BackyardBrewer wrote:I use wolfram alpha's website and this equation:
http://wolframalpha.com/1*((95/40)-1)=
Just adjust the first 1 for the amount of pure spirit and the 95 for the reported abv as your alcometer shows it.
Post if you need help. Next we'll talk about temp correcting for extra accuracy:-)
I must point out that the formula above is incorrect. It has too many brackets. The calculation will work out if you have 1 litre or 1 gallon but won't work for any other volume.
Should read 1*(95/40)-1=
To explain the maths slightly differently,
ABV of your uncut spirit (say 95%) divided by ABV you require (say 40%) ie 95/40
Multiply this by the volume you currently have say (5 litres)
Now we have 5 * 95/40 = 11.875 litres which will be your final volume of 40%
All you do is add water until the final volume comes up to 11.875 litres (everything at 20 deg C)
Sorry to crap on a bit, but you might think the amount of water to add is
11.875 - 5 = 6.875 litres, that is probably good enough but the real answer is 7.21 litres to add because when adding pure alcohol to water the total volume shrinks. The maths for that is very complex.
Many home distillers work out how much to add, ie 5 * (95/40) - 5 = 6.875 which is probably good enough, but there is more maths and less accuracy.
ps if you leave the extra brackets the apparent answer is
minus 13.125