About this ratio calculator
Two jobs from the same four boxes. Enter A and B on their own and the ratio is reduced to its lowest terms — 12:18 becomes 2:3. Enter any three of A, B, C and D and the fourth is solved, which is what a proportion is: 3:4 = 9:? gives 12. That second case is the one most people arrive wanting, whether they are scaling a recipe, working out a mix, or resizing something while keeping its shape. The answer appears with the completed proportion beside it so you can check it reads correctly, and the arithmetic runs in your browser.
How the ratio calculator works
Simplifying divides both sides by their greatest common divisor, found by the Euclidean algorithm, so 12:18 reduces to 2:3. Solving a proportion uses the cross product: A : B = C : D means A × D = B × C. Rearranging that for whichever box you left empty gives D = B × C ÷ A, C = A × D ÷ B, B = A × D ÷ C, or A = B × C ÷ D. Where that rearrangement would divide by zero the proportion has no answer, and it says so rather than returning something meaningless.
Ratio Calculator examples
A 12, B 18, C and D blank
2:3 — the ratio in lowest terms
A 3, B 4, C 9, D blank
D = 12, shown as 3 : 4 = 9 : 12
A 2, B 3, C 7, D blank
D = 10.5 — values that do not divide evenly are kept as decimals
A blank, B 4, C 9, D 12
A = 3 — any one of the four can be the unknown
Limitations & privacy
Simplifying takes whole numbers from 0 to 1 billion; solving a proportion accepts decimals in the same range. Results are rounded to six decimal places. It handles ratios between two quantities. It is not a calculator for the named ratios that have their own formulas — gear ratio, compression ratio, golden ratio, debt-to-income — each of which needs inputs this does not ask for. A ratio with more than two parts, like 2:3:5, is outside what this solves.
Inputs and results stay in this browser. Only tool identifiers are stored for your recently used tools. You can clear that history from the directory.
Ratio Calculator: frequently asked questions
How do I solve a ratio with a missing number?
Fill in the three values you have and leave the unknown box empty. For 3:4 = 9:?, enter A 3, B 4, C 9 and leave D blank; the answer is 12. It works whichever of the four is missing.
How do I simplify a ratio?
Enter the two numbers as A and B and leave C and D empty. Both sides are divided by their greatest common divisor, so 12:18 comes back as 2:3.
What is a proportion?
Two ratios that are equal: A : B = C : D. It holds when the cross products match, so A × D = B × C. That identity is what lets any one of the four be worked out from the other three.
Can it work out a gear ratio or compression ratio?
No. Those share the word but not the calculation — a gear ratio comes from tooth counts, a compression ratio from swept and clearance volumes, and each needs inputs this does not ask for. This handles the ratio between two quantities you already have.
Can I use decimals?
Yes, when solving a proportion. Simplifying to lowest terms needs whole numbers, since a greatest common divisor is only defined for integers.
Can it handle three-part ratios like 2:3:5?
No. It works on two quantities. A three-part ratio is usually handled by taking it two at a time, or by scaling each part against the total.
Updated October 19, 2026