Guide
How to Calculate Rectangle Area and Perimeter
A rectangle's area is length × width; its perimeter is 2 × (length + width). A room measuring 8 meters by 5 meters has an area of 40 square meters and a perimeter of 26 meters. The two answers measure different things.
Area measures the space inside
Imagine covering a floor with squares that are each 1 meter by 1 meter. An 8 m by 5 m rectangle fits eight squares across and five rows deep, so it contains 8 × 5 = 40 squares. That is 40 m², pronounced square meters.
The same formula works for a square because a square is a rectangle with equal sides. A 4 m by 4 m square has an area of 16 m². It does not work unchanged for triangles, circles, or other shapes.
Perimeter measures the edge
A rectangle has two sides of each length. Add all four: 8 + 5 + 8 + 5 = 26 m. The shorter formula is 2 × (8 + 5) = 26 m. Perimeter is a length, so its unit is meters rather than square meters.
Use area when estimating floor covering or a tabletop surface. Use perimeter for edging or the total length around a boundary. For skirting boards, subtract doorway widths separately; the calculator gives the full geometric perimeter.
Convert measurements before multiplying
Both dimensions must use the same unit. This calculator expects meters. If a space measures 250 cm by 180 cm, enter 2.5 and 1.8. The area is 2.5 × 1.8 = 4.5 m², and the perimeter is 2 × (2.5 + 1.8) = 8.6 m.
Converting area uses a squared factor: 1 m² = 10,000 cm², because 1 m × 1 m = 100 cm × 100 cm. Dividing an area in cm² by only 100 produces an answer 100 times too large. Converting each side first avoids this mistake.
Measure irregular rooms as smaller rectangles
For an L-shaped floor, split it into two non-overlapping rectangles, calculate each area, then add them. A 4 m by 3 m section plus a 2 m by 2 m section gives 12 + 4 = 16 m². Do not count the same corner twice.
Do not add the two perimeters to find the outer boundary: that would count shared internal edges. Measure the outside boundary separately. Flooring purchases also need allowances for cuts, pack sizes, and the installation pattern; the area result alone is not an order quantity.
Use the rectangle calculator
Enter Length (m) and Width (m) to see Rectangle area in m² and Perimeter in m. Decimal inputs let you use measurements such as 2.5 m without rounding them to whole meters.
Check that the shape is rectangular and the measurements describe the surface you actually need. Negative lengths are rejected. A zero dimension produces zero area, which is mathematically valid but does not represent a usable floor.
Try it yourself
Put this guide into practice with our free Rectangle Area Calculator — no sign-up, no download.
Open the Rectangle Area Calculator