Enter three known values of a proportional relationship to find the fourth.
Formula
x = B × C / A
Worked example
If 4 kg cost 10, then 6 kg cost 10 × 6 / 4 = 15.
Frequently asked questions
When can I use the rule of three?
Only when the two quantities are proportional, such as weight and price at a fixed price per kilo.
What is an inverse rule of three?
When one quantity rises as the other falls, such as workers and time. Then x = A × B / C.
Why is it also called cross-multiplication?
Because you multiply the diagonal values B and C, then divide by A.
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Results are estimates for information only. Check important figures with a professional.