INTERACTIVE TOOL
Rule of three calculator
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How to use this tool
- Enter the known pair A and B.
- Enter C in the same unit as A.
- Calculate the corresponding value in the unit of B.
Formula or method usedIf A corresponds to B and C corresponds to X, then X = B × C ÷ A.
Worked example
If three units cost 15, seven units at the same unit price cost 35.
Assumptions and limits
This is direct proportion, not inverse proportion or a nonlinear relationship.
Frequently asked questions
Does it work for inverse proportions?+
This version calculates direct proportions. For an inverse relationship, rearrange the values first.
What happens to my inputs?+
Calculation inputs are processed locally in your browser. Favorites and preferences may be saved on this device. The site may perform separate network requests for live panels; these do not include the values entered into this local calculator. Do not paste secrets into public tools.
How should I check the result?+
This is direct proportion, not inverse proportion or a nonlinear relationship.