I've just found out I've been calculating ABV wrong all these years!

[moto748]

I've always subtracted the bit of the final gravity from original gravity after the decimal point (the mantissa, I remember they used to call it in Maths lessons), and divided by 7.36.

Now I understand I should be using the whole number for the subtraction, and multiplying by 131.25.

Example: OG 1050, FG 1010

Method 1: (50-10)/7.36 = 5.43%

Method 2: (1.050-1.010) x 131.25 = 5.25%

Now I realise neither method will be spot-on. What does the panel think. is the variation in answers "within the margin of error" anyway? What formula do you guys use?

I'm wondering if I should change the sum in my spreadsheet now...

 

[chrisb8]

I always use the brewer's friend calculator and that gives 5.25% for your example. I'm sure the difference is within margin for error anyway and how important the fraction of abv is to you

 

[Slid]

The calculations are of enormous significance if you are paying excise duty and sell your beer.
I know the "Method 1" works better for weaker beers and less so for stronger beers, because that Maths degree comes in handy once in a Blue Moon.
Also I know that the inaccuracies are really down to using a "guessing stick" which is the HB hydrometer and the irritating fact that "an estimate minus an estimate" is just an informed guess.

 

[johncrobinson]

Back when I were a wee lad brewers never quoted %abv just OG and FG, Even spirits were in degrees proof.not abv.

Most hydrometers for homebrew are cheap and nasty.I have seen laboratory grade hydrometers and they are a world apart from what we use.

Our Hydrometers simply cannot calculate down to a few hundredths of a percent.Which is the figures being discussed here.

 

[An Ankoù]

Back when I were a wee lad brewers never quoted %abv just OG and FG, Even spirits were in degrees proof.not abv.

Most hydrometers for homebrew are cheap and nasty.I have seen laboratory grade hydrometers and they are a world apart from what we use.

Our Hydrometers simply cannot calculate down to a few hundredths of a percent.Which is the figures being discussed here.

As I recall, it was just the OG that was quoted. We had no idea of the attenuation and so were unable to calculate the abv, even if we wanted to. The former system made much more sense, to my mind, in the context of cask beer: surely a realy fresh cask is still a bit green and has some unfermented sugars, while towards the end of a "slow to move" cask all that sugar will have fermented and the beer will pour a bit flat, but be stronger for it. How much stronger, I don't know, to be frank. Equally, I don't know how the body responsible for collecting duty on beer can know how much to charge on a cask of still-fermenting ale.

We're not talking about a few hundredths of a percent, but a few tenths. I think, in the context of the above, our hydrometers are plenty accurate enough provided we use them properly.

 

[Northern_Brewer]

The calculations are of enormous significance if you are paying excise duty and sell your beer.

In which case you use the official HMRC equations and look-up table, whereby you use a different factor depending on the strength of the beer. Given that this involves cash to the government, the rules for this kind of thing are laid out in detail :
https://www.gov.uk/government/publi...-notice-226-beer-duty--2#calculation-strength
But also in this case you'd be using professional hydrometers, which can measure gravity far more accurately than homebrew ones - there's no point calculating to within .001 if your input data is nowhere near that accurate.

See eg Homebrew Dad's Alcohol by Volume Calculator for more discussion

 

[DavidDetroit]

Method 2

That's the method I got off the kit sheet--Brewer's Best Double IPA--when I made my first beer. I know it's not perfect because it's less accurate the higher the ABV, if I remember right.
If someone has an exact formula, I'd use it but nothing involving more than a hydrometer. I would spring for a really good hydrometer though.
Edit: I'm fine being pretty accurate.

 

[moto748]

I realise of course that simply reading off a cheap hydrometer and applying the values into some formula is ever going to be approximate at the best of times. I really wondered which method most posters used, or whether there was consensus about one being better than the other.

I know the "Method 1" works better for weaker beers and less so for stronger beers,

but if that's the case, I'll stick with what I've got and won't change my spreadsheet. Thanks!

 

[Griff097]

I have used method 2 as from research that seemed correct.
Never heard method 1 before.

 

[DavidDetroit]

Let me bump the idea--is there a sliding scale that someone knows about for ABV calculations with homebrew?
Again, I'm okay with being close but if I can be a little more accurate on ABV, I'm on board.

 

[JohnB]

John Palmer says:
ABV = 76.08*(og-fg)/(1.775 - og)*(fg/0.794)
This is for anything over about 5% ABV the simple calcs are fine for lower ABV, but this one is a bit more accurate for higher ABV's.

 

[Cwrw666]

I made a table of OGs minus FGs and their respective ABVs which I've got in the front of my brewing book. Never have to do the calculation again. Based it on the OPs second method, except that I ignore the decimal point in the gravity readings and x by 0.131

 

[foxbat]

I've always subtracted the bit of the final gravity from original gravity after the decimal point (the mantissa, I remember they used to call it in Maths lessons), and divided by 7.36.

Now I understand I should be using the whole number for the subtraction, and multiplying by 131.25.

Example: OG 1050, FG 1010

Method 1: (50-10)/7.36 = 5.43%

Method 2: (1.050-1.010) x 131.25 = 5.25%

Now I realise neither method will be spot-on. What does the panel think. is the variation in answers "within the margin of error" anyway? What formula do you guys use?

I'm wondering if I should change the sum in my spreadsheet now...

See the table I just posted in this thread. It contains the mutlipliers that HMRC want commercial breweries to use.

 

[moto748]

Thanks foxbat, that seems just about the last word on the subject!

 

[NotSure]

The HMRC formula using the multipliers doesn't seem to be consistent. I just plugged-in some number of beers I made and some come out with higher ABV, some lower. "F" looks like it may be a fudge factor to fit observation, but I could be mistaken. However, accurate or not, if you have to play by HMRC rules, you use it.

As far as I am aware, the most accurate formula for %ABV is from: Hall, Dr. Michael. “Brew By The Numbers – Add Up What’s In Your Beer.” Zymurgy, Summer 1995: 54-61.

This is the formula that Homebrew Dad uses, and it's also the formula I use on my website:
Calculator for Alcohol By Volume (ABV) and Attenuation (brewingcalculators.com)

Although Brewer's Friend uses the main formula from the Hall paper, they use a simplified conversion of extract to gravity which gives an inflated result.

 

[CD]

Back when I were a wee lad brewers never quoted %abv just OG and FG, Even spirits were in degrees proof.not abv.

Most hydrometers for homebrew are cheap and nasty.I have seen laboratory grade hydrometers and they are a world apart from what we use.

Our Hydrometers simply cannot calculate down to a few hundredths of a percent.Which is the figures being discussed here.

When I were a lad Brewers kept the strength of their beers a secret, and the existence of the OG only came to peoples notice thanks to Camra.

 

[The-Engineer-That-Brews]

But also in this case you'd be using professional hydrometers, which can measure gravity far more accurately than homebrew ones

The way I can tell, with perfect accuracy, is how many times I have to correct my spelling when trying to write a coherent post. It needs to be some measure like 'percentage of mistyped characters vs. correctly typed characters' with the effect of autocorrect removed