Have another person stand at one corner of your gazebo and grip the end of the tape measure.
Measure one side of the gazebo from the corner where your friend is standing, to the corner to his right. Pull the tape measure tight to ensure an accurate measurement. Make a note of the distance.
Measure the adjoining side of the gazebo, from your friend's corner to the corner on his left. Make note of the measurement.
Choose a gazebo cover based on the measurements. If you gazebo is square, the measurements you took will be the same, such as 10 feet by 10 feet. A rectangular gazebo will have a measurement like 10 feet by 12 feet.