Measuring guide

A retrofit fails on millimetres, not on software. These are the measurements, how to take them, and how much error you can afford.

Tools

A digital calliper is worth having; a steel rule is enough for everything except cylinder length. Photograph each measurement with the rule in frame — you will want the evidence when a part does not fit.

The six measurements that matter

1. Backset

Door edge to the centre of the keyhole or cylinder.

Tolerance: about ±1 mm. Above that you are looking at a different lock, not a badly measured one.

2. Centre distance

Centre of the spindle (handle) hole to centre of the cylinder hole. This is the dimension that decides whether a handle can be fitted at all, and it has essentially no tolerance.

• 72 mm — European interior door
• 78 mm — some older European series
• 85 mm — common on Asian mortise sets
• 92 mm — European exterior door, and most UK uPVC doors

If you are replacing a lock on a fire door, redrilling to change this number is not an option.

3. Cylinder length

Remove the single central screw on the door edge and pull the cylinder out. Measure from the centre of the screw hole to each end and quote both numbers — 35/35, 30/10, 40/50.

Never order a cylinder by total length alone. Two cylinders with the same total length and different splits do not interchange.

For a retrofit motor, also record how far the cylinder protrudes past the escutcheon on the inside — usually 3–10 mm is what a clamp needs.

4. Faceplate

The strip visible on the door edge. Record width × length (24 × 235 mm is a common European size) and whether the ends are square or rounded. A rounded faceplate will not sit in a square pocket.

5. Spindle

The square bar the handle turns. European hardware is 8 mm; older and some North American hardware is 7 mm or 5/16 in. Measure across the flats, and note whether the corners are worn — a worn spindle slips in a new coupler.

6. Door thickness

Measure at the lock, not at the corner, and not including any veneer that has lifted. European interior doors are 38–50 mm; Singapore fire-rated leaves are typically 42–50 mm; North American doors are 1-3/8 to 2 in.

Two measurements people forget

Handle screw spacing — top screw hole to bottom screw hole. Essential on uPVC doors, where 205 mm and 215 mm both exist and look identical.

Clearance to the gate — on any door that opens onto a metal security gate, close the gate and measure from the door handle to the gate handle. If it is under 35 mm, a deep push-pull handle will not fit, no matter what the datasheet says.

Recording it

Write it as a spec sheet, in this form:

Door:        timber, interior, 40 mm thick
Lock family: euro profile cylinder + mortise case
Backset:     55 mm
Centre dist: 72 mm
Cylinder:    35/35 (total 70 mm), emergency function: yes
Faceplate:   24 x 235 mm, rounded ends
Spindle:     8 mm square
Standards:   DIN 18251 case, DIN 18252 cylinder
Fire rated:  no

That is enough information for a designer to specify an adapter without ever seeing the door, and it is what the identification wizard puts on your clipboard.

Questions

Do I need to remove the lock to measure it? No, except for the cylinder. Everything else can be measured with the door open and the lock in place.

My measurements are between two standard values. Then measure again — the standards are round numbers, and a 73 mm reading is a 72 mm lock measured with a bent rule. If it is genuinely between, you have a non-standard or badly worn lock, and the case should be replaced rather than adapted.

What is the measurement that causes the most failed retrofits? Cylinder length. It is quoted as two numbers, people record one, and the clamp ends up the wrong distance from the door face.