How this calculator helps
What the calculator is for
The rule of three calculator finds an unknown value from three known numbers. It is useful for prices, recipes, consumption, performance, scales and many everyday situations with direct or inverse proportion.
How the calculation works
In direct proportion, both values move in the same direction. In inverse proportion, one value rises while the other falls. The calculator applies the selected relationship and calculates the unknown X.
Practical example
Example: if 3 pieces cost CZK 120, how much do 8 pieces cost? Use direct proportion. For inverse proportion, you can solve work-time examples such as how long a task takes with more people.
What to watch out for
The most common mistake is mixing direct and inverse proportion. Not every relationship is linear, so discounts, tiered pricing or physical formulas may need a more specific model.
FAQ
When should I use direct proportion?
When both values move in the same direction, for example more pieces mean a higher price.
When should I use inverse proportion?
When an increase in one value means a decrease in the other, such as more workers reducing time.
Can the calculator round the result?
Yes. The result can optionally be rounded.
Can I copy the result?
Yes. Clicking the result field prepares it for copying.