Window bays project out from a wall and form an area distinct from the main part of the room. The shape varies according to the architectural style of the building; rectangular, semicircular, triangular and trapezoidal shapes are commonly used. Calculating the square footage of a window bay, even an irregularly shaped one, is a straightforward task involving basic math and no previous building or decorating experience.

### Step 1

Divide the window bay floor into simple shapes, the areas of which are easily calculated. Envisage the overall shape broken into separate squares, triangles, segments of circles and rectangles.

### Step 2

Mark out the separate areas on the floor. Use chalk on hard floor surfaces and tack pins lightly pressed into carpets. For example, a square window bay with a narrowing and pointed end may be divided into a square and a triangle.

### Step 3

Calculate the area of each of the shapes within the bay using the following formulas: Area of a rectangle or square: Width x depth Area of any triangle: Half the depth x width Area of a semicircle: (Pi x radius squared) / 2 Area of a trapezoid: (shortest parallel side + longest parallel side)/2 x depth (Pi is a transcendental number usually shortened to 3.1415.)

### Step 4

Add together the individual areas to establish the overall floor area of the window bay. Using the example window bay consisting of a square and a triangle, if the square was 12 feet square and the triangle was 6 feet square, the square footage of the window bay would be 18 square feet — 12 + 6 = 18.