If you want to know how much water to add to dilute your alcohol
down, just multiply the amount of spirits you have by (strong/weak) - 1
eg: if you want to dilute 2L of 75% alcohol down to 40%, you
will need to add 2 x ((75/40)-1) = 1.75L of water
If you want to know how much alcohol to use to make a known quantity,
multiply the final amount by (weak/strong)
eg: to make 1.125L of 40% alcohol using spirit at 75%, you will need
to use 1.125 x (40/75) = 0.6L of the 75% spirit, then top it up to 1.125L
Proof or Abv ?
Note that there are a couple of different ways of describing the alcohol strength. Some people tend to talk about "Proof" whereas others (myself included) tend to stick to "ABV" - the amount of Alcohol By Volume. Basically 100 proof = 50% abv.
But its not always so ... Harry explains ...
In the US a very simple relationship is defined between 'proof' and %
AbV. It is that proof = 2 × %AbV
So, 180 proof (US) = 90 %AbV.
It the UK it was laid down by an Act of Parliament in 1816 that "a
quantity of 100 proof liquor would have the same weight as 12/13
ths of the same volume of pure water at 51° F." (That is twelve
100 proof (UK) = 57.06 %AbV
200 proof (US) = 100 %AbV = 175.25 proof (UK)
100 proof (US) = 50 %AbV = 87.6 proof (UK)
To avoid confusion, most alcohol for export from the UK is labelled
A bit of history...
What is Proof?
Proof is another (older) measure of the strength of an alcoholic
liquid. It had its origins in days when a simple test was needed
that the liquor did indeed contain a *correct* measure (or more) of
alcohol. And it was indeed a simple test.
Some of the liquor was poured over a little gunpowder and ignited.
If the alcohol content was adequate, then it would burn 'just right'
with a steady blue flame and eventually ignite the gunpowder. If
there was insufficient alcohol then it would fizzle out and the
gunpowder would be too wet to burn. The 'just right'
condition 'proved' the liquor and it was declared to be '100 proof'.
This simple test was clearly cumbersome to perform and was later
replaced by using a specially graduated hydrometer to measure the
specific gravity. This was far more objective and allowed precise
statements to be made as to how much different it was from being
100 proof. This gave rise to "under-proof" and "over-proof"
Keep your powder dry (or wet it with the right stuff!)
At one time (in the days of sailing ships, cannons and gunpowder)
the makers of Plymouth Gin distilled a special gin for the Royal
Navy. It was 57%AbV or 100 proof. Why?
In order to keep it secure it was stored in the magazine close by
the gunpowder. So, even if it leaked and wetted the gunpowder, at
100% proof the gunpowder would still explode.
Though that need has been long gone, they still market the stuff!