Skip to content The rule of three is a useful mathematical tool for calculating a ratio of different values. So that you don't have to do this laboriously in your head, we have created a Online rule of three calculator developed.
With the rule of three calculator, you can quickly and easily calculate a ratio, which is useful for percentage calculations, for example.
In our online rule of three calculator, all you have to do is enter your known values and the type of calculation.
If you are unsure about the type of calculation, you will find explanations and examples for both cases below.
Then you can simply click on "Calculate rule of three" and get the result.
With a rule of three Values calculatedwhich in a certain Relationship with other values. In principle, three values are known and the fourth is to be calculated.
A distinction is made between a proportional and one antiproportional Calculation. The former applies in most cases and basically means "the more, the more". The second case stands for "the more, the less".
A rule of three is a mathematical calculation in which you put three known quantities in relation to each other and thus determine a fourth quantity. The best-known application for the rule of three is calculating percentages.
The name of the rule of three could have two origins. It could either come from the fact that three values are used to calculate a third. However, many textbooks also explain that the calculation is carried out using three sets:
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The Formula for the rule of three consists as said of three sets and basically looks like this:
In concrete terms, it looks like this:
If 12 apples cost CHF 3.60, how much do 8 apples cost?
A table is often used for the calculation. This allows you to visualise how the values relate to each other in a proportional and antiproportional calculation.
This is how it looks for the example just mentioned with the apples:
As the rule of three is somewhat difficult to understand in theory, we will give a few more examples here. However, the easiest way to do the calculation is of course with the rule of three calculator.
Proportional means that the first value is in relation to the second, like the third to the fourth. The best way to illustrate this is with an example.
Let's assume that 3kg of potatoes cost CHF 5. How much will 2kg cost?
The ratio is therefore as follows:
The result is CHF 3.33.
3kg is in relation to CHF 5, just as 2kg is in relation to CHF 3.33. If you have more potatoes, they will also cost more CHF.
The easiest way to proportional realise that the doubling of one value leads to the doubling of the other.
Antiproportional is the opposite of proportional and means that the ratio is opposite. Here is another example:
Let's assume you drive 50km/h and reach your destination in 4 hours. How many hours do you need if you drive 100km/h?
The rule of three then looks like this:
The result is 2h.
50km/h is therefore in inverse proportion to 4h, as 100km/h is to 2h. If the km/h increase, the hours decrease.
The easiest way to antiproportional realise that the Doubling one value leads to halving the other.
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The rule of three is always needed when you want to put different values in relation to each other. This is often used for percentage calculations or when you want to know how much a certain number is worth if you know a different quantity (e.g. how much do 350g of strawberries cost if you know the price per kg?).
The rule of three can also be used particularly well for calculating percentages. To do this, always calculate 1% first and then multiply it by the number of per cent you are looking for. An example:
If a TV costs CHF 800 new and is now written down by 30%, how much does it cost?
CHF 800 equals 100%, so now you have to calculate CHF 800 : 100 to know the price for 1%. With a 30% discount, the TV now costs 70% of the original price, so you calculate CHF 8 * 70. With the discount, it only costs CHF 560.
Here, too, the rule of three calculator is of course much faster.
Eric is the founder of Schwiizerfranke.com and certified IAF wealth advisor. Since 2019, he has been helping Swiss citizens to organise their finances comprehensibly, independently and efficiently.
📌 Note: This article is for information purposes only and does not constitute personalised investment advice.