Lock Backset, Bore, Stile, and Strike Layout

The two standard backsets exist because of door and stile geometry, and mixing them up is the common error. Bore at 2 3/8 in for a lock supplied at 2 3/4 in and the latch face sits 3/8 in shy of the door edge; there is no adjustment that fixes it, and confirming which one the lock is before the hole saw touches the door takes ten seconds. THE STILE IS THE CHECK NOBODY MAKES. A 2 3/8 in backset with a 2 1/8 in cross bore reaches 3 7/16 in into the stile, so a narrow-stile door will not take the lock at all -- which is why glass and aluminium doors use mortise or narrow-stile hardware instead, and why finding that out at the door is expensive. The edge bore has its own margin: a 1 in bore in a 1 3/4 in door leaves 3/8 in of material each side, and a thinner door or a fatter bore eats it. The strike location is where layout errors show up. It sits at the same height as the cross bore, but its horizontal position depends on the door stop location and the door's clearance in the frame, so it is laid out from the CLOSED door rather than measured from the jamb edge -- a strike set from the wrong reference produces a door that either will not latch or that rattles. Height is the third check: 38 in to centre is comfortably inside the 34 to 48 in operable-parts range, and a lock set at residential height on a commercial door does not comply, which is a layout decision made before boring rather than a hardware one. Handing and bevel are determined before boring too. A layout aid using standard dimensions. It does not replace the lock manufacturer's own template, which is supplied with the lock and which governs -- manufacturers vary in bore sizes, latch face dimensions, and required clearances, and mortise and multipoint hardware follow entirely different preps. It does not address frame preparation, electrified hardware raceways, access control preps, or the reinforcement a door needs to carry the hardware. On a rated door, field modification is restricted and any prep beyond the listing voids the label. NFPA 80, the manufacturers' listings and templates, the adopted accessibility standard, and the AHJ govern.

cross bore reaches backset + bore diameter / 2 into the stile, which is the minimum stile the prep needs; edge bore leaves (door thickness - edge bore) / 2 of material each side; the strike sits at the cross bore height, positioned horizontally from the closed door.

The standard cylindrical-lock prep dimensions by name -- a 2 3/8 in or 2 3/4 in backset to the cross bore centre, a 2 1/8 in cross bore, a 1 in edge bore -- with the lock manufacturer's template and NFPA 80 named as governing. The template supplied with the lock is the authority.

Addition and subtraction on standard trade dimensions the user enters; no manufacturer template is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: backset_in, cross_bore_dia_in, edge_bore_dia_in, door_thickness_in, lock_height_in, stile_width_in, accessible_min_in, accessible_max_in, cross_bore_far_side_in, min_stile_required_in, edge_bore_center_from_face_in, edge_bore_side_wall_in, backset_mismatch_in

• Confirm the backset before the hole saw a lock supplied at the other standard leaves the latch face short by the difference, unfixablylocksmithing practice
• The stile can rule the lock out the cross bore reaches backset plus half its diameter into the stilelocksmithing practice
• The strike is laid out from the closed door not measured from the jamb edge, because the stop and the frame clearance set itlocksmithing practice