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Proportion calculator
Solve A/B = C/X for the missing number (cross multiplication).
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- No sign-up
- Works on any device
About Proportion calculator
Solves A/B = C/X for the missing number by cross multiplication: X = B x C / A. Use it for scaling recipes, map scales, currency at a fixed rate and similar triangles. Keep the same units on both sides, or the ratio will quietly give the wrong answer.
How to use Proportion calculator
- Fill in the three numbers you know in A/B = C/X, leaving the unknown empty.
- Check that the numbers are in the right positions.
- Read the missing value, found by cross multiplication.
Why use this tool?
- Private: everything happens in your browser. Nothing you enter is uploaded.
- Free, with no sign-up, no watermark and no limit on how often you use it.
- Works on phones, tablets and computers: Android, iPhone, Windows, Mac and Linux. Nothing to install.
- Free to embed on your own website, with a ready-made code to copy.
- Available in 10 languages.
Also known as
Frequently asked questions
How do I solve 3/5 = 9/x?
Cross multiply: 3 x X = 5 x 9, so 3X = 45 and X = 15. Check by simplifying 9/15, which reduces back to 3/5.
How does cross multiplication work?
Multiply the numbers diagonally and set the products equal: A times X equals B times C. Then divide to get X. The tool does this step for you.
Where is proportion useful in real life?
Scaling recipes, converting map distances, working out prices for a different quantity, or resizing an image while keeping its shape. Each uses the same A/B = C/X idea.
Is Proportion calculator free?
Yes. Proportion calculator on Toolipedia is free to use, with no sign-up and no watermark.
Is it safe to use?
Yes. Everything is worked out in your browser, and nothing you enter is sent to our server.
Does it work on my phone?
Yes. It works in any modern browser on Android and iPhone, as well as on tablets and computers. There's nothing to install.
Do I need an account?
No. Just open the page and use the tool. There's no sign-up and no email needed.
Can I put Proportion calculator on my own website?
Yes. Use the Embed on your site button under the tool to copy a short piece of code and paste it into your page. It is free, needs no account, and the tool keeps working inside your page.