Proportion Calculator (a : b = c : x)
Math ToolsSolve the missing number in a proportion
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All history →Proportion Calculator (a : b = c : x) — at a glance
Find the missing number when two ratios are equal.
2 : 5 = 6 : x
Solve Proportion
x = 15
How to use it — step by step
Three known numbers a, b and c.
1
Enter your values
Fill in the fields. Default numbers are shown so you can see how it works.
2
Press Solve Proportion
The answer appears at once as large result cards.
3
Copy or share
Copy the result, or share a link that opens with your numbers filled in.
Pro tips
Small things that make the result better.
✦Also called the rule of three.
✦Keep units consistent on both sides.
✦Use it to scale recipes up or down.
Quick facts
Freeno signup
Privateruns in your browser
Instantresult in a second
No limituse it as often as you like
Frequently Asked Questions
How is x found?+
Multiply b by c and divide by a.
What is the scale factor?+
c ÷ a, the number you multiply the first ratio by.
Is my data safe?+
Yes. Everything runs inside your own browser. Your text, files and images are never uploaded to any server.
Is this tool really free?+
Yes — completely free, no signup, no watermark and no usage limit.
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