Everyday
Rule of Three Calculator
Solve a direct rule-of-three problem when A corresponds to B and C corresponds to the unknown X.
- Free to use
- Runs in your browser
- Level A · Standard formula
- Last reviewed 2026-08-16
Calculator guide
How to use the Rule of Three Calculator
Solve a direct rule-of-three problem when A corresponds to B and C corresponds to the unknown X. Enter the requested values and select Calculate. The result and supporting values are produced locally in your browser.
Formula
Worked example
If 4 units correspond to 10, then 7 units correspond to 17.5.
What to keep in mind
- This calculator assumes a direct proportional relationship.
- For inverse proportional relationships, the setup must be adjusted.
Methodology
This calculator uses deterministic arithmetic and performs the calculation locally in your browser. Verify assumptions, units, and source values for your specific use case.
Review standard: Level A · Standard formula. How calculator reviews work.
Frequently asked questions
What does this calculator calculate?
Solve a direct rule-of-three problem when A corresponds to B and C corresponds to the unknown X.
How is the result calculated?
X = B × C ÷ A.
Can I use the result for an important decision?
Use the result as a calculation aid and verify the inputs, assumptions, and applicable professional or regulatory requirements before relying on it for an important decision.
Does CalculateMeasure store the values I enter?
The calculator performs its arithmetic in your browser. The calculator itself does not need to send your entered values to a server to produce the result.