Rule of Three Calculator

To use the rule of three calculator, enter three numbers & one unknown variable "x", and hit calculate button


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Rule of Three Calculator

Rule of Three calculator is a free online tool that solves simple proportions using the mathematical rule of three. This calculator takes three values from the user and calculates the fourth unknown value with just one click. 

What is the Rule of Three?

The rule of 3 is a simple but useful method to solve the proportions. It helps to find a fourth term using three known values based on the proportions. This rule involves multiplying and dividing numbers to obtain a missing value. It is very helpful in finding the cost of an item, calculating the speed of a moving object, or determining the amount of ingredients needed for a recipe.

Mathematically, if we have three values (i.e. a, b, and c) and want to determine the unknown value (x), we can set up proportions (a/b = c/x) and solve for the unknown.

The formula for the rule of the three is given in the figure.

rule of 3 formula

The Rule of Three can be used to solve problems related to percentages, ratios, and proportions.


The above rule of three calculator is an easy-to-use tool that can solve proportion problems quickly and efficiently. If you prefer to determine the fourth unknown value manually, some examples can help you learn how to use the rule of three.

Example 1:

Find the value of x in the following proportion.

10 ⟶ 5

03 ⟶ x


Here, a = 10, b = 3, c = 5, and x =?

Write the given problem in the form of a/b = c/x

I.e. 10/3 = 5/x

To find the value of x, use the formula of the rule of three.

∴ x = (b. c) /a

x = (3 * 5) /10

x = 15/10

x = 1.5

Example 2: (Real Life Example)

In 25 minutes a train travels 20km, how far will it travel in 5 minutes?


Let x be the distance covered by the train in 5 minutes.

Time (minutes)   :   Distance (kilometer)

         25                                     20

         05                                       x

Write in proportional form.

25 / 5 = 20 / x

Use the rule of three to find x.

x = (5 * 20) /25

x = 4 km

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