Ratio Calculator

Simplify a ratio, scale it by a factor or solve for a missing value in A : B = C : D.

Ratio calculator

What do you want to do?

A B

C D Missing value

Scale factor

Result: —

Decimal form: —

—

Ratios are reduced with the greatest common divisor, so 12 : 18 becomes 2 : 3.

Choose whether to simplify a ratio, solve a missing value or scale by a factor.
Enter the numbers you know in the boxes.
The simplified ratio or the missing value appears instantly, with the working shown.

Common questions about the free online ratio calculator.

Enter the two terms in the simplify mode. The tool divides both by their greatest common divisor, so 12 : 18 becomes 2 : 3.

Choose the solve mode, enter the three values you know and leave the unknown blank. The tool cross multiplies and fills in the answer.

It means the two ratios are equivalent. The product of the outer terms equals the product of the inner terms, so A times D equals B times C.

Yes. Decimal terms such as 2.5 : 3.75 are scaled to whole numbers first, then reduced, giving 2 : 3.

It shows the ratio expressed against 1, so 2 : 3 becomes 0.666667 : 1. It is handy when comparing ratios of different sizes.

A ratio such as 0 : 5 simplifies to 0 : 1. The tool reports that 0 : 0 is undefined, and it warns you when a division by zero would be needed.

Use the scale mode and enter a factor. A ratio of 2 : 3 scaled by 3 gives 6 : 9, which is the same proportion.

Yes. Negative terms are allowed and the sign is kept on the first term of the simplified ratio.

No. All calculations run locally in your browser, so nothing is ever uploaded.

Last updated: October 10, 2026