Backset is the horizontal distance from the edge of a door to the centerline of the lock’s operating point. Depending on the product, that point may be the center of a lever spindle, keyhole or cylinder. Correctly identifying Door Lock backsets ensures that the lock body fits the prepared opening and that handles, cylinders and other trim align with the door design.
| Lock application | Common backset range | Typical consideration |
|---|---|---|
| European Mortise Lock | 50–80 mm | Door stile and handle position |
| Narrow Stile Lock | 20–40 mm | Limited aluminum profile space |
| Tubular latch | 60 or 70 mm | Residential and light commercial doors |
| Panic exit device | Model-specific | Strike and trim coordination |
| Security deadlock | 50–70 mm | Deadbolt position and door construction |
These figures are common reference points rather than universal sizes. Buyers should use the manufacturer’s technical drawing for final door preparation.
For a mortise lock, measurement starts at the finished edge of the door or the face of the installed forend. It extends horizontally to the center of the lever follower or cylinder, according to the lock drawing.
The measurement should not begin from the frame, strike plate or decorative trim. Mixing these reference points can produce an incorrect door cutout.
When replacing an existing lock, buyers should remove the edge plate where practical and compare the complete lock case dimensions. Two locks with the same backset may still have different case heights, depths, center distances or fixing positions.
A deeper backset moves the handle and cylinder farther away from the door edge. This can improve hand clearance around the frame and create a more balanced appearance on wide doors.
A shorter backset is often needed for narrow stiles, glazed doors or doors with restricted internal space. However, the selected lock case must still have enough structural support and clearance from glazing channels or reinforcement.
The standard mortise lock backset depends on regional practices, door construction and lock function. European commercial lock bodies commonly use measurements such as 50, 55, 60, 70 or 80 mm.
Before confirming a size, buyers should check:
Door material and stile width
Lock case depth
Lever and cylinder centerlines
Glazing and reinforcement locations
Required handle clearance
Existing door preparation
Applicable fire-door evidence
A fire-rated door should not be cut for a different lock size without confirming that the modification remains within its approved construction.
Backset and center distance describe different measurements. Backset runs from the door edge to the operating center. Center distance is the vertical spacing between the lever follower and the cylinder or keyhole.
A lock with a 60 mm backset and 72 mm center distance cannot be replaced automatically by one with a 60 mm backset and 85 mm center distance. The handle and cylinder openings would not align.
Most Commercial Mortise Locks use a fixed backset because fixed internal geometry supports consistent operation and precise door preparation. Adjustable products are useful when one lock must fit more than one common preparation, although available functions and certifications may be more limited.
D&D supplies mortise lock options with several backsets for wooden, metal and fire-rated doors. Technical drawings should be approved before machining begins.
An experienced adjustable lock body supplier should clearly state the available adjustment range, compatible door types and certification limits. Accurate backset selection prevents site modification and ensures that the lock, lever, cylinder and strike operate as a coordinated assembly.
Previous: How Does a Deadbolt Work?