TopToolsAiSimplify a ratio to its lowest terms or solve a proportion A : B = C : D for the missing value. Decimals are supported and the cross-multiplication working is shown step by step.
Last updated: 21 Sep 2026
How to use Ratio Calculator
- To simplify a ratio, fill in only A and B and leave C and D empty.
- To solve a proportion, fill in any three of the four boxes and leave one empty.
- Click Calculate to see the answer and the working.
Frequently asked questions
How do I simplify a ratio with decimals?
The calculator multiplies both sides by a power of 10 to remove the decimals, then divides by the GCD. For example, 2.5 : 10 becomes 25 : 100 and then 1 : 4.
How does solving a proportion work?
It uses cross-multiplication. Because A/B = C/D, it follows that A × D = B × C, so the missing value comes from multiplying and dividing the three known numbers.
Is this calculator free to use?
Yes, it is completely free, with no signup or account required.
Does my data get uploaded anywhere?
No. Everything is calculated in your browser and nothing is sent to our servers.
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