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Ratio Calculator

Fill in three values and leave one blank to solve A : B = C : D.

A
:
B
=
C
:
D
Result
2 : 3 = 4 : 6

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A ratio calculator works with proportions of the form A : B = C : D. Leave one of the four values blank and it finds the missing term; or enter two numbers to reduce a ratio to its simplest whole-number form. Everything is computed in your browser.

How to use

  1. To solve a proportion, type three of the four values and leave one box empty.
  2. Read the missing value and the equivalent decimal below.
  3. To reduce a ratio, switch to Simplify and enter two numbers.

How it works

A proportion means two ratios are equal, so the cross products are equal: A × D = B × C. From that single rule the calculator rearranges to find whichever term you left blank. To simplify a ratio it divides both numbers by their greatest common divisor (GCD), giving the smallest whole-number ratio that means the same thing.

The proportion A to B equals C to D, with cross products A times D equal to B times C.A : B=C : DA × D = B × C

Features

Solve any term

Leave any one of the four values blank and get the answer instantly.

Simplify ratios

Reduce ratios like 1920:1080 to 16:9 using the greatest common divisor.

Decimal view

See the ratio as a decimal so you can compare values quickly.

Private & instant

All math runs in your browser; nothing is uploaded.

When to use it

Recipes

Scale ingredient amounts up or down while keeping proportions.

Screens & images

Work out aspect ratios and resize dimensions like 16:9.

Maps & models

Convert between real distances and scaled drawings.

Mixes & splits

Split amounts or mix solutions in a fixed ratio.

Notes

  • Decimals are allowed; results are rounded to six places.
  • Simplifying only reduces whole-number ratios to lowest terms.
  • A blank that would require dividing by zero can't be solved.
  • All calculations run on your device; nothing is sent anywhere.

FAQ

What is a proportion?
Two equal ratios, written A : B = C : D. If three values are known, the fourth is determined.
How do I find a missing value?
Enter the three known numbers and leave the unknown box blank — the answer appears automatically.
How does simplifying work?
Both numbers are divided by their greatest common divisor, so 1920:1080 becomes 16:9.
Can I use decimals?
Yes for solving proportions. Simplifying to whole-number terms works best with whole numbers.
Is my data uploaded?
No. Every calculation happens in your browser and nothing leaves your device.

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