Ratio Calculator

Solve proportions and ratios easily. Find the missing value in A:B = C:D and simplify ratios online for free.

Enter 3 values, leave one empty

: = :
Missing Value: —
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Simplified: —
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Scale by
Scaled Result: —
Quick Reference
2 : 3 = 4 : ?= 6
1 : 4 = ? : 20= 5
3 : 5 = 9 : ?= 15
Simplify Ratio: 12 : 8= 3 : 2
Simplify Ratio: 15 : 25= 3 : 5
Scale Ratio: 2 : 3 × 4= 8 : 12

How It Works

  1. Enter two or more values to express as a ratio (e.g. 4 and 6).
  2. The calculator simplifies the ratio to its lowest terms using the greatest common divisor.
  3. Optionally solve a proportion: enter three known values to find the fourth.
  4. Results appear instantly as you type.
  5. Copy the simplified ratio or missing value with one click.

Formula

Simplify:     divide all parts by GCD
Proportion:   a/b = c/d  →  d = (b × c) ÷ a
GCD (Euclid): gcd(a, b) = gcd(b, a mod b) until b = 0

Example

Input: Ratio: 12 : 18
Result: Simplified: 2 : 3

FAQ

It divides all parts of the ratio by their greatest common divisor (GCD), giving the simplest equivalent ratio.

A proportion states that two ratios are equal, e.g. 2:3 = 4:6. The calculator can solve for an unknown value when three values are known.

No. All calculations run entirely in your browser. No data is sent to or stored on our servers.

The standard method for finding the GCD: repeat gcd(a, b) = gcd(b, a mod b) until b = 0. The last non-zero remainder is the GCD.