Proportion Calculator
Solve A:B = C:D. Leave exactly one field blank, then calculate the missing value using cross multiplication.
A : B = C : D
Privacy: calculations run locally in your browser. No inputs are stored or transmitted.
How it works
Cross multiplication: A:B = C:D ⇒ A×D = B×C
Solve the missing term by rearranging:
- If A is missing: A = (B×C) ÷ D
- If B is missing: B = (A×D) ÷ C
- If C is missing: C = (A×D) ÷ B
- If D is missing: D = (B×C) ÷ A
Examples
- 2:3 = 10:D → D = 15
- A:8 = 3:12 → A = 2
- 5:B = 20:4 → B = 1
FAQ
- What is a proportion?
A proportion states that two ratios are equal, written as A:B = C:D.
- How do you solve a proportion?
Use cross multiplication: A×D = B×C. If one value is missing, rearrange to solve for it.
- How many blanks can I leave?
Leave exactly one field blank. If more than one is blank, the problem is underdetermined.
- Can I use decimals and negative numbers?
Yes. Decimals and negatives are supported. Negative values can represent directional changes or signed quantities.
- What if a divisor would be 0?
Some cases are undefined (division by zero). The tool will warn you if the missing value cannot be solved with the given inputs.
- Is this the same as percentage?
Not always. Proportions compare ratios. Percent is a ratio out of 100. You can convert a proportion to percent in some cases (e.g., part/whole).