Ratio Calculator
Simplify a ratio to its lowest terms, or solve a proportion A : B = C : D for the missing value.
Reviewed by the WorldCalcs team · Methodology · Last reviewed: June 2026
Simplify a ratio
Result
3 : 4
Decimal: 0.75
Solve a proportion
Result
8
What is a ratio?
A ratio compares two quantities, showing how much of one there is relative to the other. It is written with a colon, like 3 : 4, and read "three to four". Ratios are used for mixing, scaling recipes, aspect ratios, maps and odds. Like a fraction, a ratio can be simplified to its lowest terms, and two equal ratios form a proportion.
How ratios are simplified and solved
To simplify a ratio, divide both numbers by their greatest common divisor — so 18 : 24 becomes 3 : 4, because both divide by 6. To solve a proportion A : B = C : D when one value is missing, cross-multiply: A × D = B × C. For example, in 1 : 2 = 4 : ?, the missing value is (2 × 4) ÷ 1 = 8.
Example
The ratio 1920 : 1080 simplifies to 16 : 9 (both divide by 120), the common widescreen aspect ratio, and equals about 1.78 as a decimal. To solve 3 : 4 = ? : 20, cross-multiply: the missing value is (3 × 20) ÷ 4 = 15.
All calculations happen in your browser. Nothing is sent, stored, or tracked.
Results are estimates and may contain errors — for general information only, not professional advice. Always verify before relying on them. Disclaimer
How to use
Use Simplify a ratio to reduce two numbers to their lowest terms and see the decimal. Enter whole numbers; zero in the second term is flagged.
Use Solve a proportion to find the missing value in A : B = C : D. Leave exactly one field blank and the calculator fills it by cross-multiplication.