Free Proportion Calculator
Working with a proportion a/b = c/d? Enter any three values and leave the unknown blank — the calculator cross-multiplies and solves it, then verifies both sides match.
How to Use the Proportion Calculator
- Fill in three of the four values (a, b, c, d).
- Leave the unknown one blank.
- Click Solve Proportion.
Features
Applies cross-multiplication (a×d = b×c) to isolate the missing variable, validates that exactly one field is blank, and shows both ratios as a self-check.
Benefits & Use Cases
Students solve ratio homework, cooks scale recipes (“if 2 cups serve 4, how much for 10?”), and map readers convert scale distances.
Tips
Write the proportion so matching units align vertically (cups over servings = cups over servings) — misaligned setups are the top error.
Frequently Asked Questions
How do you solve a proportion?
Cross-multiply: a×d = b×c, then divide to isolate the unknown.
What if I fill all four values?
The tool asks you to leave exactly one blank — it solves for unknowns, it doesn’t verify complete proportions.
Can it handle decimals?
Yes — decimals and fractions-as-decimals work in any position.
Why must b and d be non-zero?
Division by zero is undefined, and a zero denominator breaks the proportion.
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