Rule of Three

This tool helps you quickly and easily solve proportionality problems. Simply enter three of the four necessary values (A, B, C, D) and let the calculator find the fourth one for you. Ideal for daily calculations in economics, cooking, DIY, and more.

`A/B = C/D`

Introduction to the Rule of Three

The rule of three is a simple mathematical method for solving problems of proportionality. It is often used in situations where you need to calculate an unknown fourth value when three other values are known and proportionally related.

How Does the Rule of Three Work?

The rule of three is based on the principle of proportionality. If two quantities are proportional, the ratio of the first two is equal to the ratio of the next two. This equality allows for the missing value to be found.

Example: If 10 apples cost 5 euros, how much do 15 apples cost?

Data Entry

To use the calculator, enter three out of the four terms A, B, C, and D. The terms A and B are linked by a proportional relationship, as are the terms C and D. For example, if A is to B what C is to D, then by entering A, B, and C, the calculator will determine the value of D. If you enter B, C, and D, then the calculator will determine A.

Calculation of the Missing Term

The calculator uses the formula of the rule of three to determine the missing term. If you have entered A, B, and C, the value of D is calculated by multiplying B by C and then dividing the result by A.

Concrete Example: If A = 10, B = 5, and C = 15 (as in the previous example), then D will be calculated as (5 * 15) / 10, which gives 7.5.

Practical Applications

The rule of three is useful in many contexts, such as in cooking to adjust ingredient quantities, in economics to calculate prices, or in DIY for proportional measurements.

Tip: Always check the consistency of the units used in your calculations to avoid errors.


The rule of three is a fundamental mathematical tool for solving proportionality problems in everyday life. Feel free to use this calculator to simplify your calculations.

FAQ (Frequently Asked Questions)

Find here answers to frequently asked questions about the rule of three and the use of this calculator.

Q1: What values should I enter in the fields A, B, C, and D?
A: Enter any three of the four values among A, B, C, and D, regardless of their combination. The calculator will determine the missing value by applying the rule of three.
Q2: Can I use decimal numbers, or should I limit myself to integers?
A: The calculator accepts both integers, decimal numbers, and even fractions. Just make sure to enter the values consistently in terms of units and format.
Q3: How is the rule of three applied in this calculator?
A: The rule of three is used to find an unknown value based on the principle that two pairs of values are proportional. If you know three of the values A, B, C, and D, the calculator uses proportionality to find the fourth.
Q4: What should I do if I get an unexpected or incorrect result?
A: First, check that you have entered the values correctly and that the units are consistent. If the problem persists, review the principles of the rule of three or contact support for help.
Q5: Can this calculator be used for complex calculations, or is it only for simple calculations?
A: While the calculator is primarily designed for simple proportionality calculations, it can be used in more complex contexts as long as the relationship between the values remains proportional.

See also

Algebra Calculators
Mathematics Calculators