Ratio and proportion calculator
If three values of a proportion are known, the fourth follows from cross multiplication. Leave the unknown field empty and the tool fills it in.
The rule of three only applies to directly proportional quantities. It does not hold for non-linear relationships: if one bricklayer builds a wall in eight hours, two do not necessarily take four.
Frequently asked questions
What about inverse proportion?
That is when one quantity rises while the other falls in the same ratio. There you use A × B = C × D, not cross division.
Can I scale a recipe with this?
Yes: A is the reference number of servings, B the recipe quantity, C the servings you want, and D comes out as the new quantity.
How do I work out a percentage with it?
Put 100 in A, the whole in B, the percentage in C; D is the corresponding part.