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Percentage Calculator
Find a percentage of a number, what percent one number is of another, or the whole from a part.
Proportion Calculator
Solve a proportion a/b = c/x for the missing value with cross-multiplication.
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How to use this calculator
Enter your values. The result updates as you type; press Calculate to confirm and jump to the result on mobile. Use Share to send the calculator with your result.
Solving proportions
Two ratios are proportional when a/b = c/d. Cross-multiplying gives a·d = b·c, so the missing value d = b·c ÷ a. Example: 3/4 = 9/x → x = 4 × 9 ÷ 3 = 12.
Proportions scale recipes, maps and unit prices. The Spanish and Portuguese editions call this the “regla de tres” / “regra de três”.
Frequently asked questions
What is cross-multiplication?
Multiplying the numerator of each ratio by the denominator of the other: a/b = c/d ⇒ ad = bc.
About this tool
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- Last updated
- 31 Oct 2026
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