Maths idiot in stupid alcohol content question shocker

[Luther]

Hello,

A quick question from a mathsophobe.

On me Coopers instructions, it suggests OG minus FG divided by 7.46 = alcohol content + add a bit for secondary fermentation.

So... can someone confirm or deny that what I deduce from that is that you get 1% alcohol from every 7.46 drop in gravity?

Therefore a drop of 30 points of gravity = a 4% ale. Is that a reasonable rule of thumb or am I a dunce?

Wahey! We've had a new central heating boiler installed - the airing cupboard is now a clement 23 degrees and my Coopers IPA is frothing like mad! Mind you, a brewbelt would have been cheaper...

Cheers

[Jim]

There's a variety of figures that I've seen used as the constant in that formula. A recent thread on here mentioned that HMRC use 7.75 (except they multiply by the reciprocal of that figure). So that makes the formula:

ABV = (OG-FG)/7.75

I use 8.06 on my recipe formulation page, but I'm going to change it to the above (in the fulness of time ).

[Luther]

(except they multiply by the reciprocal of that figure).

Ahem... er... exactly. Fancy multiplying by a reciprocal... the idiots.

I think I followed that, thanks!

[edit1now]

HMRC have a range of fudge factors - I think Aleman put them up on JBK, but here they are again:

Value of factor 'f' for various alcoholic strengths

(OG - PG) % ABV Factor

Up to 6.9 Up to 0.8 0.125
7.0 - 10.4 0.8 - 1.3 0.126
10.5 - 17.2 1.3 - 2.1 0.127
17.3 - 26.1 2.2 - 3.3 0.128
26.2 - 36.0 3.3 - 4.6 0.129
36.1 - 46.5 4.6 - 6.0 0.13
46.6 - 57.1 6.0 - 7.5 0.131
57.2 - 67.9 7.5 - 9.0 0.132
68.0 - 78.8 9.0 - 10.5 0.133
78.9 - 89.7 10.5 - 12.0 0.134
89.8 - 100.7 12.0 - 13.6 0.135

Sorry about the formatting!

All you need to do is take your Present Gravity (PG) away from your OG (Original Gravity), say 1011 away from 1044, which gives 33.
Look up 33 in the list, and it falls between 26.2 and 36.0, so you use the fudge factor of 0.129.
Multiply 33 times 0.129 and you get 4.257% ABV. Put the figures into a spreadsheet - it's easier!

The figures come from the HMRC website, and the fudge factors allow brewers (and inspectors) to assess ABV without lugging some huge piece of kit around.

[PaulR]

Hope noone minds, but I was bored at work and tweaked the table formatting (also rounded up the ABV column).

Code: Select all

(OG - PG)       Multiplication     Potential
                Factor             ABV

Up to 6.9       0.125              Up to 0.9%
 7.0 - 10.4      0.126              0.9% - 1.3%
10.5 - 17.2     0.127              1.3% - 2.2%
17.3 - 26.1     0.128              2.2% - 3.3%
26.2 - 36.0     0.129              3.4% - 4.6%
36.1 - 46.5     0.130              4.7% - 6%
46.6 - 57.1     0.131              6.1% - 7.5%
57.2 - 67.9     0.132              7.6% - 9%
68.0 - 78.8     0.133              9% - 10.5%
78.9 - 89.7     0.134              10.6% - 12%
89.8 - 100.7    0.135              12.1% - 13.6%

[Jim]

Nice one. Edit1now, do you mind if I use that data when I update my page?

EDIT: Duh, seeing as it came from HMRC, I'll take it as read I can.

[edit1now]

I got it from somewhere on JBK