Ratio Calculator

Solve proportions (A:B = C:D) for any missing value and simplify ratios to lowest terms. Free, instant, runs entirely in your browser.

Solve Proportion

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Leave one field empty to solve for it

Simplify Ratio

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Reference

What is a ratio and proportion?

A ratio compares two quantities (e.g., 3:4 means "3 for every 4"). A proportion states that two ratios are equal: A:B = C:D. Proportions are solved using cross multiplication: if A/B = C/D, then A×D = B×C. This tool solves for any one missing value and can also simplify ratios to their lowest terms by dividing both sides by their greatest common divisor (GCD).

Proportion formula

Cross multiplication: A/B = C/D → A × D = B × C

Solve for missing value:
A = (B × C) / D
B = (A × D) / C
C = (A × D) / B
D = (B × C) / A

Simplify ratio: Divide A and B by their GCD.
Example: 12:8 → GCD(12,8) = 4 → simplified = 3:2

Common use cases

  • Cooking — scale recipes up or down while keeping ingredient proportions correct.
  • Maps & models — calculate real-world distances from scale ratios (e.g., 1:50,000).
  • Design — maintain aspect ratios when resizing images, screens, or layouts.
  • Chemistry — solve stoichiometric proportions in chemical equations.
  • Finance — calculate proportional allocations (e.g., splitting costs, equity shares).

Privacy

All calculations run 100% in your browser. No data is sent to any server.

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