Rule of three calculator
Direct proportion solver: if A relates to B, then C relates to X.
Find the unknown in a proportional relationship.
How to use this tool
- Enter the first pair (A and B).
- Enter the third value C.
- Read X, the proportional answer.
- Adjust for variations and compare.
Typical use cases
- Scaling a recipe up or down.
- Finding paint or fertilizer coverage from a pilot patch.
Limits & caveats
- Assumes a linear (direct) proportional relationship.
- Does not handle compound rule of three with three or more pairs.
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The rule of three solves daily proportional questions like recipe scaling, paint coverage and unit pricing without a calculator app. Inputs stay in your browser.