When should I use direct rule of three?

Use direct proportion when both quantities increase or decrease together, such as quantity and price.

When should I use inverse rule of three?

Use inverse proportion when one quantity increases while the other decreases, such as workers and time.

Rule of Three Calculator Online

Quick answer

The rule of three calculator finds the fourth value in a proportion from three known values. Use direct proportion when values grow together and inverse proportion when one grows while the other decreases.

How to use it

Select direct or inverse proportion.
Enter value A and its corresponding value B.
Enter value C to find X.
Check the result and formula used.

Formulas used

Direct rule of three: X = (B x C) / A

Inverse rule of three: X = (A x B) / C

Practical example

If 4 products cost 20, then 7 products cost 35 in a direct proportion: X = (20 x 7) / 4.

In inverse proportion, if 4 people take 20 days to finish a task, 8 people would take 10 days: X = (4 x 20) / 8.

How to interpret the result

X is the value that corresponds to C in the proportion you entered. It can represent price, time, distance, quantity, consumption or any proportional quantity.