Use this simple proportion calculator to find an unknown value from three known values. Choose direct or inverse proportion, enter A, B and C, then select Calculate.
Simple Proportion Calculator
Find X in the proportion A : B = C : X
How to use the calculator
The calculator solves A : B = C : X, where X is the unknown value. Choose the type of proportion, enter A, B and C, and select Calculate. Use Clear to start again.
What is a simple proportion?
A simple proportion is a mathematical method for finding a fourth unknown value when three known values are proportional. It involves two quantities and is useful for prices, unit conversions, recipes and time estimates.
Direct proportion
Two quantities are directly proportional when they change in the same direction and at the same rate.
Formula: X = (B × C) ÷ A
Example: a car travels 120 km with 10 litres of fuel. For 300 km, X = (10 × 300) ÷ 120 = 25 litres.
Inverse proportion
Two quantities are inversely proportional when they change in opposite directions.
Formula: X = (A × B) ÷ C
Example: 8 workers build a wall in 15 days. With 12 workers, X = (8 × 15) ÷ 12 = 10 days.
How to identify direct or inverse proportion
Ask whether the second value also increases when the first increases. If yes, the proportion is direct. If one increases while the other decreases, it is inverse.
Practical examples
| Situation | Type | Example |
|---|---|---|
| Product price | Direct | 3 kg cost $18; 7 kg cost $42 |
| Construction time | Inverse | 4 workers take 20 days; 10 take 8 days |
| Recipe | Direct | 4 servings use 300 g; 10 use 750 g |
Frequently asked questions
What is a simple proportion?
It finds an unknown value from three known proportional values.
What is the difference between direct and inverse proportion?
In a direct proportion both quantities change in the same direction; in an inverse proportion they change in opposite directions.
Which formulas are used?
For a direct proportion, X = (B × C) ÷ A. For an inverse proportion, X = (A × B) ÷ C.
How do I identify the type?
If both quantities increase or decrease together, it is direct. If one increases while the other decreases, it is inverse.
Can I use decimals and any units?
Yes. Decimals are accepted, but the units within each quantity must be consistent.
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