Missing Side from Diagonal
The diagonal is the hypotenuse of a right triangle, so Pythagoras solves it. Subtract the known side squared from the diagonal squared and take the square root.
Your numbers
Side b
120cm
Area
6,000cm²
Perimeter
340cm
Formula
Side b = √(Diagonal² − Side a²)
If the diagonal is shorter than the known side, no such rectangle exists — check which length you actually measured.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Diagonal (cm)Corner to opposite corner. | 130 | 0 and up |
| Side a (cm)The length of the first side. | 50 | 0 and up |
Step by step
Quick reference table
Results when only Diagonal (cm) changes and everything else stays put.
| Diagonal (cm) | Side b (cm) | Area (cm²) | Perimeter (cm) |
|---|---|---|---|
| 65 | 41.53 | 2,076.66 | 183.07 |
| 97.5 | 83.7 | 4,185.17 | 267.41 |
| 130 | 120 | 6,000 | 340 |
| 195 | 188.48 | 9,424.04 | 476.96 |
| 260 | 255.15 | 12,757.35 | 610.29 |
What each result means
| Result | At default values |
|---|---|
| Side b (cm)The length of the second side. | 120 |
| Area (cm²)The space a shape covers — length squared. | 6,000 |
| Perimeter (cm)The total length around the outside of a shape. | 340 |
Common mistakes
If the diagonal is shorter than the known side, no such rectangle exists — check which length you actually measured.
Glossary
- Diagonal
- Corner to opposite corner.
- Side a
- The length of the first side.
- Side b
- The length of the second side.
- Area
- The space a shape covers — length squared.
- Perimeter
- The total length around the outside of a shape.
Frequently asked questions
Q. How is Missing Side from Diagonal calculated?
Side b = √(Diagonal² − Side a²) — The diagonal is the hypotenuse of a right triangle, so Pythagoras solves it. Subtract the known side squared from the diagonal squared and take the square root.
Q. Can you walk through an example?
With Diagonal 130cm, Side a 50cm, the answer is Side b 120cm.
Q. What do I need to enter?
Enter Diagonal, Side a. The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Diagonal (cm) moves the answer to Diagonal (cm) 65 → Side b (cm) 41.53 and Diagonal (cm) 260 → Side b (cm) 255.15. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
If the diagonal is shorter than the known side, no such rectangle exists — check which length you actually measured.
Related calculators
Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.