You have bought a smart lock. Will it fit your door?

Most people buy the lock first. They see a fingerprint reader or a keypad, decide they are done with keys, and order one. Then the box arrives and the instructions assume you already know what a backset is.

The good news is that whether it fits comes down to four measurements and one question, and you can do the whole check in about five minutes with a tape measure and your phone. None of it requires taking the door apart.

This is the question that decides everything else, and it splits UK front doors into two groups.

Multipoint doors. Open the door and look at the edge that goes into the frame. If you see several bolts, hooks or rollers spaced along it, and lifting the handle throws them all at once, that is a multipoint mechanism. Nearly every uPVC and composite door in the country is one.

Single-point doors. One latch and possibly one deadbolt, and no hardware along the edge. Most older timber doors are like this.

It matters because a multipoint door is locked by the handle and the cylinder together. A smart lock that replaces the whole handle set can drive it. One that only turns the cylinder generally cannot, no matter what the listing implies.

1. Door thickness. Measure the edge of the door. Most are 44mm; composite doors are often 48mm or more. Nearly every smart lock states a range, and a thick door is the most common reason the supplied screws are too short.

2. Backset. From the edge of the door to the centre of the handle spindle. In the UK it is usually 35mm, 45mm or 60mm. Get this wrong and the handle sits in the wrong place or fouls the frame.

3. Centres. On a multipoint door, the distance from the centre of the handle spindle to the centre of the keyhole. 92mm and 62mm cover most doors. This is the measurement people most often skip, and the one that most often stops the job.

4. Cylinder length. If the lock replaces your euro cylinder, measure from the fixing screw hole to each end separately — cylinders are frequently offset, like 40/50, not symmetrical. A cylinder that protrudes more than a couple of millimetres past the handle is a security problem in its own right, regardless of how clever it is.

Fit is mechanical. Whether it works for you is not, and three things decide that:

Where the WiFi reaches. Many locks talk Bluetooth to your phone and need a separate gateway plugged in nearby to do anything remotely. Bluetooth range through a solid door and a wall is roughly 10-15 metres in practice, not the number on the box. If your router is at the back of the house and the door is at the front, find that out now rather than after fitting.

Whose account it lives in. Set it up in your own, always. The most common mess we are called to fix is a lock configured under somebody else's account, where the owner cannot add a code without ringing them.

What happens when the batteries die. They warn you for weeks, and almost every lock worth buying keeps a mechanical key override. Check yours does, then put that key somewhere that is not inside the house.

Often, yes — and we would rather say so than pretend otherwise.

If you have a single-point timber door, the measurements match, and you are comfortable with a screwdriver and a chisel, a straightforward retrofit is a genuine afternoon job. Plenty of people do it well.

It is worth getting somebody in when the door is multipoint and the mechanism has to be changed rather than just the cylinder; when the existing cut-out is damaged or was made for a different lock; when there is glazing near where the new lock body goes, because drilling near it is how doors get cracked; when it is a fire door, where the certification depends on what is fitted to it; or when it is a rental or a communal entrance and somebody else has to agree to it first.

The failure that costs money is not a lock fitted slightly crooked. It is a door damaged in a way that cannot be undone — and doors cost considerably more than locks.

If you would rather just know, send three photographs and we will tell you: the face of the door around the handle, the edge of the door with it open so the mechanism shows, and the lock you have bought, still in its box.

We will tell you whether it fits, what it needs, and whether it is a job you can reasonably do yourself. No charge, and no obligation — if the answer is that you can manage it in an afternoon, that is a perfectly good outcome and we will say so.

Send us a photo, or read more about what fits a uPVC or composite door.