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Key Features
Instant Calculation
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Cross multiplication solves proportions of the form a/b = c/d for whichever single value is unknown. This tool accepts any three of the four values and solves for the missing one.
How it works
Cross multiplying a/b = c/d gives a×d = b×c. Whichever of a, b, c, d is left blank, the tool rearranges this equation to solve for it directly.
- Enter a (leave blank if unknown).
- Enter b (leave blank if unknown).
- Enter c (leave blank if unknown).
- Enter d (leave blank if unknown).
- Click Calculate to see your results.
Examples
A recipe scaling proportion
If 3 cups of flour makes 4 servings, how many cups make 6 servings? Setting up 3/4 = x/6 and cross-multiplying gives x = (3×6)/4 = 4.5 cups.
Who should use it
- Scaling recipe ingredient quantities.
- Solving similar-triangle or map-scale ratio problems.
Industry applications
- Education (algebra and ratio coursework)
- Cooking and recipe scaling
- Cartography and scale modeling
Advantages
- Solves for any of the four unknowns in a proportion.
- Shows the cross-multiplication equation explicitly.
Limitations
- Only solves simple proportions — does not handle systems of multiple simultaneous proportions.
Common mistakes to avoid
- Setting up the proportion with mismatched units or in the wrong order (e.g. flipping which quantity is the numerator vs. denominator) before cross-multiplying.
Best practices
- Always double-check that both ratios in the proportion are set up in the same order (e.g. both as "flour over servings") before solving.
Tips
- Always set up your proportion with corresponding quantities in the same position (numerator vs. denominator) on both sides before cross-multiplying.