Math & School

Rule of Three Calculator

Solve proportional and anti-proportional problems.

Updated on Apr 21, 2026 Calculator, calculation path and examples on one page

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Calculator

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Calculation path

Step by step

  1. Choose the appropriate formula

    Result: x = b × c / a

  2. Use values

    Formula: x = b × c / a

    Values inserted: 12.00 × 5.00 / 3.00

    Result: 8 p.m

Instructions

How to use this calculator correctly

How to use the calculator

  • Choose directly or inversely proportional.
  • Enter a, b and c to match the task.

Common mistakes

  • Misjudging proportionality.
  • swap a and c.

Limits of the calculation

  • The calculator assumes clean proportional relationships.

Related guide: Solve the rule of three, percentage calculations and fractions in an understandable way

Examples

Typical calculations

3 pieces cost 12 euros, 5 pieces cost x euros.

Directly proportional

Result x: 8 p.m

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3 workers need 12 days, 6 workers need x days.

Inversely proportional

Result x: 6.00

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4 liters are enough for 10 servings, 7 liters for x servings.

Quantity calculation

Result x: 17.50

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FAQ

Frequently asked questions

When is a rule of three directly proportional?

When more of one size means more of the other size.

When is a rule of three inversely proportional?

If more of one size means less of the other, say more workers and less time.

Why can't a be 0?

Because otherwise a division by 0 would occur.

Where is the rule of three used in everyday life?

For example, recipes, prices, quantities, speeds or working times.

Is the rule of three the same as percentage calculation?

Not quite, but percentage problems can often be presented as a rule of three.

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Sources and notes

Rule status and context

Formulas
Direct: x = b × c / a, reverse: x = a × b / c