Ratio Calculator
Solve proportions (A : B = C : D) for a missing value and simplify ratios.
Reviewed October 7, 2026 Runs in your browser — inputs are never stored
Solve A : B = C : D. Leave C or D blank to solve for it.
D =
12
Simplified A : B = 3 : 4
This tool performs standard mathematical operations for educational and reference purposes. Always double-check results for critical academic or professional work.
About the Ratio Calculator
The ratio calculator solves proportions of the form A : B = C : D. Enter any three values and it finds the fourth by cross-multiplication. Proportions appear in recipes, maps and scale drawings, screen aspect ratios, mixing concentrates and many school problems. Leave the unknown field empty and the calculator fills it in.
How to use it
- 1Enter three of the four values in A : B = C : D.
- 2Leave the value you want to find empty.
- 3Press Calculate to solve for the missing value.
Worked example
3 : 4 = 9 : D gives D = 4 × 9 ÷ 3 = 12. If a recipe uses 3 cups of flour for 4 servings, 9 cups makes 12 servings.
How it works
How it works
Cross-multiplication solves the proportion: if A/B = C/D then D = B·C/A.
Who it is for
Students, cooks, designers resizing images and anyone scaling quantities.
Why it helps
Cross-multiplication is easy to set up the wrong way round; the calculator keeps the terms in order.
Common use cases
- Scaling recipes up or down
- Resizing images while keeping the aspect ratio
- Working out map or model scales
Frequently asked questions
A:Enter the three known values and leave the unknown (C or D) blank.
A:Yes, it reduces A : B to lowest terms.
Reviewed for accuracy · Last updated October 7, 2026
Calculations use standard, well-established mathematical identities and algorithms with no approximation beyond normal floating-point precision.
CalcWisee Editorial Team
Calculator development and formula review
CalcWisee is an independent site founded by Ayman. Every calculator is built from a published formula or official definition, tested against hand-worked examples before it goes live, and corrected promptly when a reader reports an issue to ayman@calcwisee.com.
Expertise: Financial mathematics, Health and fitness formulas, Applied mathematics, Unit conversion standards
References & sources
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