Doorknob measurements are fairly standardized, but there are still a number of variables to consider. The standard measurements for doorknobs will depend on the answers to these questions:
- Is the door interior or exterior?
- Is the door blank or pre-drilled?
- Are you replacing an existing doorknob?
To speak the language of doorknob installation, these are the terms to know:
- The bore hole is the doorknob hole.
- The cross bore is the hole that goes from the narrow edge of the door to the doorknob hole.
- The backset is the distance from the edge of the door to the center of the bore hole.
- The latch is the part of the hardware that goes into the cross bore hole. These come in three varieties: drive-in, rounded corner and square corner.
- The shank, or spindle, is the part of the hardware that goes through the door, connecting the knobs on either side.
- A handleset is hardware for an exterior door that includes a keyed deadbolt separate from the knob or lever.
When choosing a doorknob — which generally comes in a set including knobs, shank and latch — focus on three key measurements: the thickness of the door, the backset and the size of the bore hole. Most interior doors are 1-3/8″ thick, while exterior doors are typically 1-3/4″ thick. The most standard backset for a door is 2-3/8″, but some doors (particularly exterior) have a 2-3/4″ backset. The standard cross bore has a diameter of 1″. The standard shank size of a doorknob is 1/4″.
The standard diameter of the bore hole is 2-1/8″. If you will be using a hole saw to drill the bore hole, the standard height is a bore hole centered on 36″ from the bottom of the door. In new construction, doors are hung so that the bore hole is centered on 40-5/16″ above the finished floor.
If using a separate deadbolt for an exterior door, the bore hole should be either the standard 2-1/8″ or 1-5/8″ diameter. Handlesets that are one piece require two bore holes — one above the other and typically 5-1/2″ apart center to center.
It’s not unusual to encounter variations on standard measurements when replacing existing doors or knobs. The most common variation is height from the floor; if you are replacing a door, before buying one that is pre-drilled it is wise to measure the knob heights of your existing doors. Since doors are often trimmed to fit, and floors may be laid over older floors, if your existing doorknobs are more than a half-inch off the standard it’s better to buy a blank door and drill the bore hole to match your existing doors, so as not to be visually jarring.
If you are replacing a knob set on a door that has a nonstandard configuration (different door thickness, different hole relationship for handlesets), look for hardware manufacturers that offer variable configuration.