CalcWhenRatio Calculator
If 2 : 3 = 10 : d, then d = 15 — the same proportion scaled by five, which is the whole trick of recipes, maps and models.
How it works
Two jobs in one tool. Solve mode completes a proportion: you know three of the four values in a : b = c : d and it finds the fourth — doubling a recipe from its ratio, scaling a photo, converting a model's dimensions. Simplify mode reduces a ratio to lowest terms, the form screens, gears and mix instructions are quoted in (1920 : 1080 becomes 16 : 9).
The solve panel shows the cross-multiplication check — a × d equals b × c — so the answer arrives with its own proof attached.
The method
Solve: d = b × c ÷ a (equivalent fractions). Simplify: divide both sides by their greatest common divisor (Euclid's algorithm). Decimal and fraction forms are shown alongside for cross-checking.
Frequently asked questions
How do I scale a recipe with a ratio?
If the original serves use 2 : 3 (flour : liquid) and you have 10 cups of flour, put a=2, b=3, c=10 — d=15 cups of liquid keeps the bake identical, just bigger.
What is 16 : 9 as a ratio about?
It is 1920×1080 (and 3840×2160) reduced by the common factor 120. Simplify mode is how such marketing ratios are verified.
Can ratios have decimals?
Yes — the solver accepts decimals throughout. Simplify mode rounds to whole numbers first, since lowest-terms ratios are a whole-number concept.