Always use a steel measuring tape. Consider any obstructions on or around your window opening. Window cranks, latches, security devices, or removable screens can interfere with installation or operation. When furnishing an entire room, order all materials at once to ensure matching dye-lots and components.
Outside mount hangs outside the window opening.
Ensure measurment extends over the window to maintain cover. Inside mount hangs inside the window opening just covering the window,therefore measure the recess only.