Rule of three calculator

3 rolls cost $4.50 – what do 7 cost? Enter three values and the fourth is calculated.

→ → ?

First row: known ratio. Second row: new value → result.

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How to use

  1. Choose whether the relationship is direct or inverse.
  2. Enter the three known values, optionally with units.
  3. The result x appears instantly – including the calculation.

Direct or inverse proportion?

Direct: the more, the more. Twice as many rolls cost twice as much. Formula: x = B × C ÷ A.

Inverse: the more, the less. 3 workers need 12 days – 6 workers need only 6 days. Formula: x = A × B ÷ C.

Frequently asked questions

When is a proportion inverse?

When one quantity decreases as the other increases – e.g. workers and time, or speed and travel time.

Can I enter decimals?

Yes, with a dot or comma as decimal separator.

Where is the rule of three used in daily life?

Unit prices, scaling recipes, fuel consumption, currencies or working hours.