Area of a rectangle and perimeter: in short
Multiply the length by the width: a room of 5 m by 4 m has an area of 5 × 4 = 20 m². For the perimeter, double the sum of the two sides: 2 × (5 + 4) = 18 m.
The formulas
Area = length × width. Perimeter = 2 × (length + width). Diagonal = the square root of (length² + width²). Both sides must be in the same unit: in metres the area comes out in square metres.
Room examples
| Length | Width | Area | Perimeter |
|---|---|---|---|
| 3 m | 2.5 m | 7.5 m² | 11 m |
| 4 m | 3 m | 12 m² | 14 m |
| 5 m | 4 m | 20 m² | 18 m |
| 6 m | 4 m | 24 m² | 20 m |
| 7 m | 5 m | 35 m² | 24 m |
| 10 m | 8 m | 80 m² | 36 m |
What the perimeter is for
For skirting boards, a border or a fence you count the distance around the room or plot, minus the doors for skirting. For paint or wallpaper you use the perimeter times the wall height.
When the room is not a rectangle
An L-shaped room splits into two rectangles: work out each and add them. A room with an alcove is a big rectangle minus the alcove. In the editor you draw the room as it is and the area appears by itself.
Figure 1. One floor of 7.5 × 5 m. The rooms add up to 37.5 m².
About this page · Written by the Squaremetry team. Last checked on 7 October 2026. Found a mistake? Send us a message.