TOOL · CALCULATOR
Rule of three calculator
If 3 kilos cost 27, how much do 5 cost? Fill in A, B and C and X appears instantly — with the math written out so you can see where it came from.
Direct: both quantities grow together — more kilos, more money.
Processed in your browser — your files never leave your computer.
How it works
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Fill in the three values
A is to B as C is to X. Enter the numbers you already know in A, B and C — comma or dot for decimals, both work.
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Pick direct or inverse
Direct when both quantities grow together; inverse when one grows while the other shrinks. The page explains which to use as you switch.
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Read X — and the math
The result shows up highlighted, with the formula filled in step by step so you can follow (and check) the path.
Frequently asked questions
When do I use the direct rule of three?
When both quantities grow together: more kilos, more money; more hours worked, more pay. It is the most common case.
And the inverse — when?
When one quantity grows while the other shrinks: more workers, fewer days of work; more speed, less travel time. The math changes in the inverse case — and the tool sets up the right formula for you.
How is the calculation done?
Direct: X = (B × C) ÷ A. Inverse: X = (A × B) ÷ C. The tool shows the formula already filled in with your numbers, exactly as you would write it on paper.
Does it work for scaling recipes?
Absolutely. Recipe serves 4 and you need 6? Direct rule of three on each ingredient and you are done.
Can I type decimals with a comma?
Yes — comma or dot, both work. The result follows your page language.
Are the numbers I type sent anywhere?
No. Everything happens in your browser — no account, no history, no network. Close the tab and it is gone.