Proportion Calculator

A proportion is a simple but powerful idea: two ratios that are equal to one another, written as a/b = c/d. It says that the first pair of numbers relates in exactly the same way as the second pair, and that single fact lets you scale recipes, convert between units, resize drawings, read maps, and work out percentages without ever guessing. This calculator solves a proportion for whichever value you do not know. Enter three of the four numbers, leave the fourth box empty, and the calculator finds the missing value, shows you the completed proportion, and displays both cross-products so you can see that the equation balances. The method behind it is cross multiplication. Because the two ratios are equal, multiplying diagonally across the equals sign gives equal results: a times d must equal b times c. From that one relationship you can isolate any of the four letters. If d is unknown, d equals b times c divided by a; if a is unknown, a equals b times c divided by d; and the same pattern handles b and c. The calculator works out which box you left blank, applies the right rearrangement, and guards against dividing by zero so you always get a sensible answer or a gentle prompt. Cross multiplication is the same rule you used at school to compare fractions, just put to work in reverse. This tool is built for students learning ratios and proportions, cooks adjusting quantities, makers scaling plans, and anyone who needs to find a missing value quickly and check the working at a glance. Type your three known values below and the answer updates instantly.

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