Sitemap > Maths > Rule of three Rule of three The rule of three to calculate the corresponding values of many sizes proportionally. In the form below, you just have to enter the three values that you know and you will get the fourth. The result is to 2 what 3 is to 1 1 -> <- 2 3 -> <- Result Examples of applications You draw the plans of your future house and you take as a starting point an existing plan but wish to increase it or reduce it, while keeping the proportions. In digital photography, you wish redimensionner an image of which you know initial dimensions, for example: 3840 points length by 2880 points height, in a smaller image of 800 points length. Which has to be its height? Rule of three Do you know the resolution of your monitor, for example 1280 X 800, and do you want to know if it is one 4/3, 16/10ème or 16/9ème? In kitchen, you know the quantities of the ingredients of a receipt but you wish to make some twice more or twice less. In do the building, when it is a question of making a batch, you know the proportions of sand, cement and water, but which quantity necessary is each ingredients if you wish to make a volume 5 times more important? When you do the shopping, a pot of 750 G Nutella is worth 4$. Should this rate there, how much cost a pot of 3 kg? For each one of these cases, it is enough to make a simple rule of three.