Rectangle formulas
- Area = length × width
- Perimeter = 2 × (length + width)
- Diagonal = √(length² + width²)
Worked examples
A room 4 m by 3 m has an area of 12 m², a perimeter of 14 m (the length of skirting board needed, minus door openings) and a diagonal of 5 m.
A 12 ft × 10.5 ft bedroom is 126 ft². Flooring is sold by area, so buy about 10% more, roughly 139 ft², to allow for cuts and waste.
Painting a wall
A wall 5 m long and 2.7 m high is 13.5 m². Subtract windows and doors, multiply by the number of coats, and divide by the paint's coverage per litre to get the litres needed.
How this works
The page multiplies length by width for the area, adds the two sides and doubles the sum for the perimeter, and uses the Pythagorean theorem for the diagonal. The unit you choose labels the results, with square units for the area. Inputs must be positive. Everything runs in your browser.
Frequently asked
How do I calculate the area of a room?
Multiply its length by its width. For L-shaped rooms, split them into rectangles and add the areas.
What is the area of a square?
Side × side: a 5 m square has 25 m².
How much extra flooring should I buy?
Usually 5–10% more than the area, more for diagonal patterns.
How do I convert m² to ft²?
Multiply by 10.7639, or use the square meters to square feet converter.
Sources
Reviewed Oct 17, 2026 · test vectors fixtures/calculators/math/area/rectangle/