I often find that people are confused about the proper height for the Mezuzah to be placed on the door.  Many people like to put the Mezuzah up very high on the door near the top.  Some others will put the Mezuzah very low on the entrance to child’s room in order that the child will be able to reach the Mezuzah to kiss it

The Shulchan Aruch says that the Mezuzah should be at the "bottom of the top third" of the door.  If it is lower than that, it is invalid.  If it is higher it is still Kosher as long as a handbreadth remains between the top of the door and the Mezuzah.  (A handbreadth is about 3.5 inches.)  Based on this, if you put the Mezuzah very low on a child’s room, the Mezuzah is not valid.  If you place it a bit too high it is still Kosher as long as it is not right near the top.

The ideal thing to do is to measure the door, divide by three, and place the Mezuzah in the exactly correct spot

EXCEPTIONS

There are some exceptions to this rule. 

1. If a door is very tall and the Mezuzah is going to be up above your head, the Mezuzah should not be placed in the top third.  Rather the Mezuzah should be placed at shoulder hight.
2. If the door is not rectangular such as an arch or any non-rectangular shape, the rules are more complicated.  Look out for a post about this in the near future.