Rule of three formula
For direct proportion, X = (B × C) ÷ A. For inverse proportion, X = (A × B) ÷ C. Use direct when both values move together and inverse when one rises while the other falls.
Rule of three calculator
Enter three values in a proportion to find the fourth. Choose direct or inverse proportion and get the result instantly.
Quick example
If 2 items cost $10, then 5 items cost $25 in direct proportion. If 2 people finish a task in 10 hours, 5 people finish it in 4 hours in inverse proportion.
When to use this calculator
Use it for proportional prices, recipes, scale, productivity, speed, time, consumption, studying, and any simple proportion problem. It helps avoid the common mistake of mixing direct and inverse proportion.
Frequently asked questions
What is the rule of three?
It is a method for finding an unknown value in a proportion using three known values.
When should I use direct proportion?
Use it when both quantities increase or decrease together. Example: more items cost more money.
When should I use inverse proportion?
Use it when one quantity increases while the other decreases. Example: more people can finish the same task in less time.
Can the first value be zero?
No. The base value of the proportion must be different from zero for the calculation to make sense.
Can this calculator solve percentage problems?
Yes, many percentage problems can be solved with the rule of three. For dedicated percentage cases, use the percentage calculator too.
Is the result rounded?
The result is displayed with up to 8 decimal places to keep it readable without hiding precision.