Ratio Calculator

Solve a proportion of the form A:B = C:D by leaving the unknown value blank.

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How to use the Ratio

Fill in each input field with your own numbers.
Results calculate automatically and update as you change any value.
Read the result cards below the inputs for the full breakdown.
Adjust any field to explore different scenarios instantly.

Privacy: this tool runs entirely in your browser. Your input is never sent to, received by, or stored on any server — there are no uploads and no tracking of what you enter.

About Ratio

What is a proportion

A proportion states two ratios are equal, such as 2:3 = 10:15. If you know three of the four values, you can find the fourth.

How it works

Enter the three known values and leave C or D blank; the tool cross-multiplies to find the missing one.

Where it helps

Scaling recipes, mixing ratios, map scales, and resizing while keeping proportions.

Frequently asked questions

How do I solve a ratio?

Cross-multiply: if A:B = C:D then A x D = B x C, so you can solve for any unknown.

Which field do I leave blank?

Leave exactly one of C or D blank for the tool to solve it.

Can I scale recipes with this?

Yes — set the original ratio and one new quantity to find the other.