Measure your existing door knobs from the floor up.
Standard door handle sizes.
Two standard backsets are 2 3 8 and 2 3 4.
Other backsets include 2 used by national and lori lock and 5 used by weiser weslock kwikset and others.
If they are more than inch off the standard height you will want to match all of the pre existing door handle heights in your home.
However the standard door handle size also considers the convenience of using it.
Standard exterior doors the standard size for an exterior door is 80 inches 6 feet 8 inches by 36 inches 3 feet.
While the commercial door handle height is already 34 to 48 inch which standard you should install the knob and handle carefully if you choose to buy a slab door.
Dummy handles are surface mounted and may be located anywhere on the door you choose but they re usually located to match the appearance of nearby operating locks.
Next you need to determine which direction the door opens in out left right swings.
This creates a more harmonious appearance.
It is more important that your door handles be consistent than that they match the standard door handle height.
To measure your existing door you need to take a measuring tape and measure from the bottom of the door to the top of the door most common doors measure.
There are also extensions for some brands like schlage that can extend the latch enough to have the lock located in the center of the door.