Rule of Three
a / b = c / x → x = (b·c) / a
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About this calculator
Free, fast and accurate online calculators for math, finance, health, time, statistics and unit conversion. Built for the web — in your language.
Rule of Three is part of Digital Form — a free, fast, and minimal collection of online calculators built for everyday use. It runs entirely in your browser, so your inputs never leave your device.
Frequently asked questions
What is the Rule of Three?
The Rule of Three is a free online tool from Digital Form that helps you compute results quickly and accurately, in your browser, with no signup required.
Is the Rule of Three free to use?
Yes. All Digital Form calculators, including the Rule of Three, are 100% free with no limits.
How accurate is the Rule of Three?
It uses standard mathematical formulas and runs entirely in your browser, so results are deterministic and accurate.
Do I need to create an account?
No account is needed. Sign in only if you want to save favorites or your calculation history.