Current for 2026Last updated: 2026-06-09How we verify data: Methodology →

Proportion Calculator — Rule of Three

Updated: 9 June 2026

Enter three known values of a proportion (a:b = c:x) and the calculator finds the fourth. Useful in maths, cooking and finance.

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How we calculate the proportion

The calculator applies the rule of three: x = (b x c) / a. Enter the values a, b, c and read off the result x.

x = (5 x 12) / 3 = 20.

What is the rule of three?

It is a method for finding an unknown value in a proportion: a:b = c:x, so x = (b x c) / a.

How do I solve a:b = c:x?

Multiply b by c and divide by a; the result is the missing value x.

Where is the rule of three useful?

In recipe scaling, currency conversion, discount calculations and many everyday maths tasks.

What is the difference between direct and inverse proportion?

In a direct proportion both values grow together; in an inverse one, when a grows, x falls.

Can a be zero?

No, you cannot divide by zero, so the value a must always be non-zero.

Is the calculator free?

Yes, the proportion calculator is free and works instantly online.

The result is indicative. Do not divide by zero.

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