Proportion calculator
Solve direct and inverse proportions using the rule of three.
For a : b = c : x choose Direct. For inverse relationships use a × b = c × x.
Result
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Method, examples and limits
The direct formula is x = b × c ÷ a: if 2 items cost 10, then 5 cost 25. The inverse formula is x = a × b ÷ c: 2 people working 10 hours correspond to 5 people working 4 hours, assuming equal productivity. The divisor must be nonzero. Use a dot or comma for decimals, without thousands separators.
Example and interpretation
For a direct proportion, enter three values using consistent units. If 3 items cost 12 euros, 5 items cost 20 euros: use a=3, b=12 and c=5. Choose inverse proportion when increasing one quantity decreases the other. Decide which relationship describes your problem before using the answer.
Frequently asked questions
When should I use inverse proportion?
When the product of the two quantities stays constant: the model uses a × b = c × x.
Is this data saved?
The values you enter stay on this page. Reloading or closing it clears them.