Calculating the dimensions of a gazebo depends largely on what shape the unit is. Measuring the height will be the same no matter what, but finding the square footage can be performed in a few ways. If the gazebo is a square, for example, find the length and the width. If the gazebo is a circle, find the circumference. These dimensions can be found with a tape measure.
Find the height of the gazebo by measuring from the bottom base of the unit to the top, where the ceiling begins. If the gazebo is sitting on a concrete slab, do not measure from the bottom of the slab but measure from the base of the gazebo's frame. Similarly, if the gazebo is raised off the ground, do not measure from the ground but measure from the base of the gazebo's frame.
Find the length and width of the gazebo (if it is square). Measure two of the perpendicular walls of the gazebo. If you measure in inches (for example, 65 inches) divide by 12 to find the measure in feet (65 inches divided by 12 would be 5.4 feet).
Find the diameter of the gazebo (if it is circular). Measure across the center of the gazebo from one side of the unit to the other. As with the length and the width, convert this measurement to feet by dividing the measure you get in inches by 12. Divide the diameter by 2 to find the radius.
Find the square footage of the gazebo. For square gazebos, multiply the length by the width. For circular gazebos, multiply the radius by the radius and multiply that product by 3.14 to find the square footage.