Proportion Calculator
Solve proportions using the Rule of Three for both direct and inverse proportions. Find the unknown value in A : B = C : X.
Enter values and click Solve Proportion to see results
Formula
What is the Rule of Three?
The Rule of Three is a mathematical method for solving proportions. Given three known values, it finds the fourth unknown value by setting up a proportion equation.
Direct vs Inverse Proportion
In a direct proportion, as one quantity increases, the other also increases at the same rate. The formula is:
A : B = C : X → A × X = B × C
In an inverse proportion, as one quantity increases, the other decreases. The formula is:
A × B = C × X
Real-World Examples
Direct proportion: If 5 apples cost ₹100, how much do 8 apples cost? (A=5, B=100, C=8 → X=160)
Inverse proportion: If 6 workers finish a job in 10 days, how long would 4 workers take? (A=6, B=10, C=4 → X=15 days)
Related Tools
Explore our Ratio Calculator for simplifying and dividing ratios, or the Percentage Calculator for quick percentage conversions.
Frequently Asked Questions
What is the Rule of Three?
The Rule of Three is a method to find an unknown value when three values are known and related proportionally. If A:B = C:X, then X = (B × C) / A.
Is this calculator free, and do I need to sign up?
Yes, it is completely free with no sign-up. There are no usage limits and no paywalls.
Are my inputs saved or sent to any server?
No. Every calculation happens entirely in your browser. Nothing you type is uploaded, logged, or shared with us or any third party.