Ratio calculator
Calculator
The calculator needs JavaScript. The tables below give the same numbers for common cases.
How it is worked out
- Cross-multiply: A × x = B × C, so x = (B × C) ÷ A.
- x = (4 × 9) ÷ 3 = 12.
- Check: 9 ÷ 12 = 0.75 and 3 ÷ 4 = 0.75.
| Inputs | Answer |
|---|---|
| A: 3; B: 4; C: 9 | If 3 : 4 = 9 : x, then x = 12. |
| A: 2; B: 5; C: 14 | If 2 : 5 = 14 : x, then x = 35. |
| A: 1; B: 8; C: 3 | If 1 : 8 = 3 : x, then x = 24. |
How to use it
Two ratios are equal when their cross products are equal: A × x = B × C. Solving gives x = (B × C) ÷ A. If 3 : 4 = 9 : x, then x = 4 × 9 ÷ 3 = 12.
The calculator also shows A : B in simplest form when both are whole numbers. Use it for scaling recipes, maps and mixtures.
Frequently asked questions
How do I solve a proportion?
Cross-multiply and divide: for A : B = C : x, x = B × C ÷ A.
How do I simplify a ratio?
Divide both numbers by their greatest common factor: 12 : 18 simplifies to 2 : 3.
What is the difference between a ratio and a fraction?
A ratio compares two quantities (3 : 4); a fraction (3/4) is a part of a whole. They are written with the same numbers but mean different things.
Method: standard arithmetic shown step by step; no outside data is used. Checked 2026-11-12.
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General information only. Results are plain arithmetic on the numbers you enter; check anything important (money, health, school work) against your own records or the official source.