A ratio describes a relationship

A ratio of 12:18 means the first quantity has 12 parts for every 18 of the second. Both numbers can be divided by 6, giving 2:3. The scale changes, but the relationship does not. The tool uses the greatest common divisor to find the smallest positive whole-number pair. The two original parts must be whole numbers from 1 to one billion.

Scaling keeps the same proportion

If the ratio is 2:3 and the first part becomes 10, the second becomes 15. The calculation is new first part × original second part ÷ original first part. A scaled value may contain decimals even when the original ratio is a whole-number pair. Make sure both quantities use the units you intend; a ratio alone does not establish a physical conversion factor.

Part-to-part is different from part-to-whole

In a mixture with a part-to-part ratio of 2:3, the total has five parts. The first ingredient is 2/5 of the total, not 2/3. That distinction matters when you know the final batch size rather than the size of the first component. This tool scales from the first part; it does not assume that the target field is the total. Use it for general arithmetic, and independently verify specialized formulations.

Worked examples

Original ratio	Simplest form	If first part is 10
12:18	2:3	Second part = 15
1920:1080	16:9	Second part = 5.625

Good questions.

Can the ratio contain zero?

This tool requires both original parts to be positive so proportional scaling is well defined.

Is the new value the total?

No. It is the new first part. For a 2:3 ratio, a first part of 10 gives a second part of 15 and a total of 25.

Can the scaled result have decimals?

Yes. A proportional result is not necessarily a whole number.

Updated 5 November 2026 · Methods & corrections

Ratio calculator