.

So... quick recap, you just need to determine what the theoretical OG is when you add two liquids together ((Original Liquid Gravity x Volume) + (New Liquid x Volume)) / New Volume - and then do the same for the FG of the combined liquid. This isn't exact, but it will come close enough!




Offline KCguy

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Re: ABV calculator based on volume?
« Reply #17 on: August 25, 2019, 02:15:56 pm »
holy crap!!  Thanks Jverduin!   That totally makes sense, and I never thought about including the starter gravity in the equation. 

Yeah, I was incredulous at my final gravity, I guess Im doing something right with my pitching...or temps...or everything.  Ive never quite had much of a problem getting nice low FG's under 1.010, no matter the styles...not bragging, but I am a bit proud and happy of that aspect of my brewing. 
Drinkin Tripels, Seeing Double, Feelin Single...

Michael B
Kansas City

Offline jverduin

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Re: ABV calculator based on volume?
« Reply #18 on: August 25, 2019, 02:28:47 pm »
Happy to be helpful.

Offline Robert

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Re: ABV calculator based on volume?
« Reply #19 on: August 25, 2019, 02:40:30 pm »
One thing to note, those "calculations" based on "OG - FG = ABV" are only loose approximations, and get increasingly inaccurate as gravity goes up.  The relationship is not linear;  OG 10.050 and FG 1.010 does not result in the same ABV as OG 1.060 and FG 1.020, even though the change in gravity is the same.   Really alcohol can only be measured, but pretty accurate estimates can be made with a complex cubic equation.   If you're brewing big beers (or any beers really) and want a reasonable idea of your alcohol content, it would be easiest to use a calculator.  The online calculator at Brewer's Friend, if you specify the "alternate" formula, gives you as accurate a figure as you can get,  and does all the work for you.
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Offline Bob357

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Re: ABV calculator based on volume?
« Reply #20 on: August 25, 2019, 06:26:55 pm »
Ya know, capiche is a word the kids just don't use enough.  Thanks, Bob.  Now everything is copacetic, another word we need more of.
[/quote]

Ya mean everything ain't gnarly or rad? Man, that sucks :)

You're right, but most of them couldn't pronounce either word, or even know what they mean.
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Offline KCguy

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Re: ABV calculator based on volume?
« Reply #21 on: August 25, 2019, 06:44:04 pm »
One thing to note, those "calculations" based on "OG - FG = ABV" are only loose approximations, and get increasingly inaccurate as gravity goes up.  The relationship is not linear;  OG 10.050 and FG 1.010 does not result in the same ABV as OG 1.060 and FG 1.020, even though the change in gravity is the same.   Really alcohol can only be measured, but pretty accurate estimates can be made with a complex cubic equation.   If you're brewing big beers (or any beers really) and want a reasonable idea of your alcohol content, it would be easiest to use a calculator.  The online calculator at Brewer's Friend, if you specify the "alternate" formula, gives you as accurate a figure as you can get,  and does all the work for you.

Yeah, an approximate abv figure is fine with me, it just looked like it might deviate by 3% points when I changed it from 5 to 6 gallons.  I wasnt sure...
Drinkin Tripels, Seeing Double, Feelin Single...

Michael B
Kansas City