About the Ratio Calculator
Ratio Calculator has four modes. Simplify a ratio such as 12:18 to its smallest whole numbers and see it as a decimal ratio and as percentages. Find the missing number in a proportion a : b = c : d by leaving one box empty. Share an amount in a ratio, for example 100 split 2:3, with rounding that keeps the shares adding up to the total. Or scale a ratio by any factor. Ratios can have two to six parts, and decimals up to six places are handled. Ratios do not carry units, so 2:3 is just a comparison. When shares are rounded, the leftover cent is added to the largest share, so one share can differ slightly from a plain rounding.
Updated September 2026
How to use the Ratio Calculator
- 1Choose Simplify a ratio, Find the missing number, Share an amount or Scale a ratio.
- 2Type the ratio as 12:18, 12 to 18 or with spaces, or fill the boxes.
- 3For a proportion, leave exactly one of the four boxes empty.
- 4For sharing, enter the total and pick how to round the shares.
- 5Read the answer and copy it if you like.
What a ratio is
A ratio compares quantities of the same kind. If a class has 12 girls and 18 boys, the ratio of girls to boys is 12:18, which simplifies to 2:3. It says that for every 2 girls there are 3 boys, without saying how many students there are in total.
A ratio is different from a fraction or a percentage, which describe parts of a whole. The ratio 2:3 becomes the fractions 2/5 and 3/5 of the whole, and the percentages 40 and 60. The percentage line in the results shows this link.
Simplifying step by step
- Write the parts as whole numbers; if there are decimals, multiply all parts by 10, 100 or 1,000.
- Find the greatest common factor of the parts.
- Divide each part by it.
- Check that no number greater than 1 divides all the parts.
- For the numbers themselves, GCF and LCM Calculator shows the factors step by step.
Cross-multiplying proportions
A proportion states that two ratios are equal. To find the missing value, multiply the two numbers on the diagonal that are known and divide by the number opposite the missing one. In 2 : 3 = 10 : x, the diagonal 3 × 10 is divided by 2 to give 15.
The calculator handles a missing number in any of the four places. It refuses to divide by zero and tells you which box is the problem.
Splitting amounts, recipes and mixes
Use sharing for money, costs and quantities: rent between flatmates in the ratio of their room sizes, or paint mixed in the ratio 3:1. Scale mode multiplies a ratio, which suits recipes: a 2:3 mix for twice the batch is 4:6.
When you round shares, one share may be one cent different from what plain rounding gives, so the total still matches. For percentages of a single amount, use Percentage Calculator, and for fractions use Fraction Calculator.
Features
- Simplify ratios of two to six parts
- Decimal form and percentage shares
- Missing number in a proportion
- Share an amount so the shares add up exactly
- Scale a ratio by any factor
- Accepts colons, slashes, the word to, and spaces
- Runs in your browser
Frequently asked questions
How do I simplify a ratio?
Divide every part by the greatest number that divides them all. For 12:18 that number is 6, giving 2:3. If a part has a decimal, multiply everything by a power of ten first, so 1.5:4.5 becomes 15:45 and then 1:3. Type the ratio above to see the result.
How do I solve a proportion?
Cross-multiply: in a : b = c : d, the product a × d equals b × c. To find d, compute b × c ÷ a. If 2 : 3 = 10 : x, then x = 3 × 10 ÷ 2 = 15. Choose Find the missing number and leave one box empty.
How do I split money in a ratio?
Add the parts of the ratio, divide the total by that sum, and multiply by each part. Sharing 100 in the ratio 2:3 gives 100 ÷ 5 = 20, so the shares are 40 and 60. The calculator does this and keeps the rounded shares adding up to the total.
What is the ratio of 12 to 18?
It is 2:3 in its simplest form. Twelve is two thirds of eighteen, and as decimals the ratio is 1 : 1.5. In percentages, 12 is 40 percent of the total 30 and 18 is 60 percent.
How do I turn a ratio into a percentage?
Divide each part by the sum of all the parts and multiply by 100. For 1:3 the sum is 4, so the shares are 25 percent and 75 percent. The simplify mode shows these shares under the result.
Can ratios have three parts?
Yes. A ratio can compare three or more quantities, such as 2:3:5 for a mix of three ingredients. The tool accepts up to six parts, and simplifies, scales and shares them the same way as two-part ratios.