12.7 mm Installation Width for Telescopic Slides: Measuring Side Clearances Accurately

December 2, 2025

An installation width of 12.7 mm is the industry standard clearance for ball-bearing telescopic drawer runners, which must be maintained precisely between the drawer side and the cabinet internal wall.

Anyone building bespoke furniture knows that when fitting drawers, every millimetre determines functionality. Precise clearance calculations are essential, especially in compact designs such as custom desks for small spaces or shallow built-in cabinets. A deviation from the standard side installation width inevitably leads to jamming runners or premature ball-bearing wear. This guide provides the technical fundamentals for calculating, selecting, and installing telescopic runners with the standard 12.7 mm dimension.

Key Purchasing Criteria for Telescopic Drawer Runners

When selecting the right runner technology for furniture making, mechanical specifications and spatial requirements must be coordinated precisely.

Installation Width and Tolerance

The side installation width of exactly 12.7 mm per side is the most critical dimension for standard ball-bearing slides. For design and assembly, this means the external width of the drawer must be exactly 25.4 mm narrower than the internal clearance of the cabinet carcass. If the drawer is wider than this, excessive pressure is placed on the ball bearings, causing them to seize. If the drawer is narrower, the runners risk slipping out of their tracks.

Extension Type: Full Extension vs Single Extension

The mechanism defines how far the drawer extends from the carcass. A full extension runner consists of three profile sections and extends to 100% of its nominal length. This is essential for accessing the entire storage space right to the back. A single extension (partial extension) runner consists of two profiles and stops at around 75% of its travel. For maximum visibility and accessibility in deep drawers, full extension drawer runners are always the preferred technical choice.

Load Capacity and Dynamic Load

The load capacity of a runner is specified in kilograms and refers to the combined weight of the drawer box and its contents when fully extended. Standard models with a 12.7 mm installation width are typically rated for 30 kg to 45 kg. For workshop cabinets or heavy larder pull-outs, heavy-duty drawer runners must be installed. Please note: for heavy-duty runners (rated from approx. 80 kg), the required installation width often increases to 19.1 mm.

Nominal Lengths

The runner length must match the internal depth of the cabinet. Standard lengths range from 250 mm to 800 mm. For shallow furniture, such as space-saving hallway cabinets, short lengths are used. For these applications, 250 mm drawer runners or 300 mm drawer runners are suitable. For standard chests of drawers, 450 mm drawer runners or 500 mm runners are most commonly used.

Overview of Types and Options

The standard 12.7 mm dimension is primarily found in ball-bearing runners. Within this category, various mechanical features offer different levels of operating convenience.

Soft-Close: An integrated damper slows the drawer down over the final centimetres and pulls it shut silently. Soft-close drawer runners are the standard in modern furniture construction.
Self-Closing: A purely mechanical spring pulls the drawer shut over the last few centimetres without hydraulic dampening. Self-closing drawer runners close audibly, keeping the front flush against the carcass.
Push-to-Open: A spring-loaded mechanism pushes the drawer open with a light press on the front panel. Push-to-open drawer runners are essential for handleless fronts. Important: soft-close and push-to-open are opposing mechanisms and cannot be combined in standard ball-bearing runners.
Alternative Runner Types: Beside ball-bearing runners, there are roller runners, which typically require an installation width of 12.5 mm. For concealed mounting beneath the drawer base, undermount drawer runners are used. These require entirely different construction dimensions and do not use a 12.7 mm side clearance.

Which Runner for Which Application?

Choosing the correct runner determines the long-term durability of your furniture project.

Constructing a compact workspace: In desks for small spaces, drawers are usually shallow and short. A full extension runner with a nominal length of 350 mm or 400 mm is recommended to make pens and paperwork fully accessible. The 12.7 mm installation width ensures a slim profile.
Planning handleless fronts: Choose push-to-open runners. Ensure a gap of approx. 2 mm to 3 mm is left between the drawer front and the cabinet carcass to allow for the release travel of the spring mechanism.
Moving heavy loads: For deep kitchen pull-outs or workshop units, standard ball-bearing runners are often insufficient. Heavy-duty runners should be selected instead. Always check the technical data sheet for the exact installation width, as it often increases to over 19 mm on models rated from 100 kg upwards.
Building simple storage carts or lightweight boxes: Standard roller runners are sufficient here. They are more forgiving of minor measurement discrepancies, but do not offer full extension and have lower load capacities.

Installation Guide: Measuring and Fitting Clearances Accurately

Fitting a telescopic runner is like fitting a piston in a cylinder: a millimetre of excess play causes jamming and misalignment. Absolute precision during measuring and screwing is mandatory.

Required Tools

Cordless screwdriver
Tape measure or folding rule
Joiner square
Pencil
Spacers or shims (0.5 mm and 1 mm thickness)

Step-by-Step Installation

Determine internal cabinet dimensions: Measure the clear internal width of the cabinet carcass precisely at the height where the runner is to be fitted.
Calculate drawer width: Subtract exactly 25.4 mm (2 x 12.7 mm) from the internal carcass measurement. This gives the exact external width required for the drawer box.
Mark the height: Use a square to mark the horizontal centre line for the runner on the cabinet interior. In professional furniture making, the System 32 grid is standard, where holes are spaced 32 mm apart and start 37 mm from the front edge.
Separate the runners: Disengage the release lever on the telescopic runner to detach the inner drawer profile from the outer carcass profile.
Fixing: Screw the runners in place. Use countersunk chipboard screws (usually 3.5 mm or 4 mm diameter) with a PZ or TX drive. Screw heads must sit fully flush in the runner countersunk holes, as protruding heads will block the ball-bearing cage.
Adjustment: Insert the drawer. If the clearance is slightly too wide, thin shims or spacers can be placed between the carcass and the runner to achieve the exact 12.7 mm clearance.

FAQ – Frequently Asked Questions about Installation Widths and Telescopic Runners

What happens if the 12.7 mm installation width is exceeded? If the gap is larger than 12.7 mm (e.g. 14 mm), the runner profiles will not interlock deeply enough. The ball bearings lose proper contact, the drawer will wobble side-to-side, and the slides may pull out of their tracks under load.

Can I adjust a drawer that is too narrow using washers or shims? Yes, minor deviations of 1 mm to 2 mm can be compensated for by packing out the runner from the cabinet side with thin shims. However, it is technically preferable to build the drawer box to the exact required width (internal cabinet dimension minus 25.4 mm) from the start.

Does the 12.7 mm dimension apply to undermount runners? No. Undermount runners are fitted concealed beneath the drawer base. The side clearance between the drawer side and the carcass is typically only 4 mm to 6 mm, depending on the manufacturer and system.

Which screws are suitable for fitting ball-bearing runners? Use countersunk chipboard screws with a diameter of 3.5 mm or 4 mm and a length of 16 mm (for 18 mm carcass material). A countersunk head is essential so that the screw sits flush within the profile. Alternatively, 5 mm diameter Euro screws can be used in pre-drilled System 32 hole patterns.

How do I calculate drawer depth relative to runner length? The external length of the drawer box should correspond exactly to the nominal length of the runner. If installing a 450 mm nominal length runner, the drawer box from the front edge to the back wall must also measure 450 mm deep (excluding any overlay front panel).