At what height should interior door handles be fitted?

Normally you measure up 36" from the bottom of the door, not floor.

You look at the knob package to find the offset, that's how far in from the door edge.

This will either or be 2 3/8" or 2 3/4" or sometimes on newer sets can be either.

Good luck Roger

You can get a template from many box stores or hardware stores that will position cabinet hardware in the exact spot where it should be placed. The ones I've seen are a bluish-green color and can be found in the section where hardware is found. Very easy to use, with a variety of placement options for you to choose from this one tool. Costs under $5.00.

Ignore above post. Just saw you were talking about door handles, not cabinet doors. Sorry.

No absolutes here. The standard pre-hung interior door comes with lock set hole drilled 36” from bottom, 44” from the top. Measuring from top eliminated any question about trimming door bottoms. Just replaced 8 interior doors for customer. Old doors were drilled at 40”, exactly centered top to bottom. Manufacturer probably did that so doors could be hung either left or right (hinges were also equally spaced) by turning door top to bottom.