About the Ratio calculator
This ratio calculator solves proportions of the form A : B = C : D. Fill in any three boxes and leave one empty, and it works out the missing value by cross-multiplying. Fill in all four and it simplifies both ratios and tells you whether they are equal.
Ratios come up everywhere: scaling a recipe, mixing paint or concrete, reading a map scale, resizing an image, or sharing money in a set proportion. Students use it to check proportion problems, and the steps show the cross-multiplication with your own numbers.
How to use it
- Enter the first ratio in A and B.
- Enter the second ratio in C and D, leaving the one you want to find empty.
- Read the missing value and the full proportion.
- To simplify or compare two ratios, fill in all four boxes.
The formula
Two ratios are equal when their cross products are equal:
A : B = C : D ⇔ A × D = B × C
So the missing value is:
- D = B × C ÷ A
- C = A × D ÷ B
- B = A × D ÷ C
- A = B × C ÷ D
To simplify a ratio of whole numbers, divide both parts by their greatest common factor: 12 : 18 = 2 : 3. The scale factor C ÷ A tells you how much bigger (or smaller) the second ratio is than the first.
Worked example: scaling a recipe
A recipe uses 3 cups of flour for every 4 eggs. How many eggs do you need for 15 cups of flour? Set it up as 3 : 4 = 15 : D.
Cross-multiplying gives 3 × D = 4 × 15, so D = 60 ÷ 3 = 20 eggs. The scale factor is 15 ÷ 3 = 5, so everything in the recipe is multiplied by 5.
The same method works for maps. On a 1 : 50,000 map, 4 cm represents 4 × 50,000 = 200,000 cm, which is 2 km. It also works for prices: if 3 notebooks cost $4.50, then 8 cost 4.50 × 8 ÷ 3 = $12. For comparing value across pack sizes, see the unit price calculator.
Tips and common mistakes
Keep the order the same. If the first ratio is flour : eggs, the second must also be flour : eggs. Swapping the order on one side gives the wrong answer.
Part-to-part or part-to-whole? A class with 12 girls and 18 boys has a girls-to-boys ratio of 12 : 18 = 2 : 3, but the girls are 12 out of 30 students, or 2 : 5 of the whole class.
Decimals. When a ratio contains decimals, the calculator shows it as a value to 1, so 1.5 : 2.5 appears as 0.6 : 1. To get whole numbers, multiply both parts by 10 first and enter 15 : 25, which simplifies to 3 : 5.
Frequently asked questions
How do I solve a proportion?
Cross-multiply and divide. For 2 : 5 = x : 30, x = 2 × 30 ÷ 5 = 12.
How do you simplify a ratio?
Divide both numbers by their greatest common factor. 24 : 36 has a GCF of 12, so it simplifies to 2 : 3.
How do I know if two ratios are equal?
Multiply across: if A × D equals B × C, the ratios are equal. Fill in all four boxes and the calculator checks this for you.
How do you split an amount in a given ratio?
Add the parts, divide the amount by that total, then multiply by each part. To share $100 in the ratio 2 : 3, each part is $100 ÷ 5 = $20, so the shares are $40 and $60.
Can a ratio have more than two parts?
Yes, like 1 : 2 : 3 for a mix. This calculator solves two-part proportions, but you can handle each pair separately.
Last updated October 24, 2026. Results are estimates for education and planning — see our disclaimer.