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Calculator · Maths
Proportion Calculator — Rule of Three
Enter three known values of a proportion (a:b = c:x) and the calculator finds the fourth. Useful in maths, cooking and finance.
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.
Example: 3:5 = 12:x
x = (5 x 12) / 3 = 20.
Frequently asked questions
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.
Indicative result
The result is indicative. Do not divide by zero.
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