Rule of Three Calculator
Find an unknown value in a proportion with the calculation shown.
Value X20
Step by step: X = (12 × 5) / 3 = 20.
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How is this result calculated?
The calculator starts from the proportion A / B = C / X, then isolates the unknown value using cross-multiplication.
The formula used is X = B × C / A. The steps are shown so the result can be checked.
Frequently asked questions
What is the rule of three?+
The rule of three finds an unknown value in a proportion. If A corresponds to B, then C corresponds to X.
What formula is used?+
The calculator applies cross-multiplication: X = B × C / A. That means A cannot be equal to 0.
Can I use decimal numbers?+
Yes, the fields accept decimals with either a dot or a comma as the decimal separator.
Does this replace a spreadsheet?+
It is mainly useful for a quick, checkable calculation. For long lists of proportions, a spreadsheet is still better.