Surface-Mounted Rim Locks with 15 to 40 mm Backset: Comparing Common Sizes

December 16, 2025

The backset of a surface-mounted rim lock describes the exact distance from the centre of the keyhole or cylinder to the outer edge of the lock from which the bolt extends.

When fitting a lock to furniture or replacing a faulty unit, you will inevitably encounter the concept of the backset. When choosing suitable furniture locks, this measurement is the most critical technical parameter. If the wrong dimension is selected, the lock cylinder will not align with the existing hole in the furniture front, or the bolt will not engage cleanly with the striking plate. This technical guide compares common sizes from 15 mm to 40 mm and explains the specific constructions they are designed for.

What Is a Surface-Mounted Lock and How Is It Measured?

A surface-mounted rim lock is screwed flat onto the inside of a door or drawer—unlike a mortice lock, which is recessed into the timber. It consists of a housing, a bolt, and the locking mechanism. The mechanism can be operated via a standard lever key or a furniture lock cylinder.

To determine the backset correctly, use a tape measure or calliper. Measure from the faceplate edge (the side of the lock casing where the bolt shoots out) straight to the exact centre of the cylinder bore or keyhole. On an existing piece of furniture requiring a replacement lock, simply measure from the door edge to the centre of the hole drilled in the wooden front.

Much like the wheelbase of a vehicle determines the fixed position of the axles, the backset dictates the exact horizontal positioning of the lock on the timber surface. Replacing a lock with one of a different backset is not possible without making structural alterations to the front (drilling a new hole and filling the old one).

Comparison of Common Backset Sizes

Different standard dimensions have become established depending on the furniture type, frame width, and historical construction methods. The range typically spans from 15 mm to 40 mm.

15 mm to 20 mm Backset: Compact Solutions

Locks with a very small backset are used where space along the edge of the furniture front is strictly limited.

Application: Narrow timber frames, small drawers, delicate boxes.
Special features: The cylinder sits extremely close to the door edge. This requires precise installation, as the material near the edge (especially in chipboard) can easily split or break out.
Related types: Similarly compact dimensions are often found on a letterbox lock or a dainty display cabinet lock, where space is inherently restricted.

25 mm to 30 mm Backset: The Modern Standard

In contemporary furniture production, dimensions between 25 mm and 30 mm are the standard standard.

Application: Standard office pedestals, modern chests of drawers, cupboard doors made from board materials.
Special features: This distance offers an ideal compromise. It is large enough to prevent the drill hole from tearing out at the edge, yet small enough to keep the bolt travel short and efficient. A typical drawer lock for a desk generally features a 25 mm backset.

35 mm to 40 mm Backset: Solid Timber and Period Furniture

Larger backset dimensions are required when the door construction features a wide frame or when dealing with historic pieces.

Application: A surface-mounted lock for an antique wardrobe, heavy panelled doors, solid timber furniture.
Special features: On antique furniture, the keyhole was often positioned deeply within the face to bypass expansive fittings or decorative mouldings at the border. Anyone restoring period furniture will almost always require locks with a 35 mm or 40 mm backset.

Selection Criteria: Which Lock for Which Scenario?

Choosing a specific dimension depends on whether you are building a new piece of furniture or sourcing a replacement part for an existing build.

Scenario 1: Repair and Replacement

When replacing a defective lock, the existing front dictates the required size. Measure the distance from the door edge to the centre of the hole. If this distance is 30 mm, you must select a lock with an exact 30 mm backset. A discrepancy of just 5 mm means the cylinder will not fit through the front.

Scenario 2: Building New Furniture

When constructing your own furniture, you have full flexibility. Here, the dimension depends on the door construction:

If the door consists of a solid panel (e.g. MDF), a standard backset of 25 mm or 30 mm is recommended.
If it is a framed panel door, the cylinder must sit centrally within the stile timber. If the frame timber is 80 mm wide, a 40 mm backset is ideal.
For tall cupboards requiring multi-point locking, an espagnolette cupboard lock with locking rods is often fitted instead of a standard surface-mounted lock. Here too, the backset must correspond accurately with the front drilling.

Installation: Fitting a Surface-Mounted Lock Correctly

Installing a surface-mounted lock requires precision. Incorrect positioning can cause the door to bind or prevent it from latching properly.

Tools and Materials Required

Cordless drill driver with a suitable wood drill bit (often 18 mm or 19 mm for the cylinder)
Forstner bit (if the cylinder is to be recessed flush)
Tape measure and pencil
Screwdriver
Suitable wood screws and furniture screws

Countersunk screws with a diameter of 3.0 mm or 3.5 mm are suitable for securing the lock to engineered wood. Depending on your screwdriver drive, you can use wood screws 3.5mm or alternatives such as wood screws PZ2. The screw length must be selected so that it does not pierce the front surface. For a standard 19 mm furniture door with a 2 mm lock base plate, screws measuring 15 mm to 17 mm in length are ideal.

Step-by-Step Instructions

Measure and mark: Mark the horizontal position (backset) and vertical height on the inside face of the door.
Drill: Drill the hole for the cylinder or keyhole from the outside to avoid tear-out on the visible face. For veneered or laminated panels, apply masking tape over the area before drilling.
Position the lock: Place the lock on the inside and guide the cylinder through the drill hole.
Fasten: Secure the lock casing with wood screws. Pre-drilling (for example, with a 2 mm drill bit) prevents the timber from splitting.
Mount the striking plate: Extend the bolt and mark the exact position for the striking plate on the cabinet carcase. Then screw the plate on flush.

Frequently Asked Questions (FAQ)

What happens if I choose the wrong backset? If the backset of the lock does not match the hole in the furniture front, the lock cannot be fitted. The cylinder or key pin will miss the hole in the wood and hit against the solid inside face of the door instead.

Can I replace a 25 mm surface-mounted lock with a 30 mm one? A direct replacement is not possible. To fit a larger backset, a new hole must be drilled through the furniture front. The old hole would then need to be filled and disguised using wood filler or timber plugs.

How do I measure the backset on an antique wardrobe if the lock is missing? On the inside of the door, measure horizontally from the closing edge straight to the exact centre of the existing keyhole. This measurement in millimetres is your required backset.

Are surface-mounted locks available for different door thicknesses? The backset only dictates the horizontal positioning. For varying door thicknesses, the cylinder length is the deciding factor. A lock for 19 mm thick chipboard requires a shorter cylinder (often 22 mm) than a lock designed for a 30 mm solid timber door.