Planning to swap the rusty key-and-knob set on your vintage door for a sleek smart lock? Before you press “Buy Now”, grab a ruler and check these 5 critical door measurements. Get them right and the install takes 15 min; get them wrong and you’ll be drilling new holes.
Introduction: Why “looks right” is never enough
Every wooden or metal door on the planet is a custom snow-flake—warped by weather, trimmed by carpenters, drilled by previous locks. The good news? You only need five numbers to know if the lock you want is the lock you need. We’ll show you exactly where to measure, what the jargon means, and the red-flag ranges that scream “call a locksmith”.
1. Door thickness : the first gatekeeper
- How to measure: Open the door, measure the cross-section at the lock height. Do NOT assume the stated thickness on the blue-print is still accurate—old doors swell and get re-planed.
- Smart-lock tolerance: Most units accept 35 mm–55 mm (1⅜”–2⅛”). If your door is <35 mm, the interior escutcheon will protrude and may hit the frame. If it’s >55 mm, the included screws and spindle will be too short.
Pro tip: If you’re on the edge, order a “thick-door kit” before installation day.
2. Lock-case size (a.k.a. lock body depth & height)
Smart locks bolt onto the existing mortise or tubular latch. Two sub-numbers matter:
a. Case depth – distance from the door edge to the back of the lock body. Common sizes are 60 mm (2⅜”) and 70 mm (2¾”).
b. Case height – top-to-bottom height of the metal box. Smart locks expect 100–120 mm clearance inside the door cavity.
Measure with the old lock removed; if the cavity is smaller you’ll need to route extra wood or choose a retrofit cylinder instead of a full dead-bolt replacement.
3. Backset / edge distance: where the keyhole meets the frame
Definition: distance from the centre of the keyhole/cylinder to the leading edge of the door.
- Standard residential: 44 mm (1¾”) or 60 mm (2⅜”).
- Smart-lock requirement: within ±2 mm of the advertised backset or the bolt will miss the strike pocket.
Quick check: measure the old keyhole centre to the door edge. If you get 48 mm, a lock designed for 44 mm will sit crooked and jam.
4. Faceplate (forend) length & width
The shiny metal strip that shows after the lock is mounted. Smart locks usually ship with a 200 mm × 20 mm faceplate.
Red flags:
- Vintage doors with ornate 150 mm plates may expose raw wood when you swap.
- Narrow stile metal doors may have a 15 mm-wide faceplate groove—too thin for the new lock’s 20 mm plate.
Fix: purchase an adapter plate or file the smart-lock plate to fit.
5. Existing bore spacing (bolt-through & cross-bore diagram)
Pull out your phone and sketch the hole pattern. Three distances decide plug-and-play success:
a. Cross-bore diameter – the big hole for the dead-bolt barrel. Standard = 54 mm (2⅛”). Anything 1 mm smaller requires re-drilling.
b. Latch bore diameter – the hole for the latch tongue. Must be 25 mm (1″) or the new latch won’t slide in.
c. Bolt-through holes – two small holes that accept long screws to sandwich the front and back escutcheons. Centre-to-centre spacing is usually 38 mm (1½”) or 44 mm (1¾”). Measure horizontally from the keyhole centre. If your old lock used 32 mm spacing you’ll need to drill new holes or choose a lock with adjustable screw posts.
Checklist:
| Measurement | Your Door (mm / inch) | Smart-Lock Spec (from manual) | Match? |
|---|---|---|---|
| Door thickness | ✅ / ❌ | ||
| Case depth | ✅ / ❌ | ||
| Case height | ✅ / ❌ | ||
| Backset | ✅ / ❌ | ||
| Faceplate L × W | ✅ / ❌ | ||
| Cross-bore diameter | ✅ / ❌ | ||
| Latch bore diameter | ✅ / ❌ | ||
| Bolt-through spacing | ✅ / ❌ |
