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

Back to category: Math & School

Calculator

Type of rule of three

Reset Load example

Examples

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

Result x: 8 p.m

Load this example

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

Result x: 6.00

Load this example

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

Result x: 17.50

Load this example

FAQ

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.

Related calculators

Calculate percentage, percentage, base value and percent change.

Reduce, expand, add, subtract, multiply and divide fractions.

Calculate daily and weekly working time from start time, end time and breaks.

Sources and notes