Ratio Calculator
Enter A and B to simplify the ratio. Add C or D as well, leaving the other empty, and we solve the missing number.
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The formula
A : B = C : D, so D = (B x C) / A and C = (A x D) / B.
Simplifying a ratio
Divide both sides by their greatest common factor. 12 : 18 becomes 2 : 3 because 6 goes into both. A ratio in simplest form is easier to compare and to scale.
Solving for a missing number
This is the classic "if 3 is to 4 as 9 is to what?" question. Cross-multiply: 3 x D = 4 x 9, so D = 12. Fill in A = 3, B = 4, C = 9 and leave D empty.
Everyday uses
- Recipes: a sauce uses 2 cups of stock for 3 servings. How much for 8? 2 : 3 = x : 8 gives about 5.33 cups.
- Mixing: paint, concrete or garden feed often come as a ratio such as 1 : 4.
- Maps and models: a scale of 1 : 50,000 means 1 cm on the map is 500 m on the ground.
- Screens: 16 : 9 is how wide a video is compared with its height.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| A: 3; B: 4; C: 9 | D: 12 |
| A: 12; B: 18 | Simplest form: 2 : 3 |
| A: 2; B: 5; D: 20 | C: 8 |
| A: 16; B: 9; C: 48 | D: 27 |
Ratio Calculator: frequently asked questions
What is the difference between a ratio and a fraction?
A ratio compares parts to each other (boys : girls = 3 : 2). A fraction compares a part to the whole (3/5 of the class are boys). You can convert between them by adding the parts.
Can I use decimals?
Yes. If the numbers are not whole, the simplest form is shown as a decimal out of 1, for example 2.5 : 1.
How accurate is this calculator?
It uses standard double-precision arithmetic, accurate to about 15 significant digits. Results are rounded only for display.
Is what I enter in this calculator saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this calculator free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026