DIN 18251 Mortise locks for doors — case dimensions and requirements
Mortise lock cases for timber, steel and tubular-frame doors. Defines case dimensions, backsets, follower size and classes 1–8 by duty.
What it actually decides
- Backset values — 55 mm and 60 mm are the standard ones; 45, 65, 70 and 80 mm also exist
- Centre distance — 72 mm for interior doors, 92 mm for exterior doors; 78 mm on some series
- Follower (spindle) size — 8 mm square
- Faceplate sizes — 20 × 235 mm and 24 × 235 mm are the common ones
- Duty class, which drives cycle life and fire performance
Applies to these lock types
Euro profile cylinder in a mortise case (DIN) · Multipoint locking system (uPVC / composite)
Why a retrofit designer cares
This is the single most useful document for anyone designing a European retrofit adapter, because it is what makes the parts interchangeable in the first place.