Proportion Calculator
Fill in any three values and leave the fourth blank. The calculator solves the proportion by cross-multiplying.
A / B = C / D
Leave exactly one box empty.
Cross-multiplication
Two ratios are in proportion when they are equal. Multiplying both sides of A/B = C/D by B × D gives the cross-product rule:
A × D = B × C
A = B × C ÷ D
B = A × D ÷ C
C = A × D ÷ B
D = B × C ÷ A
Worked example
3 / 4 = 9 / D
- Cross-multiply: 3 × D = 4 × 9 = 36
- Divide by 3: D = 36 ÷ 3 = 12
D = 12, so 3/4 = 9/12.
Real-world examples
- If 3 notebooks cost 4.50, how much do 7 cost? 3/4.50 = 7/D → D = 10.50.
- A map scale of 1 cm : 250 m — how far is 6.4 cm? 1/250 = 6.4/D → D = 1,600 m.
- A recipe for 4 people uses 300 g of flour; for 6 people: 4/300 = 6/D → D = 450 g.