Rule of Three Calculator
Solve simple proportions with rule of three. Fill 3 values to find the 4th.
About this tool
The Rule of Three Calculator solves simple proportions in the form A/B = C/X. Fill in any three of the four values — A, B, C, and X — and the fourth is calculated automatically. The visual layout in fraction form makes the relationship being calculated immediately clear.
To use it, fill in the fields with the values you know and leave blank the one you want to find. The result appears instantly in place of the empty field, with visual highlighting to distinguish it from the inputs. A clear button resets all fields for a new calculation.
The rule of three is one of the most widely used mathematical calculations in practice, with applications across virtually every field: calculating the proportional price for a different quantity of a product, figuring out how much of an ingredient to use when scaling a recipe up or down, converting scales on maps and architectural drawings, calculating proportional dosages in nursing or agriculture, verifying equivalences in nutritional tables, and solving speed, time, and distance problems for travel planning.
All processing is local — no values are sent to servers. The tool is free and works on any device with a modern browser.
Frequently asked questions
Is the rule of three calculator free?
Yes, completely free.
Are the values I enter sent to any server?
No. The calculation happens locally in the browser. No data is transmitted.
Does it work on mobile?
Yes. The interface adapts to small screens and works in any modern browser.
Does the tool handle inverse proportions?
The tool solves direct proportion (A/B = C/X). For inverse proportions, rearrange the values in the equation before entering them.
Can I leave any of the four fields blank?
Yes. Fill in the three known values and leave the unknown blank — the tool automatically solves for any one of the four fields.