Proportion Calculator

Proportion Calculator solves a/b = c/d for whichever of the four terms you leave blank. A proportion is simply a statement that two ratios are equal, and cross multiplication turns it into an equation with one multiplication and one division: a x d = b x c, so the missing term is the product of the two terms diagonally opposite it divided by the one beside it.

This is the single most useful piece of arithmetic in everyday life, even though it is rarely called by its name. Scaling a recipe, converting a currency at a known rate, working out a map distance, enlarging a photo while keeping its shape, mixing a solution to a given strength -- all of them are a proportion with one term unknown.

The check rows exist because a proportion that has been solved correctly has two equal cross products, and comparing them catches a mistyped figure immediately. The scale factor is shown too: it is the number that multiplies the first ratio into the second, which is often the figure you actually wanted.

The page shows its working, not just its answer. Where there is a formula it is printed with your own numbers substituted in, so you can check the result rather than take it on trust -- which is the difference between a calculator you can use for something that matters and one you cannot.

Everything runs inside your browser. What you type is never uploaded, never logged and never stored, and the calculator keeps working if your connection drops after the page has loaded.

• Shows missing term, its value, the ratio, scale factor and cross products together, so you are not running it twice
• A results table you can read across
• Prints the formula with your own figures substituted in
• Updates as you type, with no button to press and no page reload
• Copy the result in one click
• Free, no sign-up, and it runs entirely in your browser

The proportion calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

• Scaling a recipe from four servings to seven
• Finding a real distance from a map scale
• Resizing an image while keeping its proportions
• Working out a dose or dilution from a known ratio