When alcohol comes out of a still, it is usually fairly strong. Far too strong, in fact, to drink. The strength is also variable, depending on what you are distilling, what sort of still you use and how far into the distilling process you are. Therefore diluting (or "cutting") alcohol down to a consistent strength suitable for drinking is a standard part of producing distilled spirits.

Well, that should be simple. Just add some water, mulitplying and diviing to calculate the final volume and strength. Add one volume of water to one volume of alcohol and you'll end up with twice the volume at half the strength. Right?

Wrong. Try it: add one litre of alcohol at, say, 96% strength by volume to one litre of pure water. You'd expect to end up with two litres of 48% alcohol by volume. But you don't. When you measure carefully, you will note you now have less than two litres (almost 80ml. less) and the alcohol strength of the mixture is closer to 50% by volume!

So what's going on here?

One plus one does not always equal two

Simply put, mixing alcohol and water is a lot like mixing concrete: you start with a certain amount of stones and a certain amount of sand and cement, but when you mix them, the sand and cement fill up the gaps between the stones. With water and alcohol something similar happens. This has to do with the way alcohol and water molecules interact. The physics are complicated and involve terms like hydrogen bonds and the polar properties of hydroxyl groups, so let's forget about all that. Instead, think of it this way:

Imagine you have a jar filled with stones. Then add the same volume sand to the jar, shaking it to mix the two. Soon the sand begins to fill up the empty spaces left between the stones, so that the total volume of the sand/stone mixture ends up being less than the sum of the separate volumes you started with. While this is a gross (and not entirely accurate) simplification, it does illustrate how water and alcohol molecules behave somewhat in the same way.

Incidentally, you may also notice that mixing alcohol with water increases the temperature of the mixture somewhat. This is because the density of the alcohol increases, and just like air gets hot when it is compressed (i.e. when it has its volume reduced) the alcohol/water mixture warms up when its volume is reduced during the mixing process.

Estimating volume and strength after dilution

Unfortunately this does complicate things somewhat for the distiller. Since the changes in volume and alcohol strength are not linear, we can't simply multiply or divide to arrive at the actual figures. While it is possible to calculate them, this requires complex mathematics and some familiarity with molecular physics, neither of which appeal very much to the average distiller.

Fortunately there are easier ways to deal with this.

Since 100% water and 100% alcohol both have their full volume, it is not surprising that the greatest reduction in volume occurs when the two are blended more or less in a 50/50 ratio. At this point the maximum reduction in volume is just under 4%. In other words, if you mix 500ml.  of pure alcohol with 500 ml. of water, you would end up with about 960ml. of mixture. The more you move away from a 50/50 mixing ratio, either adding more water or more alcohol, the lower the reduction in volume becomes. Therefore most distillers estimate their volume reduction on the basis of how much water they plan to add to their alcohol, but the loss in volume will always be between 0 and 4%.

Predicting the exact alcohol strength after dilution is a little more complicated. You could of course add water to alcohol bit by bit until your alcoholometer indicates the desired strength of the mixture, but that is a bit of a pain. Fortunately, a lot of alcohol has been diluted with water since distilling was first invented, and a a lot of notes have been taken. Tables have been drawn up, and distillers routinely refer to these tables in order to arrive at the right blending ratio, depending on the strength of the alcohol coming out of the still and the desired final strength of the mixture.

So simply refer to the table below the next time you dilute your alcohol! It may save you some hassle.