Everyday utility

Ratio Calculator

Enter two ratio parts and a comparison value to simplify the ratio, read equivalent forms, and solve the missing side of a proportion.

Last reviewed May 18, 2026 by ToolSpilo Editorial Team.

Review method: Reviewed against the implemented ratio simplification logic and examples, displayed formulas, and worked examples.

Calculator tool

How this calculator works

Use the explanation to understand the formula, assumptions, and practical limits behind the calculator result.

What a ratio tells you

A ratio compares two quantities in the form A:BA:B. The calculator simplifies the pair by dividing both parts by their greatest common factor, then shows the decimal value A/BA/B, the percentage of AA relative to BB, and the missing value in an equivalent proportion.

For example, 12:8 simplifies to 3:2. If 12:8=18:x12:8 = 18:x, then x=12x = 12 because both sides keep the same relationship.

When ratio outputs become undefined

Some ratio-derived values need division. If both parts are zero, the ratio itself is undefined. If B=0B = 0, decimal and percentage forms cannot be calculated. If A=0A = 0, the equivalent proportion A:B=C:xA:B = C:x cannot solve xx by dividing through AA.

Use the simplified ratio when comparing mixtures, map scales, recipes, or relative quantities. Use the warnings when a zero input changes which outputs are mathematically meaningful.

Frequently asked questions

What does it mean to simplify a ratio?

It means dividing both parts by the same common factor until no larger shared factor remains. 12:8 becomes 3:2.

Why can a ratio output be undefined?

Because decimal, percentage, and proportion forms require division. Division by zero has no valid finite result.

Is a ratio the same as a fraction?

They are closely related, but a ratio compares two quantities while a fraction is often read as part of a whole. The same numbers can be useful in both forms depending on the question.

How do I solve an equivalent proportion?

Keep the same relationship on both sides. If A:B=C:xA:B = C:x and AA is non-zero, then x=C×B/Ax = C \times B / A.