Unit of measurement for the volume fraction of a substance in the total volume of a mixture. Mixing 100 litres of ethanol with 100 litres of water gives 192.75 litres of mixture; this phenomenon is called volume contraction. Since this 192.75 litre mixture contains 100 litres of alcohol, 100 litres of mixture contains 51.8 litres of alcohol. Thus the alcohol content of the mixture is 51.8 percent by volume. Another unit of measurement is the percentage by mass (weight percent). The 100 litres of water weigh 100 kg, but the 100 litres of alcohol weigh only 79.45 kg. The weight of the mixture is therefore 179.45 kg, the alcohol content is 44.2 percent by weight (instead of 51.8 percent by volume). The alcohol thus has a low relative density.