Solve a proportion A : B = C : D for the missing value, simplify a ratio to its lowest terms and see the scale factor.
Last reviewed:
Enter your numbers to see a result.
With a 3, b 4, c 9, the missing value d is 12.
| A | 3 |
|---|---|
| B | 4 |
| C | 9 |
| Missing value D | 12 |
|---|---|
| A ÷ B | 0.75 |
| Scale factor (C ÷ A) | 3 |
| Simplest form of A : B | 3 : 4 |
Two ratios are in proportion when they describe the same relationship, such as 3 : 4 and 9 : 12. Enter A, B and C to find the D that keeps the proportion. This is how recipes are scaled, maps and models are read, and images are resized without distortion. Whole-number ratios are also reduced to their simplest form.
If A : B = C : D, then D = B × C ÷ A (cross-multiplication). Two ratios are in proportion when they describe the same relationship, such as 3 : 4 and 9 : 12. Enter A, B and C to find the D that keeps the proportion. This is how recipes are scaled, maps and models are read, and images are resized without distortion. Whole-number ratios are also reduced to their simplest form.
With a 3, b 4, c 9, the missing value d is 12. A ÷ B: 0.75; Scale factor (C ÷ A): 3; Simplest form of A : B: 3 : 4.
This is an estimate based on the inputs and assumptions shown on this page, not professional advice. Check anything with financial, health or safety consequences independently.
Yes. Calculations run in your browser and your numbers are not sent to a server. A share link contains the inputs you choose to share, so check it before sending it to anyone.
Cross-multiply: 3x = 4 × 9 = 36, so x = 12. The second ratio is the first one multiplied by 3.
Divide both parts by their greatest common factor. 18 : 24 has a GCF of 6, so it simplifies to 3 : 4.
If a recipe for 4 uses 300 g of flour and you cook for 6, set A = 4, B = 300 and C = 6. D = 300 × 6 ÷ 4 = 450 g.