Area of a Rectangle

Enter the two sides and the calculator multiplies them, then adds the perimeter and the diagonal — the three numbers you usually need when measuring a room, a wall or a piece of land.

How the calculation works

A rectangle has four right angles, so its area is simply one side times the adjacent side: A = a × b (often written length × width). It is the most basic area formula in geometry and the starting point for parallelograms, triangles and trapezoids. The result is shown in square metres.

Two extra values come with it. The perimeter, P = 2 × (a + b), tells you how much baseboard, edging or fencing runs around the shape. The diagonal, d = √(a² + b²), comes from the Pythagorean theorem; builders use it to check that a layout is truly square — if both diagonals match the calculated value, the corners are 90°.

Side a and side b are labelled in metres and accept decimals such as 3.45. The formula works in any unit: enter feet and you get square feet, enter centimetres and you get square centimetres. Just use the same unit for both sides.

Worked example

The calculator opens with sides of 6 m and 4 m. Area = 6 × 4 = 24 m², perimeter = 2 × (6 + 4) = 20 m, diagonal = √(36 + 16) = √52 ≈ 7.2111 m. In imperial terms, a 12 ft × 10 ft bedroom works the same way: 120 sq ft of floor and 44 ft of perimeter.

Things to keep in mind

• Never mix units. 150 inches × 10 feet must become 12.5 ft × 10 ft = 125 sq ft before multiplying.
• Square units do not convert like linear ones: 1 m² = 10,000 cm² and 1 sq yd = 9 sq ft, not 3.
• Each side field accepts values up to 100. For a 250 × 40 m lot, divide one side by 10 (25 × 40 = 1,000) and multiply the answer by 10 to get 10,000 m² (1 hectare).
• When ordering flooring or tiles, add roughly 5–10% for cuts and waste, more for diagonal patterns.
• For L-shaped or irregular rooms, split the floor into rectangles, calculate each one and add them up.