A ratio compares two or more quantities by showing how much of one quantity exists compared with another.

How to Calculate Ratio: Formula, Examples & Easy Guide

Q: How do you calculate a ratio?

To calculate a ratio, write the quantities in the correct order and simplify them by dividing both quantities by their greatest common factor when possible.

Ratios are used to compare quantities in mathematics, cooking, business, finance, science, construction and everyday life. Learning how to calculate a ratio makes it easier to compare two or more quantities and understand their relationship.

In this guide, you will learn how to calculate a ratio, simplify ratios, find equivalent ratios, compare quantities and solve common ratio problems with easy examples.

For a quick calculation, you can also use our free Ratio Calculator .

Use the Ratio Calculator

What Is a Ratio?

A ratio is a mathematical way of comparing two or more quantities. It shows how much of one quantity there is compared with another.

A ratio can be written using a colon, as a fraction, or using words.

Example:

2:3
2 to 3
2/3

These forms represent the same comparison when used in the appropriate context.

How to Calculate a Ratio

To calculate a ratio between two quantities, follow these simple steps:

Identify the two quantities you want to compare.
Write them in the correct order.
Separate the quantities with a colon.
Simplify the ratio if possible.

Ratio = First Quantity : Second Quantity

Example: Calculate a Ratio

Suppose a classroom has 12 boys and 18 girls. The ratio of boys to girls is:

Boys : Girls

12 : 18

Divide both numbers by 6:

12 ÷ 6 : 18 ÷ 6

2 : 3

Therefore, the simplified ratio of boys to girls is 2:3.

How to Simplify a Ratio

A ratio can be simplified by dividing all of its parts by the same common factor.

The greatest common factor, or GCF, is often used to simplify a ratio completely.

Example: Simplify 20:30

Ratio = 20:30

The greatest common factor of 20 and 30 is 10.

20 ÷ 10 : 30 ÷ 10

2:3

Ratio Formula

A basic ratio compares two quantities:

Ratio = A : B

Here, A represents the first quantity and B represents the second quantity.

The order matters. For example, 2:3 is not the same ratio as 3:2.

How to Find a Ratio From Two Numbers

Suppose you want to find the ratio between 15 and 25.

Step 1: Write the numbers in order.

15:25

Step 2: Find the greatest common factor.

GCF = 5

Step 3: Divide both numbers by 5.

15 ÷ 5 : 25 ÷ 5

3:5

How to Calculate a Ratio With Three Numbers

Ratios can compare more than two quantities.

For example, suppose a basket contains:

4 apples, 6 oranges and 8 bananas.

The ratio is:

4:6:8

The greatest common factor is 2.

4 ÷ 2 : 6 ÷ 2 : 8 ÷ 2

2:3:4

Equivalent Ratios

Equivalent ratios have the same relationship between their quantities.

You can create an equivalent ratio by multiplying or dividing every part of the ratio by the same non-zero number.

Starting ratio: 2:3

Multiply both parts by 2:

4:6

Multiply both parts by 3:

6:9

Therefore:

2:3 = 4:6 = 6:9

How to Find a Missing Number in a Ratio

Ratios can also be used to find an unknown quantity.

Suppose:

2:3 = 8:x

Since 2 becomes 8 by multiplying by 4, multiply 3 by 4:

3 × 4 = 12

Therefore:

x = 12

Ratios and Fractions

A ratio can often be written as a fraction.

For example:

3:5 = 3/5

However, the meaning depends on the context. A ratio compares quantities, while a fraction can represent a part of a whole.

Ratios and Percentages

Ratios can also be converted into percentages when the quantities represent parts of a whole.

For example, suppose the ratio of red balls to blue balls is 2:3. The total number of parts is:

2 + 3 = 5

Red share:

2 ÷ 5 × 100 = 40%

Blue share:

3 ÷ 5 × 100 = 60%

Ratio Example in Cooking

Ratios are commonly used in recipes.

Suppose a recipe requires flour and sugar in a ratio of 3:1.

This means that for every 3 parts of flour, you need 1 part of sugar.

Flour : Sugar = 3:1

If you use 6 cups of flour, multiply both parts by 2:

6:2

Therefore, you need 2 cups of sugar.

Ratio Example With Money

Suppose two people divide $500 in the ratio 2:3.

First, add the ratio parts:

2 + 3 = 5

Value of one part:

$500 ÷ 5 = $100

First share:

2 × $100 = $200

Second share:

3 × $100 = $300

Ratio Example in a Classroom

Suppose a classroom has 20 students, including 8 boys and 12 girls.

The ratio of boys to girls is:

8:12

Divide both numbers by 4:

2:3

The ratio of boys to girls is therefore 2:3.

Part-to-Part vs Part-to-Whole Ratios

It is important to understand what two quantities are being compared.

Type	Example	Meaning
Part-to-Part	2:3	Compares one part with another part
Part-to-Whole	2:5	Compares one part with the total

How to Compare Two Ratios

One way to compare ratios is to convert them into fractions or use cross multiplication.

Consider:

2:3 and 3:5

Compare:

2/3 and 3/5

Cross multiply:

2 × 5 = 10

3 × 3 = 9

Since 10 is greater than 9:

2:3 is greater than 3:5

Common Mistakes When Calculating Ratios

Reversing the order of the quantities.
Dividing only one part of the ratio.
Using different factors for different parts.
Forgetting to simplify the ratio.
Confusing a part-to-part ratio with a part-to-whole ratio.
Ignoring the units when quantities are measured differently.

Ratios With Different Units

Before comparing two quantities using a ratio, make sure their units are compatible.

For example, if you want to compare 1 meter with 50 centimeters, convert them to the same unit first.

1 meter = 100 centimeters

Ratio:

100:50

Simplify:

2:1

Ratio Formula Quick Reference

Task	Method
Write a Ratio	First Quantity : Second Quantity
Simplify a Ratio	Divide every part by the same common factor
Find Equivalent Ratio	Multiply or divide every part by the same number
Find Total Parts	Add all parts of the ratio
Convert Ratio to Percentage	Part ÷ Total Parts × 100

How to Use a Ratio Calculator

If you need to calculate or simplify ratios quickly, an online ratio calculator can save time and reduce manual errors.

Open the Ratio Calculator.
Enter the quantities you want to compare.
Start the calculation.
Review the calculated or simplified ratio.

You can use our free Ratio Calculator for quick ratio calculations.

Calculate a Ratio

Frequently Asked Questions

What is a ratio?

A ratio compares two or more quantities and shows the relationship between them.

How do you calculate a ratio?

Write the quantities in the correct order and simplify them by dividing all parts by the same common factor when possible.

How do you simplify a ratio?

Divide every part of the ratio by the same common factor. For example, 12:18 becomes 2:3 when both numbers are divided by 6.

What is an equivalent ratio?

Equivalent ratios have the same relationship between their quantities. For example, 2:3, 4:6 and 6:9 are equivalent ratios.

Related Math Calculators

You may also find these free calculators useful:

Final Thoughts

Ratios provide a simple way to compare quantities. To calculate a ratio, identify the quantities, write them in the correct order and simplify them when possible.

Ratios are useful in many situations, including mathematics, recipes, classroom comparisons, money, measurements and everyday problem solving.

When you need a quick calculation, use our free Ratio Calculator on Easy Online Tool.

Disclaimer: This article is provided for general educational and informational purposes. Always check the units and context of quantities before calculating or interpreting a ratio.