There is no single height that works on all doors when it comes to door-knocker installation. Instead, choose a height between 4 and 5 feet from the bottom of the door, or the ground or flooring outside the door, centering the knocker from the sides of the door.
Use an average adult eye-level height -- approximately 5.5 feet or slightly higher -- as a starting point for your door knocker placement decision. Measure from the ground-level standing position outside of the door, such as from the porch floorboards upward. Place a strip of masking tape at this height and see if it seems to be a good level to you and others sharing your home. If so, use the tape strip -- and potentially several others -- to mark dots on the door for exact knocker placement. Measure the center of the door from side to side and mark the tape at this point to ensure the center of the knocker lines up with the center of the door.
In some cases, the door's design may not allow for a knocker at the 4 to 5 foot range -- a large window or etched glass may take up much of the upper door area. In this case, find a location along the non-glass portion of the door, such as the area above the doorknob, to install the knocker at the desired height.
Installing a Surface-Mount Door Knocker
A surface-mount door knocker installs with a few screws inserted through the outside of the knocker, into the door.
Line the door knocker up at the desired height, centered from left to right.
Position a level along the top of a flat-topped doorknocker, or across two parallel points on the front of the knocker, to ensure the knocker is straight. Adjust the knocker until it's straight.
Use a pencil or marker to make dots on the door or masking tape through the screw holes in the knocker. Set the knocker, pencil and level aside.
Drill pilot holes for the screws using a bit narrower than the screws included with the knocker.
Line the door knocker up so the drilled holes in the door match the holes in the knocker. Place one screw in each hole.
Tighten the screws with a Phillips screwdriver.