Proportion Calculator
Solve proportions a/b = c/d for the fourth term — recipes, maps, dosages scale correctly. Double a 4-serving recipe to 10 without breaking ratios — free, runs locally.
How to Use the Proportion Calculator
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Frequently Asked Questions
How do I scale a recipe with proportions?
Set servings over ingredients with the unknown last: A=4, B=500g flour, C=10 gives D = (500 × 10) ÷ 4 = 1250g. The tool cross-multiplies for the fourth term — scaling that never breaks the taste balance.
What is cross-multiplication?
For a/b = c/d, a×d equals b×c — the tool solves that equation for the fourth term D only. Rearrange by hand first if a different term is the unknown.
Can it handle percentages as ratios?
Yes — 30% is the ratio 30/100. Put the unknown last: A=100, B=450, C=30 gives D = (450 × 30) ÷ 100 = 135.
Do my recipe numbers leave the browser?
No — solving runs locally in your browser. Numbers never leave your device.
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Last updated October 2026