A cannot be zero. This is cross-multiplication, not a regression.
How this tool actually works
Cross-multiply: a over b equals c over x, so x is c times b divided by a. The form runs in the browser after you press the button. Nothing is emailed, stored, or billed. If a value is missing or impossible, the result box shows an error instead of a quiet wrong number. Refresh to start over. Keep this tab open if you want to try a second set of inputs beside the first. This page is the Proportion Calculator. People reach it with the query proportion calculator. The relationship implemented in the widget is: x = c * b / a.
A worked example
Two over five equals eight over x gives x = 20. Scale a recipe the same way. Walk the fields from top to bottom once with numbers you can re-check on paper. Then change a single input and confirm only the pieces that should move actually move. That habit catches unit mistakes faster than rereading a paragraph of instructions. This page is the Proportion Calculator. People reach it with the query proportion calculator. The relationship implemented in the widget is: x = c * b / a.
Mistakes this page is built to catch
A of zero is refused because the proportion would not be defined. Blank number boxes become zero in the browser, which is a different choice than 'unknown'. Mixing units in two fields, or filling both sides of a one-way converter, is rejected or skews the line. Read the error text before assuming the formula is wrong. This page is the Proportion Calculator. People reach it with the query proportion calculator. The relationship implemented in the widget is: x = c * b / a.
Who usually needs this number
Homework and kitchen scaling both hit this keyword. These pages exist because people search for a working widget, tap an ad, and want the answer on the same screen. The copy around the form is here so the page is not a naked doorway. It is still a calculator, not advice, not a diagnosis, and not a loan offer. This page is the Proportion Calculator. People reach it with the query proportion calculator. The relationship implemented in the widget is: x = c * b / a.
Units, rounding, and limits
Any finite B and C. A must be nonzero. Four decimals on x. Each label states the unit the field expects. The printed digits are a display rounding, not a lab certificate. Currency uses US formatting. Dates are civil YYYY-MM-DD. There is no locale switcher; if you need another convention, convert before you type. This page is the Proportion Calculator. People reach it with the query proportion calculator. The relationship implemented in the widget is: x = c * b / a.
What to do with the result
Simplifying a colon ratio without solving for a missing term is the ratio page. Copy the result line if you need it elsewhere. Double-check with a second method when the number matters. Related tools on this same site cover the nearby questions people ask after they have this answer. Official rules, prices, and medical guidance still belong to the primary source, not this page. This page is the Proportion Calculator. People reach it with the query proportion calculator. The relationship implemented in the widget is: x = c * b / a.