Rule of Three Calculator

Use it when two quantities have a known proportional relationship and you know three corresponding values. For example, if 3 notebooks cost $6, you can find the cost of 9 notebooks.

Do not use it when the quantities do not have a consistent proportional relationship. Fixed fees, discounts that change by quantity, thresholds, and other nonlinear effects can make a simple proportion inaccurate.

Two quantities are directly proportional when they increase or decrease together at a constant rate. For example, at a fixed price per item, buying twice as many items costs twice as much. This calculator uses X = (B × C) ÷ A.

Two quantities are inversely proportional when one increases as the other decreases, while their product stays constant. For example, for a fixed job, more workers can mean fewer hours to finish it. This calculator uses X = (A × B) ÷ C.

Direct proportion can estimate ingredient amounts for a different number of servings, distance at a steady speed, or price at a fixed unit rate. Inverse proportion can estimate travel time at a constant distance when speed changes, or task time when the number of equally productive workers changes.