Mirrors are additions to a room that can serve a few purposes. One purpose is to give you a chance to take a look at yourself before you leave your house. No one wants to see a bad look on the way to a job interview. Another purpose is to add depth to a room. Finally, they can be used as ornamental fixtures or focal points.
Measure the height, from ground to eye level, of all members of the household. Find an average. It is best to do this in inches. Do not use children because they will skew the average to be far too low for the adults to use.
Determine the size of the mirror or cabinet you will be using. The average 18-inch-high medicine cabinet with a mirror will be set at a different height than a full mirror that is 48 inches high or a 48-inch-long sliding cabinet door model. Remember that you want your eye level to fall two-thirds of the way up the mirror.
Measure from the floor to a point on the wall where the average eye-level height is. Measure the height of the cabinet. Divide that number by 3. This will give you the amount of the cabinet that must be over the height of the average. Mark the spot where the top of the cabinet will start with a pencil.