Ratio & proportion solver
Solve A : B = C : D for any missing term, simplify ratios and scale them.
Leave one box as the unknown and it is solved for you.
Missing value
A : B = C : D → 3 : 4 = 9 : 12
Simplified ratio
As a decimal
As a percentage
Share of the total
Splitting 7 in this ratio
How it works
A proportion says two ratios are equal. Cross-multiplying turns it into a single equation you can solve for whichever term is missing.
A / B = C / D → A × D = B × C Where this shows up
- Scaling recipes and mixtures — 3 parts to 4 parts, at any batch size.
- Maps and models — 1 : 87 means 1 cm on the model is 87 cm in reality.
- Unit pricing — if 3 kg costs £9, what does 5 kg cost?
- Similar triangles — the classic shadow-and-height problem is a proportion.
Ratio versus fraction
“3 : 4” and “3/4” are not quite the same claim. The ratio compares two parts; the fraction compares a part to the whole. Mixing 3 : 4 gives seven parts total, of which the first is 3/7 — not 3/4. Getting this backwards is the single most common ratio error.