What is the Rectangle Calculator?
The rectangle calculator finds area, perimeter and diagonal from the two side lengths. It is handy for rooms, plots and sheets of material.
How to use the Rectangle tool
- Enter Length and Width.
- The result updates instantly as you type, or press Calculate.
- Use Try an example to see how it works, and Reset to start again.
Formula
Area = length × width. Perimeter = 2 × (length + width). Diagonal = √(length² + width²).
Worked example
With length 12, width 8, the area comes to 96. Perimeter: 40; Diagonal: 14.4222.
Frequently asked questions
What is the difference between area and perimeter?
Area is the space inside the rectangle. Perimeter is the distance around its edge.
How is the diagonal found?
With the Pythagorean theorem applied to the length and width.
Rectangle area, perimeter and diagonal
Area = l × w, perimeter = 2(l + w), diagonal = √(l² + w²).
| Length | Width | Area | Perimeter | Diagonal |
|---|---|---|---|---|
| 2 | 3 | 6 | 10 | 3.606 |
| 3 | 4 | 12 | 14 | 5 |
| 4 | 5 | 20 | 18 | 6.403 |
| 5 | 5 | 25 | 20 | 7.071 |
| 5 | 12 | 60 | 34 | 13 |
| 6 | 8 | 48 | 28 | 10 |
| 8 | 15 | 120 | 46 | 17 |
| 10 | 20 | 200 | 60 | 22.361 |
| 12 | 16 | 192 | 56 | 20 |
| 25 | 40 | 1,000 | 130 | 47.17 |
FAQ
How do I find the diagonal of a rectangle? Use the Pythagorean theorem: √(length² + width²).
Does a square work here? Yes, set length and width equal.
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Checked by Calcovate · Formula tested against worked examples · Updated 7 Nov 2026 · How we check our calculators.