Rule of Three Calculator is an online calculator and learning resource focused on solving proportional relationships. Its tools handle both direct proportion, where two quantities rise or fall together, and inverse proportion, where one increases as the other decreases. Users enter three known values to calculate an unknown, with formulas and step-by-step explanations to make the working easier to follow.

The site offers examples, word problems, practice exercises, and a worksheet generator with printable PDFs. Its educational content explains how to choose between direct and inverse relationships, match units, apply the formulas, and check results. Examples cover everyday scenarios such as comparing prices, scaling recipes, estimating travel time, and relating workers to completion time.

Designed for students, teachers, and anyone working with proportional calculations, the resource also includes ratio and compound calculators, games, and an embed builder. The calculator is presented as free to use and supports decimal and large-number inputs. The website is available in multiple languages, including English, Spanish, French, German, Italian, Portuguese, Hindi, Chinese, and Japanese.