Ratio Calculator
Simplify a ratio, solve a missing value in a proportion A:B = C:D, or split a total in a given ratio
Last updated: November 4, 2026
How to Use the Ratio Calculator
How to use the ratio calculator
To simplify a ratio
- Choose Simplify a ratio.
- Type the values, such as 12 and 18. Add a third value for a three-part ratio.
- Read the simplest form (2:3), the decimal value of A ÷ B and each part as a percentage.
To solve a proportion
- Choose Solve a proportion (A:B = C:D).
- Fill in three of the four boxes and leave the one you want to find empty.
- Read the missing value, the completed proportion and the cross-multiplication check.
To split a total
- Choose Split a total in a ratio.
- Enter the parts of the ratio (up to three) and the total amount.
- Read the amount for each part in the result and in the table.
Switching modes loads a ready-made example, so you can see how each one works before you type your own numbers.
Formula & Methodology
What is a ratio?
A ratio compares two or more quantities. The ratio of 12 to 18 is written 12:18 or 12/18. Like a fraction, a ratio does not change when both sides are multiplied or divided by the same number, which is why 12:18, 6:9 and 2:3 all describe the same relationship.
How to simplify a ratio
Divide every term by their greatest common factor (GCF).
Example: simplify 12:18
- List the factors: 12 has 1, 2, 3, 4, 6, 12. 18 has 1, 2, 3, 6, 9, 18.
- The greatest common factor is 6.
- Divide both sides: 12 ÷ 6 = 2 and 18 ÷ 6 = 3.
- The simplified ratio is 2:3.
For decimals, multiply every term by 10, 100 or 1,000 first so they become whole numbers. For example 0.5:1.5 becomes 5:15, which simplifies to 1:3. A three-part ratio such as 12:18:24 simplifies to 2:3:4 in the same way.
How to solve a proportion
A proportion states that two ratios are equal: A:B = C:D. Cross-multiply to find a missing value:
A × D = B × C
So if D is missing, D = B × C ÷ A.
Example: 4:7 = x:21
- Cross-multiply: 4 × 21 = 7 × x
- 84 = 7x
- x = 84 ÷ 7 = 12
The answer is exact when it does not divide evenly, so the calculator also shows it as a fraction. For example 2:3 = 5:x gives x = 7.5 (15/2).
How to split an amount in a ratio
- Add the parts of the ratio: 2 + 3 = 5
- Divide the total by that sum to find one part: 100 ÷ 5 = 20
- Multiply each part: 2 × 20 = 40 and 3 × 20 = 60
So $100 shared in the ratio 2:3 becomes $40 and $60.
Ratio as a percentage or fraction
Each part of a ratio is a share of the whole. In 2:3 the first part is 2 out of 5 parts, which is 2/5 = 40%, and the second is 3/5 = 60%.
About Ratio Calculator
Our ratio calculator is a free, easy-to-use tool designed to help yousimplify a ratio, solve a missing value in a proportion a:b = c:d, or split a total in a given ratio. If you're a professional or just need quick calculations, our calculator provides accurate results instantly.
Frequently Asked Questions
How do you calculate a ratio?
To compare two quantities, write them as A:B. To keep it simple, divide both by their greatest common factor. For example 20:30 becomes 2:3 when you divide both by 10.
How do you simplify a ratio?
Find the greatest common factor of all the terms and divide each term by it. For 12:18 the greatest common factor is 6, so the ratio simplifies to 2:3. If one of the numbers is a decimal, multiply every term by 10, 100 or 1,000 first.
How do you find a missing value in a proportion?
Cross-multiply and divide. In A:B = C:D, multiply A by D and B by C, which are equal. Then solve for the unknown. For 4:7 = x:21, x = 4 × 21 ÷ 7 = 12.
How do you split an amount by a ratio?
Add the parts of the ratio, divide the total by that sum, and multiply the result by each part. To share 100 in the ratio 2:3, add 2 + 3 = 5, so one part is 20, giving 40 and 60.
What is the difference between a ratio and a fraction?
A ratio compares parts (2:3 means 2 parts to 3 parts), while a fraction compares a part to the whole (2/5 means 2 parts out of 5 in total). You can convert between them: the ratio 2:3 means the first part is 2/5 of the total.
How do you convert a ratio to a percentage?
Divide each part by the sum of all parts and multiply by 100. For 2:3, the parts are 2/5 = 40% and 3/5 = 60%. The calculator shows these shares for you in simplify mode.
Can a ratio have more than two numbers?
Yes. A three-part ratio such as 12:18:24 compares three quantities and simplifies the same way, to 2:3:4 here. You can use up to three values for simplifying and splitting.
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