Door Lock Backset Guide for Australian Doors

A replacement lock can look right in the packet and still leave you with a door that will not latch, a handle that misses its cut-out, or a fresh hole drilled in the wrong place. This door lock backset guide explains the measurement that determines where the lock sits in the door - and why it must be checked alongside the rest of the hardware before ordering.

What is a door lock backset?

The backset is the horizontal distance from the locking edge of the door to the centre of the key cylinder, spindle hole or lock bore. Put simply, it tells you how far into the door the working centre of the lock sits.

For a cylindrical entrance set or deadbolt, the backset is normally measured from the door edge to the centre of the large round hole through the door. Common residential sizes are 60 mm and 70 mm. A 60 mm backset is often used where the lock needs to sit closer to the edge; 70 mm gives more clearance between the key or handle and the door frame.

For mortice locks, sliding-door locks and narrow-stile aluminium door locks, the measurement can be different in appearance but follows the same principle. Measure from the faceplate edge, or the door edge where the faceplate sits, to the centreline of the spindle or cylinder. These locks commonly have smaller backsets because the lock body must fit inside a narrow aluminium stile or door rail.

Backset is not a universal size label. Two locks can both have a 30 mm backset yet use different faceplates, cylinder positions, spindle heights and fixing-hole patterns. Treat it as a critical starting measurement, not the only one that matters.

How to measure a door lock backset accurately

Remove the handle or lock furniture if it obscures the holes, then open the door so you can access the edge comfortably. A steel rule or tape measure is suitable, although a vernier caliper is useful on small aluminium lock cases.

Start at the outside edge of the door, not the edge of the faceplate screw or the edge of a rounded door profile. Measure in a straight line to the centre of the relevant hole. On a keyed handle set, this may be the centre of the spindle hole and the cylinder may be positioned below it. On a deadbolt, measure to the centre of the cylinder or bolt turn. On a mortice sliding-door lock, measure to the centre of the key cylinder or thumbturn spindle specified for that lock.

If the hardware is still fitted, take the measurement from both sides where possible. Wear, loose escutcheons and oversized holes can make a single measurement misleading. Record the result in millimetres and do not round a 29 mm or 31 mm measurement to 30 mm until you have checked the manufacturer’s dimensions.

Measure from the correct reference point

This is where many replacement orders go wrong. On a tubular latch, the backset is usually from the door edge to the centre of the handle spindle. On a mortice lock, it is generally from the front of the lock case or faceplate to the spindle centre. On some sliding-door handle and lock assemblies, the published dimension may refer to the cylinder centre, not the handle spindle.

Check the product drawing or dimensions against the original part. If the old lock has a stamped code, photograph it before removing anything. A clear photo of the lock faceplate, body and handle holes gives a hardware specialist far more useful information than the brand name alone.

Common backsets on Australian doors

Many Australian hinged timber doors use 60 mm or 70 mm tubular latches and deadbolts. This does not mean every entry set uses one of those sizes. Older doors, commercial hardware and imported locksets may differ, especially where previous repairs have altered the drilling.

Aluminium sliding doors commonly use mortice locks fitted into a narrow stile. Their backsets can be 20 mm, 25 mm, 30 mm, 35 mm or another system-specific size. The lock body, hook or bolt projection, faceplate length, cylinder type and handle fixing centres are often just as significant as the backset.

Security screen doors are another case where assumptions cause trouble. A three-point lock, deadlatch or narrow-profile mortice lock may have a dedicated case depth and backset to suit the extrusion. The replacement needs to fit the existing cut-out and line up with the striker. A standard timber-door deadbolt is not a substitute simply because the key sits roughly the same distance from the edge.

Backset versus other lock measurements

A lock will only operate smoothly when its key dimensions work together. Before ordering a replacement, compare the backset with the door thickness, the lock body depth, faceplate height and width, screw-hole centres, handle fixing centres and the position of the cylinder or spindle.

For mortice locks, also check the overall case length and depth, the distance between the spindle and cylinder centres, known as centres, and the bolt type. A hook bolt for a sliding patio door is not interchangeable with a spring latch used on a hinged door. The striker or keeper on the frame must suit the bolt as well.

For a tubular latch or entrance set, confirm whether the latch is adjustable. Some replacement latches can be set to either 60 mm or 70 mm backset, which is useful when the door preparation is sound and the remaining hardware is compatible. Fixed-backset latches cannot be stretched or shortened without repositioning the bore hole.

Door thickness matters because the spindle, tailpiece, cylinder and fixing screws must reach through the door correctly. A lock intended for a 35 mm internal door may not install securely on a 45 mm external door. Conversely, hardware designed for a thick door can leave excessive spindle length or poor cylinder fitment on a thinner panel.

Why the wrong backset causes problems

A backset that is too short places the handle or cylinder too close to the door edge. On a timber door, that can weaken the area around the bore or interfere with the latch recess. On an aluminium stile, it may put the lock body hard against an internal wall of the extrusion, preventing it from seating properly.

A backset that is too long creates a different set of issues. The existing bore hole may not align, the lock may collide with a glazing bead or internal reinforcement, and the handle can sit awkwardly close to the door centre. On sliding doors, incorrect positioning can stop the hook bolt from engaging the keeper squarely, even if the door appears to close.

Do not force a lock into a cut-out that is nearly right. Enlarging an aluminium stile without understanding its internal profile can weaken the door or expose areas that the new handle will not cover. On timber doors, repeated drilling can leave loose screws and a poor-looking finish. It is usually more cost-effective to identify the correct replacement first.

Replacing a lock without creating new problems

Before removing the old lock, take several photos: one of the door edge, one of each side of the handle or cylinder, and one showing the striker or keeper. Measure the backset, faceplate, lock body and fixing centres. Include the door type - hinged timber, aluminium sliding, security screen or another system - because the same-looking lock can have a different internal arrangement.

Keep the old screws, cylinders and handles together until the replacement is tested. In some cases, the existing cylinder is compatible with the new lock body and can remain in place. In others, the tailpiece length, cam shape or cylinder profile is different, so using the old cylinder will prevent correct operation.

Fit the lock loosely first and test the action with the door open. Check that the handle returns, the key turns freely and the bolt extends fully. Then close the door gently and confirm the bolt meets the striker at the correct height. If a sliding door needs lifting to lock, adjust the door rollers before altering the lock or keeper. A worn roller can mimic a lock alignment fault.

When to get help identifying a replacement lock

Ask for assistance when the backset does not match a common size, the lock has no visible marking, or the door is aluminium, sliding or security-screen construction. This is particularly worthwhile where a replacement must match an older Stegbar, Dowell, Jason Windows, Bradnam’s, BGC, Boral, Doric or Rolltrak system.

Glideworks can help identify specialised replacement parts from clear photographs and measurements. Show the lock from the faceplate side, include a ruler in the photo where practical, and photograph the matching handle and striker. The goal is not merely to find a lock that fits into the door, but one that restores secure, reliable operation without unnecessary modifications.

A careful backset measurement takes only a few minutes. Paired with the lock body dimensions and a clear view of the existing hardware, it gives you the best chance of fitting the right replacement the first time.