Rule of three calculator
Free, no sign-up. everything runs in your browser
If A is to B, then C is to…
Direct example: 3 kg cost $27 → 5 kg cost $45. Inverse example: 2 machines take 6 hours → 4 machines take 3 hours.
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How to use
1
Fill in the fields — nothing is sent to the internet.
2
Tap Calculate and see the breakdown instantly.
3
Adjust the values as many times as you want. Done — no sign-up, no e-mail, no waiting.
Frequently asked questions
You know three values of a proportion and want the fourth. If A is to B as C is to X, then X = B × C ÷ A. E.g.: 3 kg cost $27; how much do 5 kg cost? X = 27 × 5 ÷ 3 = $45.
In direct proportion, both quantities grow together (more kilos, more money). In inverse, one grows while the other shrinks (more machines, fewer hours) — then X = A × B ÷ C. The calculator has both modes and shows the math it used.
No. Everything is computed in your browser, on your own device — nothing is sent or stored.