Two standard backsets are 2 3 8 and 2 3 4.
Standard door handle sizes.
Standard exterior doors the standard size for an exterior door is 80 inches 6 feet 8 inches by 36 inches 3 feet.
Measure from the center of the door knob hole to the edge of the doorway to find the size of the door set.
However the standard door handle size also considers the convenience of using it.
Other backsets include 2 used by national and lori lock and 5 used by weiser weslock kwikset and others.
In most cases the door set which is inside of the door and controls the door latch will.
This creates a more harmonious appearance.
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.
Measure your existing door knobs from the floor up.
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.
It is more important that your door handles be consistent than that they match the standard door handle height.
Next you need to determine which direction the door opens in out left right swings.
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.