How to Calculate the Area of an L-Shaped or Irregular Room

Method 1: Add the Parts

Draw the floor plan and divide it with a line into two or three rectangles. Measure the length and width of each, multiply, and add. For the example: Rectangle A is 12 ft × 10 ft = 120 sq ft. Rectangle B is 6 ft × 8 ft = 48 sq ft. Total area = 168 sq ft.

Check that the pieces do not overlap and that nothing is left out. A quick test is to confirm that the dimensions of the parts add up to the overall measured dimensions.

Method 2: Subtract the Notch

Imagine a bounding rectangle around the whole shape. Take another L-shaped room: its bounding rectangle is 16 ft × 14 ft = 224 sq ft, and the missing corner is 8 ft × 7 ft = 56 sq ft, so the floor area is 224 − 56 = 168 sq ft.

Both methods give the same answer, so using both is a useful error check.

Triangles, Curves, and Circles

For a corner cut at an angle, use the triangle formula: ½ × base × height. A triangular nook with legs of 4 ft and 3 ft has an area of 6 sq ft. For a semicircular bay, use ½ × π × r²: with a radius of 3 ft, that is about 14.1 sq ft.

For an irregular curve, approximate it with a series of rectangles or triangles, or use a polygon area formula with coordinates of the corners.

• Rectangle: length × width
• Triangle: ½ × base × height
• Circle: π × radius²
• Trapezoid: ½ × (a + b) × height

Using the Area

Once you have the total, add waste before ordering flooring, paint, or tile: 5% to 10% for simple shapes and more for complicated ones. Measure in the same unit throughout, and convert at the end if needed: 168 square feet is about 15.6 square meters.