Ratio Calculator — Solve A : B = C : D
Know three parts of a proportion? Find the fourth. Scale a recipe for more guests, enlarge a plan to scale, or keep a concrete mix in the right proportion.
Updated
Formula
A : B = C : D ⇒ D = B × C ÷ A
This is cross-multiplication (the unitary method taught in school).
Examples
3 : 4 = 15 : ?
3 : 4 = 15 : 20
Recipe for 4 uses 250 g rice; for 10?
4 : 250 = 10 : 625
Important information
- A 1 : 2 : 4 concrete mix means 1 part cement, 2 parts sand and 4 parts aggregate by volume — the same idea extends to three-part ratios.
Frequently asked questions
How do I simplify a ratio?
Divide both sides by their greatest common factor. 12 : 18 becomes 2 : 3.
What is a proportion?
Two equal ratios, like 3 : 4 = 15 : 20.
Can ratios have decimals?
Yes. The missing value is calculated exactly; simplification is shown for whole-number ratios.
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