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Comment-Calculer · https://www.comment-calculer.net/en/proportion-calculator/
Math & calculators

Proportion calculator (rule of three)

Scale a known amount to a different quantity. Enter the amount and quantity you know, then the quantity you need to find its proportional share.

Your inputs1

For example, a cost of 100 for 30 units. The answer uses the same unit as this amount.

Min. 0.000000000001

For the example above, enter 30. This reference quantity must be positive.

For example, enter 12. Use the same quantity unit as the reference quantity.

Decimal point or comma accepted. No registration.

Your result2

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Enter your values, then calculate.

Your calculation stays in your browser.

The formula, explained

a*c/b
a : Known amountb : Quantity for that known amountc : Quantity you need

An example to check

Using the starting values below, the calculation gives:

Known amount: 100Quantity for that known amount: 30Quantity you need: 12
40

A result proportional to the selected share — The applied factor is 0.4. It changes 100 into 40.

The rule of three assumes proportionality. Use the same unit for the two reference quantities; fixed fees or tiered pricing may not follow this model.

How to use this tool

  1. Enter the known amount, such as a cost of 100.
  2. Enter the quantity corresponding to that amount, such as 30 units.
  3. Enter the required quantity in the same unit, such as 12 units. The answer is known amount × required quantity ÷ known quantity.

A worked example

If 30 units cost 100 at a constant unit price, 12 units cost 100 × 12 ÷ 30 = 40. The multiplier is 12 ÷ 30 = 0.4. The result uses the same currency as the known cost.

When can I use the rule of three?

Use it when the amount changes in direct proportion to the quantity, such as scaling ingredients or a cost at a constant unit price. Fixed charges, minimum fees, tiered prices and contractual billing rules need separate treatment.

Can the required quantity exceed the known quantity?

Yes. For example, scaling from 30 units to 60 doubles the known amount. A required quantity of zero gives zero; the known reference quantity must remain positive.

Can I mix days, hours or other units?

Convert the two quantities to the same unit before entering them. This is a proportional calculation, not a calendar or payroll calculation.

Can I use a decimal comma?

Yes. Numeric fields accept 12.5 or 12,5 and spaces for thousands, such as 1 234.50. Do not mix a point and comma in one number.

Where are my inputs processed?

Calculations run in your browser. Form values are not sent to Analytics. Copying, printing, sharing or opening an external service is your choice.

Method and interface checked on 2026-10-15 · Report an error