Proportion Calculator
Solve proportions using the Rule of Three for both direct and inverse proportions. Find the unknown value in A : B = C : X.
✓ Runs in your browser · Updated 2026-03-31Enter values and click Solve Proportion to see results
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.