The rule of three is a method for finding an unknown value from three known values that are related proportionally. There are three types —direct, inverse and compound— and it is learned around 5th and 6th grade (ages 10-11). It is the basis of percentages, scales and many everyday calculations. This guide explains it with worked, step-by-step examples and tables.
What is the rule of three?
The rule of three is a math tool for finding a fourth value when we already know three others that form a proportion. The idea is simple: if we know how two quantities relate, we can work out the missing value.
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There are three types, depending on how the quantities relate:
- Direct rule of three: when one quantity goes up, the other goes up too.
- Inverse rule of three: when one quantity goes up, the other goes down.
- Compound rule of three: three or more quantities are involved at the same time.
In the next sections we look at each one with a worked example. Before proportions it helps to master fractions and decimal numbers, since they show up all the time when dividing and multiplying.
Tip: if your child understands why the rule of three works, they'll stop memorizing formulas. Try a free class and watch them learn with real examples.
