Local and instant calculation
Solve proportions, prices, percentages, quantities and conversions with a simple rule of three.
Enter the three known values and the tool will automatically calculate the fourth proportional value.
Free online calculator
Solve a simple rule of three for direct proportions, prices, quantities, percentages and equivalent values.
Rule of three
If A corresponds to B, calculate what corresponds to C.
Result X
250.00
Formula
X = B × C / A
Enter the three known values in the corresponding order. If A corresponds to B and C corresponds to X, the tool solves the direct proportion with X = (B × C) ÷ A and calculates the fourth value automatically.
Related values should use compatible units. For example, avoid mixing kilograms and grams unless you convert them to the same unit first.
Quick formula
If A corresponds to B and C corresponds to X, a direct rule of three can be calculated with:
X = (B × C) ÷ A
Example: if 3 units cost 150, then 5 units cost (150 × 5) ÷ 3 = 250.
If A corresponds to B and C corresponds to X, a direct proportion can be solved with X = (B × C) ÷ A.
For a direct rule of three, the common formula is X = (B × C) ÷ A, provided A is not zero and the related quantities use compatible units.
It can be used for prices by quantity, equivalent amounts, recipes, yields, estimates and other direct proportional relationships.
Yes, when the problem can be represented as a direct proportion. A percentage can be related to a total and its proportional value.
No. This tool is designed for simple direct proportion. In an inverse relationship, one quantity increases while the other decreases and a different formula is required.
Related tools