Furniture Lock Backset: Definition & Measuring Guide for Cylinder Locks

The backset on a furniture lock refers to the exact distance from the centre of the keyhole or lock cylinder to the front edge of the lock case or the edge of the door. This measurement is given in millimetres and is the critical dimension when replacing a cupboard lock or installing a new one into a furniture front.

What exactly is the backset on a furniture lock?

The backset is the most important technical parameter when choosing mechanical locking systems for cabinetry and joinery. It determines precisely the horizontal axis on which the hole for the cylinder must be drilled in the furniture door, flap, or drawer front. If this measurement does not match an existing hole, the lock cannot be fitted flush or the cylinder will tilt.

With a classic furniture lock cylinder, the measuring axis runs horizontally from the centre of the cylinder core to the forend (the visible front metal edge of the lock casing). Fitting a backset is similar to the wheelbase on a vehicle – if the distance is not accurate to the millimetre, the component will not fit into the intended geometric recess.

How to measure the backset correctly

To determine the correct dimension for new furniture locks, standard vernier callipers or a precise steel tape measure are required. The measurement is carried out in three steps:

1. Remove the lock: Remove the old lock completely from the furniture door. The measurement can only be read accurately from the dismantled lock case without distortion from overlapping wooden edges.
2. Set the reference point: Place the measuring tool exactly at the front edge of the lock casing (the side facing the door edge).
3. Measure to the centre: Read the value precisely at the centre of the cylinder hole or keyhole.

If you are retrofitting a lock onto a door where a hole is already present, measure from the edge of the door directly to the exact centre of the drilled hole.

Typical dimensions and standards for cylinder locks

Standard backset dimensions have become established in furniture manufacturing, tailored to common timber thicknesses and frame widths.

• 15 mm - 20 mm: This range is frequently used for narrow frame profiles or compact drawer locks.
• 25 mm - 30 mm: The standard choice for classic hinged doors made of chipboard or MDF (typically 16 mm or 19 mm thick).
• 40 mm and more: These dimensions are used on solid timber cupboards, deep profiles, or period furniture.

In addition to the backset, the cylinder diameter must also be taken into account for cylinder locks. Typical drill holes for the cylinder measure 19 mm or 22 mm in diameter.

Usage & selecting the right lock

Selecting the correct locking mechanism depends on the construction of the unit. A simple surface-mounted lock is screwed directly onto the inside of the door. Here, the backset alone dictates the position of the hole in the front.

For tall cupboards, an espagnolette lock (also known as a rotary rod lock) is often fitted. This system locks the door at multiple points (top and bottom) simultaneously. Here, too, an exact backset is strictly necessary so that the rods can run parallel to the door edge without rubbing against the cabinet shelves.

Tools required for installation

Specific tools are needed to drill the cylinder hole accurately for a new installation:

• Forstner bit: Essential for clean, tear-free holes in engineered wood. The diameter must match the cylinder exactly.
• Vernier callipers: For precisely transferring the backset from the door edge onto the surface.
• Bradawl: To mark the centre point of the hole so the drill bit does not slip.

Related lock types and terms

The principle of measuring to the centre applies to virtually all mechanical locking systems in interior joinery, but varies depending on the application:

• Glass and display cabinet locks: A display cabinet lock has a specific dimension tailored to the glass thickness and hinge position. These are often clamped rather than drilled.
• Cam locks and letterbox locks: A letterbox lock (cam cylinder lock) usually features a very short backset, as the lever and cylinder form a compact unit directly behind the metal front.
• Central locking systems: This system locks multiple drawers in an office pedestal simultaneously via a side-mounted locking bar.