Two ratios are proportional when they are equal. Knowing three of the four values lets you find the fourth.
How to use the Proportion Calculator
- a
- b
- c
Results update as you type, and every field accepts the units shown in its menu.
How it works
Multiply diagonally across the equals sign and divide by the remaining value.
Inputs explained
| Field | What to enter |
|---|---|
| a | A number |
| b | A number |
| c | A number |
Example values
How the main result changes when a changes and everything else stays at the example values above:
| a | x |
|---|---|
| 1.5 | 64 |
| 2.25 | 42.6667 |
| 3 | 32 |
| 3.75 | 25.6 |
| 4.5 | 21.3333 |
| 6 | 16 |
Formula
x = b × c ÷ aWorked example
Inputs — a: 3; b: 8; c: 12.
Results — x = 32.
Frequently asked questions
What is cross multiplication?
In a/b = c/x, a × x = b × c.
When do proportions not apply?
When quantities do not change at a constant rate.
What formula does the Proportion Calculator use?
x = b × c ÷ a.