Write down the lengths of each side of the floor plan. Use these lengths to calculate the area and perimeter. Note that the floor plan lists the lengths of each exterior side of the building. Find these dimensions clearly marked, usually between the arrowhead dimension lines on the floor plan, most often parallel to the lines that represent the walls of the building.
Calculate the perimeter. Total the lengths of each exterior side of the floor plan. Add the length of each side together. Check your work by making sure the number of exterior sides equals the number of dimensions you recorded and added together.
Determine the area of all the regular shapes in the floor plan. Calculate the area of each regular shape by using its geometric formula; for example, if square or a rectangle, multiply the length by the width; if a circle, multiple pi by the radius squared; if a triangle, multiply 0.5 by the base and then multiply this result by the height.
Determine the area of all irregular shapes. Divide the irregular shapes into smaller regular shapes, such as rectangles, squares and triangles, until the shape has been completely subdivided. Use the same scale as on the floor plan and measure the dimensions of the newly formed shapes. Use formulas for rectangles, triangles and regular polygons to calculate the area of each new shape.
Add the areas of each shape for the total area of the floor plan. Add up the number of shapes and see if they match the number of area calculations that you made. Verify your calculations.