Proportion Calculator
Proportion calculator: solve a/b = c/x for the missing value and see the cross-multiplication step.
Editorial review: Farrow (pseudonym; identity and professional credentials are not disclosed). This page is for educational purposes, not professional advice.
Updated
Your inputs
Results update as you type. Nothing you enter leaves your device.
Results
- Cross products
- 2 x x = 30
- Scale factor
- 3
Example with the starting values
These results use the calculator's initial inputs. Change the values above to calculate your own estimate.
Inputs
- A
- 2
- B
- 5
- C
- 6
Results
- Unknown x
- 15
- Cross products
- 2 x x = 30
- Scale factor
- 3
How does the Proportion Calculator work?
Two ratios are proportional when a/b = c/d. Cross multiplying gives a times d = b times c, so the missing value d is b times c divided by a.
The scale factor c/a shows how much the first ratio was enlarged or shrunk.
Frequently asked questions
How do you solve a proportion?
Cross multiply and divide. For 2/5 = 6/x, x = 5 x 6 / 2 = 15.
What is a scale factor?
The number you multiply the first ratio by to get the second, here c divided by a.
Where are proportions used?
Recipes, maps, unit prices, similar triangles and medicine doses.
Can a proportion have decimals or negatives?
Yes. Cross multiplication works with any nonzero first value, including decimals and negatives.