Proportion Calculator
Solve proportions and rule of three online
Proportion Calculator — Rule of Three Online Free
Solve direct and inverse mathematical proportions using the rule of three. Enter three known values (A, B, C) and the calculator automatically finds the fourth proportional (X). Useful for cooking, recipes, measurement conversions, scaling, school math and much more.
If A : B = C : X → X = (B × C) / A
How It Works
- Choose the proportion type: direct (A:B = C:X) or inverse (A×B = C×X)
- Enter the three known values (A, B and C)
- The X value is calculated automatically
- Copy the result with one click
Frequently Asked Questions
What is the difference between direct and inverse proportion?
In direct proportion, as A increases X increases proportionally too (e.g. the more km you drive, the more fuel you consume). In inverse proportion, as A increases X decreases (e.g. the more workers on a job, the fewer days needed to finish it).
How do I use the rule of three for a recipe?
Example: a recipe for 4 people requires 200g of flour. How much for 6 people? A=4, B=200, C=6 → X = (200×6)/4 = 300g. Use direct proportion.
When do I use inverse proportion?
When increasing one quantity decreases the other. Example: 3 workers finish a job in 12 days. How many days would 4 workers take? A=3, B=12, C=4 → X = (3×12)/4 = 9 days. Use inverse proportion.