Online Rule of Three Calculator - Proportions

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Calculate the fourth proportional value in a simple rule of three. Fast and free online tool for solving math and proportion problems.

First Value

Second Value

just as

Third Value

Result

How the Rule of Three Works

The rule of three compares two proportions: if A is to B as C is to x, then x = (B × C) ÷ A. How to use: enter three matching values in the two lines (first with second; third with result) and click Calculate. Example: if 2 is to 6 as 4 is to x, then x = (6 × 4) ÷ 2 = 12.