Rectangle area calculator
Calculator
The calculator needs JavaScript. The tables below give the same numbers for common cases.
How it is worked out
- Area = length × width = 8 × 5 = 40.
- Perimeter = 2 × (length + width) = 26.
- Diagonal = √(length² + width²) = 9.433981.
| Inputs | Answer |
|---|---|
| Length: 8; Width: 5 | A 8 by 5 rectangle has area 40 and perimeter 26. |
| Length: 12.5; Width: 9 | A 12.5 by 9 rectangle has area 112.5 and perimeter 43. |
| Length: 20; Width: 20 | A 20 by 20 rectangle has area 400 and perimeter 80. |
How to use it
The area of a rectangle is length × width and its perimeter is 2 × (length + width). The diagonal, from corner to opposite corner, is √(length² + width²) by the Pythagorean theorem.
Use it for floors, rooms, plots and sheets. Enter both sides in the same unit; the area comes out in that unit squared.
Frequently asked questions
What is the area of an 8 by 5 rectangle?
40 square units, with perimeter 26.
How do I find the diagonal of a rectangle?
Square both sides, add them and take the square root.
How do I convert square feet to square metres?
Multiply by 0.09290304 (1 ft = 0.3048 m exactly).
Method: standard arithmetic shown step by step; no outside data is used. Checked 2026-11-12.
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General information only. Results are plain arithmetic on the numbers you enter; check anything important (money, health, school work) against your own records or the official source.