Proportion Calculator
Solve a proportion a/b = c/d for the missing value (the rule of three) by cross-multiplication.
Your inputs
Results
D =
12
- Proportion
- 3 / 4 = 9 / 12
✓Exact calculation — rounded only for display
What this calculator does
If two ratios are equal and you know three of the four numbers, cross-multiplication finds the fourth. This is the classic “rule of three”.
Formula and method
a/b = c/d ⇒ a × d = b × c; solve for the blank value.
Worked example
Calculated by this tool with its default inputs:
Inputs
- A: 3
- B: 4
- C: 9
- D (leave exactly one field empty):
Results
- D =: 12
- Proportion: 3 / 4 = 9 / 12
How to read the results
Check that the relationship really is proportional before relying on the answer.
Assumptions and limitations
- Leave exactly one field empty.
Questions
What is the rule of three?
A method to find a fourth value in a proportion from the other three.
Methodology and verification
- Result type
- Exact mathematical calculation (rounded only for display)
- Formula version
- 1.0
- Automated verification
- 4 reference and validation test cases, checked against independently computed values. The site cannot be built if any of them fail.
- Editorial review
- Awaiting human editorial review. The explanation was drafted with AI assistance; the formula and examples are verified by the automated tests above.
Privacy
Everything you enter is calculated in this browser and is not sent to Numerith or anyone else. Inputs are added to a share link only if you choose to include them. Privacy policy