Rule of three calculation

Rule of three, Rule of proportion

A rule of three is used to calculate ratios proportional to each other. Proportional means that when a quantity A is doubled, B is also doubled. For example, e.g. A train with constant speed drives in an hour 80km so it drives in two hours 160km.

Calculation scheme for Rule of three calculation


Example: A train runs in 2 hours 140 km then 2 has to be set for A and 140 for B. The route is asked for 3 hours. So 3 for C and x is the distance searched.

2hours140km 3hoursxkm




1hours1402km 3hours31402km

1hours70km 3hours210km

So is x= 210 that means the train runs 210km in 3 hours.

Rule of three Calculator

glasses cost €. How much cost glasses?

To print pages, a printer needs minutes. How long does the printer need for pages?

How many pages can be printed in minutes?

For a distance of km needs a car liter petrol. How much petrol is needed for a distance of km?

How far can you get by the car, with liter?

Inverse Rule of three

An inverse rule of three is a calculation scheme for the calculation of ratios which are indirectly proportional to each other. Indirectly proportional, when a quantity A is halved, the quantity B is doubled. If 2 trucks need to transport a certain amount of overburden 10 days so 1 truck needs double time so 20 days.

Calculation scheme of the inverse rule of three


Example: If you need 4 trucks to remove a certain amount of overburden 10 days then 2 is set for A and 10 is for B. Looking for the time that 2 trucks is needed so C is set to 2.

4truck10days 2truckxdays




1truck410days 2truck4102days

1truck40days 2truck20days

So is x= 20 that means a truck needs 20 days.

More Calculators

Here is a list of of further useful calculators:

Index Fraction calculator Percentage calculation Per mille calculation Interest calculation