Alcohol Strength and Density
Alcohol strength is stated either by mass or by volume: If the strength of a mixture of alcohol and water is is stated as 80% by mass, it means that for every 100g of mixture, 80g is alcohol. If the strength of a mixture of alcohol and water is is stated as 80% by volume, it means that for every 100cm3 of mixture, 80cm3 is alcohol.
If you see an alcohol strength stated as, for example, 94% v/v, this means its strength is 94% by volume, correspondingly m/m means by mass.
The usual way of finding the strength of a mixture of alcohol and water is to carefully measure the density and temperature, and then to consult tables to lookup the strength either by mass or by volume. Density is measured by means of a hydrometer.
You can use the form below to perform these calculations online
Katmar Software has a program to do similar calculations available for download