Proportion Calculator (a : b = c : x)

Math Tools

Solve the missing number in a proportion

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b
c
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Proportion Calculator (a : b = c : x) — at a glance

Find the missing number when two ratios are equal.

You enter
2 : 5 = 6 : x
Solve Proportion
Result
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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