Calculator
For a/b = c/d, choose the unknown and enter the other three values. The calculator uses cross multiplication.
Find
?
d6
Completed proportion2 / 4 = 3 / 6
Show the steps
1. Cross multiply
a x d = b x c; 2 x 6 = 4 x 3.
2. Isolate the unknown
d = (b x c) / a = (4 x 3) / 2 = 6
Method and variables
Ratio & Proportion Calculator
Formula: a / b = c / d; a x d = b x c
Assumption: Three terms are known and the proportion can be expressed as a/b = c/d; the divisor involved in the calculation cannot be zero.
Worked example
Ratio & Proportion Calculator
For 2/4 = 3/d, cross multiply: 2d = 12, so d = 6.
Proportions in everyday problems
| Problem | Proportion | Answer |
|---|---|---|
| A recipe for 4 needs 300 g flour; how much for 6? | 300 / 4 = x / 6 | 450 g |
| A map scale is 1 : 25,000; 4 cm on the map is? | 1 / 25,000 = 4 / x | 100,000 cm = 1 km |
| 5 workers finish in 12 days; 6 workers (inverse)? | 5 × 12 = 6 × x | 10 days |
| Mix concrete 1 : 2 : 3 for 0.6 m³ | total parts = 6 | 0.1 / 0.2 / 0.3 m³ |
Check whether the relationship is direct or inverse before setting up the proportion. In a direct proportion both values grow together (more people, more flour); in an inverse proportion one falls as the other rises (more workers, fewer days), so the products, not the ratios, are equal.