Proportion Calculator
Solve the proportion A/B = C/x for the missing value using cross-multiplication. This free online math calculator runs entirely in your browser — no signup, no data sent anywhere.
Inputs
Results
How It Works (Formula & Method)
A proportion states that two ratios are equal: A / B = C / x. To solve for the unknown x, cross-multiply: A × x = B × C, so x = (B × C) ÷ A. The calculator also reports the decimal value of A / B and verifies that C / x reproduces the same decimal, confirming the two ratios match.
For example, if 3 / 4 = 9 / x, then x = (4 × 9) ÷ 3 = 12, and indeed 9 / 12 = 0.75 = 3 / 4.
Worked Example
Below is a worked example using the calculator's default values. The same numbers are pre-filled in the form above so you can press Calculate and see the result without typing anything.
Inputs used:
- A (in A / B): 3
- B (in A / B): 4
- C (in C / x): 9
With these inputs, the calculator computes the metrics shown in the Results panel. Change any value and press Calculate again to see how the result responds — the live widget and the chart both update instantly.
Tips & Considerations
Proportions are the backbone of scaling recipes, converting units, reading maps, and solving similar-triangle problems. If A is zero the proportion has no finite solution, since you cannot divide by zero.
Frequently Asked Questions
What is cross-multiplication?
Cross-multiplication solves a proportion A/B = C/D by multiplying diagonally: A × D = B × C. Rearranging lets you solve for whichever value is unknown. Here the unknown is x in A/B = C/x, giving x = (B × C) ÷ A.
What are proportions used for?
Proportions scale quantities up or down while keeping their relationship fixed — adjusting recipe ingredients, converting between units, computing map distances, and solving similar-triangle and mixture problems.
What does the Proportion Calculator compute?
The Proportion Calculator takes 3 input values and returns 3 results. Solve a proportion of the form A/B = C/x for the unknown value using cross-multiplication, with a decimal ratio and a verification check.
Is my data sent to a server?
No. The Proportion Calculator runs entirely in your browser using static JavaScript. Your inputs are never transmitted to CalculatorHive or any third party, and nothing is stored after you close the page.