Ratio Calculator

Simplify ratios to lowest terms, scale by a factor, or find the missing value in a mathematical proportion (A:B = C:D).

Perfect proportions and mathematical scaling. Instantly.

Operation Mode

Base Ratio (A : B)

Try ratio:

Simplified Ratio

Awaiting Ratio ValuesEnter terms A and B on the left to simplify, scale, or solve missing proportion values.

Cross-multiplication rule: In any proportion A:B = C:D, the product A × D always equals B × C.

Three ways to work with ratios

Choose the mode that matches your problem — simplification, scaling, or finding a missing value in a proportion.

Formula

Mode	What it solves	Formula
Simplify	Reduce A:B to lowest terms	A÷GCD : B÷GCD
Scale	Multiply both sides by N	A×N : B×N
Proportion	Find missing value in A:B = C:D	D = B×C÷A

Examples — One Per Mode

Simplify 12:8

GCD(12, 8)412÷4 : 8÷43:2 (factor 4)

Scale 2:3 by 5

2×5 : 3×510:15

Proportion: 2:3 = 8:?

D = (3 × 8) ÷ 2D = 12 → 2:3 = 8:12

Last reviewed August 2026

Why Use the Ratio Calculator?

Simplify any two numbers to their lowest common integer ratio
Scale recipe quantities, screen aspect ratios, and architectural models
Solve missing fourth values in proportions via cross-multiplication
Displays reduction factor (GCD) and equivalent decimal quotient
100% private, instant client-side calculation with zero data collection

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Frequently Asked Questions

Both sides are divided by their greatest common divisor (GCD). For 12:8, GCD(12, 8) = 4, so 12÷4 = 3 and 8÷4 = 2 — giving 3:2. The simplified ratio expresses the same relationship using the smallest possible whole numbers. The calculator also shows the GCD (factor) used.