Proportion Calculator
A proportion states two ratios are equal: A/B = C/D. Enter any three values and instantly solve for the fourth.
How It Works
Enter three of four values (leave one blank). Applies cross-multiplication to solve for the missing value.
Formula
A/B = C/D → A × D = B × C
Example
3/4 = X/20:
3×20 = 4×X → X = 60/4 = 15
When to Use This Calculator
Use for scaling recipes, solving math homework, converting unit rates, or any two equivalent ratio situation.
Common Mistakes to Avoid
- Setting up proportion incorrectly — ensure both ratios compare the same types.
- Forgetting units — both ratios must use matching units.
- Using proportion for non-proportional relationships — rate must be constant.
Frequently Asked Questions
What is cross-multiplication?
Multiply numerator of one fraction by denominator of the other: A/B = C/D → A×D = B×C.
How to check if ratios are proportional?
Cross products must be equal. For 2/3 and 4/6: 2×6=12 and 3×4=12. Equal = proportional.
Direct vs inverse proportion?
Direct: both increase together. Inverse: as one increases, the other decreases.
Can I enter decimals?
Yes — the calculator handles decimal values correctly.
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Last Updated: July 4, 2026