Proportion Solver

A proportion is a statement that two ratios are equal, written a : b = c : d.

Proportions are everywhere in everyday maths: scaling a recipe, converting map distances, working out unit prices, resizing images while keeping their aspect ratio, or mixing paint and chemicals to a fixed ratio.

Whenever you know three of the four quantities, the fourth is completely determined, and this solver finds it for you.

The maths behind it is cross-multiplication.

Because a : b = c : d is the same as the equation a × d = b × c, you can rearrange to isolate whichever term is unknown — for example d = b × c ÷ a.

Enter the three values you know and leave the fourth box blank, or type the letter x, to mark it as the one to solve for.

The tool fills in the missing number, shows the completed proportion, gives the decimal value of the ratio, and prints the worked cross-multiplication so you can follow the steps.

It guards the awkward cases too: if the term you are solving for would require dividing by zero, you get a clear message instead of an infinity, and non-numeric entries are flagged rather than silently ignored.

Everything runs locally in your browser, so it works offline and nothing you type is ever uploaded.