Rectangle Area Calculator

The rectangle is the shape most real measuring jobs come down to: rooms, lawns, walls, screens, sheets of plywood. Its corners are all right angles, so two measurements describe it completely. Each input here has its own unit menu, so a length in feet and a width in inches can sit side by side without any conversion by hand.

Formula

A = l × w

A

the area

l

the length

w

the width

Area from length and width

Picture the rectangle as rows of unit squares. The length says how many squares fit in a row and the width says how many rows there are, so multiplying them counts the squares. A room 14 ft by 11 ft has an area of 154 ft², which is about 17.1 square yards if the carpet is sold by the yard.

Area from the diagonal and one side

Sometimes one side is blocked but you can stretch a tape corner to corner. The diagonal and the two sides form a right triangle, so the Pythagorean theorem recovers the missing side. With a diagonal of 13 and a length of 5, the width is √(169 − 25) = 12 and the area is 5 × 12 = 60. The diagonal is always longer than either side, and the calculator flags a value that isn't.

• w = √(d² − l²)
• A = l × √(d² − l²)

Perimeter and diagonal

Perimeter is the distance around the outside, twice the length plus twice the width. You need it for baseboards or fencing, whereas area is what you need for flooring or sod. Two rectangles can share an area but differ in perimeter: 6 by 6 and 12 by 3 both cover 36, yet the perimeters are 24 and 30.

The diagonal doubles as a squareness check. If a frame's two diagonals are the same length, its corners are true right angles.

Rooms that aren't simple rectangles

Divide an L-shaped room into two rectangles, work out each one and add the results. For a closet or a chimney breast, work out its area and either add or subtract it. When ordering material, allow for cutting waste: flooring installers commonly add about 5 to 10 percent.

Frequently asked questions