The accurate calculation of a room's square footage is essential to estimating a project's costs and the amount of materials needed. Carpet, flooring, painting and drywall are just some of the building materials that are ordered and installed based on the square footage of an area, and can be determined using basic math and a tape measure.

Use a tape measure to measure the room's length and record it as dimension "A."

Measure the width of the room and record it as dimension "B."

Multiply dimension "A" and dimension "B" to get the square footage for the room. For example, a room 19 feet long and 11 feet wide has 209 square feet of floor space.

Measure areas separately for a room that is "L"- or "T"-shaped. Simply approach the room as rectangles or squares forming the shape and calculate each rectangle or square individually. Then add the totals together to get the total square footage of the room.

Measure the wall length of each section of wall in the room at the base of the wall, from corner to corner. Add the dimensions together to get a total wall length for the room. Record the total as dimension "A."

Measure the ceiling height from the base of a wall to where it meets the ceiling plane and record this as dimension "B."

Multiply dimension "A" and dimension "B" to get the square footage for the walls.

Multiply the width by the length to determine the square footage of each door or window opening. Subtract these areas from the total calculated in Step 3 to obtain the precise wall area of the room. For example, a window that is 2 feet 6 inches wide and 4 feet tall covers 10 square feet. This number would be subtracted from the total wall area.