Ratio Calculator
A proportion says two ratios are equal, and this free ratio calculator solves it for you: fill in any three of A, B, C and D in A:B = C:D, leave exactly one box empty, and the missing value appears instantly — together with the completed proportion so you can sanity-check it at a glance.
How to use
- Enter the known values into three of the four boxes A, B, C and D — the proportion is read as A:B = C:D.
- Leave exactly one box empty: that is the value you are solving for.
- The missing value appears instantly, with the completed proportion shown underneath for checking.
- Adjust any number and the answer updates immediately; nothing leaves your browser.
Frequently asked questions
How does the calculator solve a proportion?
It applies cross-multiplication: A:B = C:D means A·D = B·C. To isolate the empty slot it divides by the value diagonal to it — for example A = B·C ÷ D when A is missing. That is exactly how it is taught in school, just without the algebra steps.
Why do I get an error instead of a result?
The most common cause is a zero in the dividing position: solving for A divides by D, so D = 0 makes the answer undefined. The tool refuses to invent a number and explains which value caused the problem.
Can I use decimals or negative numbers?
Yes, decimals and negative numbers work fine. The only invalid inputs are leaving more than one box empty (nothing to solve yet), filling all four (over-determined), and zero divisors.
What can I use it for?
Any "same ratio" question: scaling a recipe from 4 to 6 people, converting a map distance, resizing a drawing, diluting a concentrate, or comparing pack sizes by unit price.
Is anything uploaded?
No. All arithmetic happens in your browser; the numbers you type never leave your device.
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About this tool
A proportion says two ratios are equal, and this free ratio calculator solves it for you: fill in any three of A, B, C and D in A:B = C:D, leave exactly one box empty, and the missing value appears instantly — together with the completed proportion so you can sanity-check it at a glance. This is the maths behind scaling recipes, resizing drawings, map scales, mixing concentrates, step-by-step unit conversion and unit-price comparisons. The engine uses cross-multiplication (A·D = B·C), the same method taught in school, and it refuses to guess when the maths is undefined: if the division needed to isolate your missing value has a zero divisor, you get an explicit error instead of a made-up number. Everything is computed as you type, entirely in your browser — your numbers never touch a server, and there is nothing to install or sign up for.