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Instant calculation
Get the result directly while you enter the data.
Online Calculator
Rule of three calculator
Quickly solve proportions by entering three known values.
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X
25
Formula: X = (B × C) ÷ A
Information
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Local processing
Data are processed in your browser and do not need to be sent to a server.
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Free
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Frequently asked questions
What's a rule of three?
It is a method for finding an unknown value from a proportional relationship between three known values.
What formula does it use?
The calculator uses X = (B × C) ÷ A.
Can I use decimals?
Yes. You can enter decimal numbers.
What if A is zero?
Division by zero is undefined, so the calculator does not show a numerical result.