Contraction means that mixtures of liquids have less volume than the sum of the separate liquids.
For example, if you mix one liter of alcohol with one liter of water, you will not get two liters of diluted alcohol, but a little less. Here is a diagram for this purpose:
"% Contraction" indicates here the ratio by which the volume of the mixture is less than that of the separate pure substances.
So, for example, if you mix 40ml of pure alcohol with 60ml of water and instead of 100ml you get only 97ml, the contraction is (100 - 97) / 100 = 3%.
A practical example: 1 liter 80%abv + 1 liter water does not give 2 liter 40%abv but 1.96 liter 40.8%abv.
So the contraction has quite noticeable effects in practice.
Dilution calculators or similar applications should therefore be able to take the contraction into account. Our calculators do this by calculating not in %abv but in %abw (alcohol by weight), and not in volume but in mass. To convert from entered liters and %abv to kg and %abw, one must be able to calculate the density of the mixture. The most accurate calculation for this, at least for the time being, is offered by the following pdf: International Alcoholmetric Tables
There, measured values of different alcohol strengths at different temperatures from numerous sources were summarized, brought into a complicated formula and then tables were created from this formula. The temperature dependence of the density of mixtures up to 40°C can also be calculated with the formula from this pdf.
By the way, increasing contraction causes heating. Therefore you should measure the temperature after each mixing or diluting, before you measure the reached alcohol content.
For example, if you mix one liter of alcohol with one liter of water, you will not get two liters of diluted alcohol, but a little less. Here is a diagram for this purpose:
A practical example: 1 liter 80%abv + 1 liter water does not give 2 liter 40%abv but 1.96 liter 40.8%abv.
So the contraction has quite noticeable effects in practice.
Dilution calculators or similar applications should therefore be able to take the contraction into account. Our calculators do this by calculating not in %abv but in %abw (alcohol by weight), and not in volume but in mass. To convert from entered liters and %abv to kg and %abw, one must be able to calculate the density of the mixture. The most accurate calculation for this, at least for the time being, is offered by the following pdf: International Alcoholmetric Tables
There, measured values of different alcohol strengths at different temperatures from numerous sources were summarized, brought into a complicated formula and then tables were created from this formula. The temperature dependence of the density of mixtures up to 40°C can also be calculated with the formula from this pdf.
By the way, increasing contraction causes heating. Therefore you should measure the temperature after each mixing or diluting, before you measure the reached alcohol content.