Single Extension or Full Extension? The Right Length for 400 mm to 600 mm Cabinet Depths

June 3, 2026

A full-extension runner extends by 100%, whereas a single-extension runner only reveals around 75% of the drawer depth due to its design.

When planning carcass furniture—whether for the kitchen, office, or workshop—selecting the right runner system is crucial for everyday ergonomics and load-bearing capacity. Especially with standard cabinet depths between 400 mm and 600 mm, joiners and installers face the question of which extension type and nominal length best fulfil the technical requirements of the project. This guide provides the hard facts on dimensions, weight capacities, and system compatibilities to help you choose the right hardware.

Key Purchasing Criteria: Nominal Length, Load Capacity, and Function

The specification of a drawer runner depends on the geometry of the cabinet carcass and the intended payload. Matching these parameters precisely prevents future jamming or mechanical failure under load.

1. Nominal Length and Internal Cabinet Depth

The nominal length of the runner defines its installed length when closed and generally corresponds to the outer depth of the drawer box. The internal cabinet depth (the clear inside measurement from the front edge to the back panel) must always be slightly larger than the nominal length of the runner to leave room for the mechanism. As a rule of thumb: internal cabinet depth minus 3 mm to 15 mm (depending on the runner type) gives the maximum nominal length.

For a cabinet with an internal depth of 420 mm, choose drawer runners 400 mm.
With an internal depth of 515 mm, drawer runners 500 mm are used.
For deep tool cabinets with an internal dimension of 620 mm, opt for drawer runners 600 mm.

2. Dynamic Load Capacity

The load capacity is specified in kilograms and refers to the combined weight of the drawer itself and its intended contents when fully extended.

Standard runners usually carry 25 kg to 35 kg.
For pots, crockery, or filing folders, versions rated for 45 kg are required.
In industrial settings or for heavy machine components, heavy duty drawer runners with a load capacity of 80 kg, 130 kg, or more are strictly necessary.

3. Motion Mechanisms: Soft-Close vs. Push-to-Open

Additional features enhance ease of use, but rely on different technical mechanisms that are mutually exclusive.

Soft-Close (Damping): A hydraulic or pneumatic cylinder cushions the momentum of the drawer over the final few centimetres, pulling it shut smoothly and silently. Drawer runners with soft close require a conventional furniture handle for opening.
Push-to-Open (Tip-On): A mechanical spring mechanism is tensioned when closed. Light pressure on the handleless front releases the catch, ejecting the drawer a short distance outwards. Push to open drawer runners provide the technical solution for handleless fronts. Combining this with soft-close damping is physically impossible in purely mechanical systems, as the spring requires momentum to latch.

Overview of Runner Types and Mounting Methods

The guide mechanism determines the running action and positioning on the carcass. Three primary systems are relevant for cabinetmaking.

Ball Bearing Runners

These runners consist of interlocking steel profiles where small steel ball bearings run inside ball cages along the flanks. They are screwed to the side of the drawer box or routed into a side groove (e.g. a 17 mm groove). Ball bearing runners offer high lateral stability and exceptionally precise guidance. They are available in lengths from 250 mm to 800 mm and beyond, serving as the foundation for most heavy-duty applications.

Undermount Runners

With this system, the mechanism is mounted invisibly beneath the drawer base. The drawer rests on the runners and is usually secured via front locking clips. Undermount drawer runners offer the advantage that the entire internal width of the carcass can be used for the drawer box, as no side clearance needs to be deducted for the runners. They are predominantly used in high-end furniture manufacturing.

Roller Runners

The simplest system consists of epoxy-coated steel profiles along which plastic rollers travel. They are screwed to one side of the carcass and along the bottom edge of the drawer. The roller runner is robust, accommodates minor inaccuracies in drawer box construction, and lifts the drawer slightly as it extends (self-closing action via an incline). It is almost exclusively designed as a single-extension runner.

Decision Guide: Which System for Which Application?

Choosing between extension types depends on accessibility requirements and the intended use.

Choose a single extension if:

The budget is strictly limited and simple installation is the priority (e.g. in basic office pedestal units).
The cabinet depth is shallow (under 400 mm) and the rear of the drawer can be easily reached by hand anyway.
You are fitting shallow drawers for pens or cutlery, where losing 25% of the extension range will not obstruct item removal.

Choose a full extension if:

The cabinet depth is 450 mm, 500 mm, or 600 mm. At these depths, up to 150 mm remains hidden inside a dark cabinet when using a single extension, making item retrieval significantly harder.
Pots, kitchen appliances, or heavy tools are stored. Full extension drawer runners allow heavy items to be lifted vertically out of the drawer without tilting them beneath the top carcass rail.
A complete overview of the entire drawer contents is strictly required.

Installation Guidelines and Tolerances

Installing runner systems demands precision. The drawer box functions like a piston in a cylinder—without the accurately calculated clearance at the sides, the entire mechanism will bind.

For side-mounted ball bearing runners, the mandatory installation clearance (the gap between the outer side of the drawer box and the inner carcass wall) is usually exactly 12.7 mm per side. If the drawer box is even 1 mm too wide, the ball bearings operate under excessive pressure, causing rapid wear to the ball cages. If it is too narrow, the runners can disengage.

Modern cabinetry utilises the System 32 hole pattern for screw positioning. This standardised drilling grid uses 5 mm diameter holes spaced 32 mm apart. The mounting holes on most fittings are aligned precisely with this grid. For fixing into wood-based panels (chipboard, MDF), Euro screws (for 5 mm holes) or classic countersunk woodscrews (Ø 3.5 mm or 4 mm) are ideal.

Frequently Asked Questions (FAQ)

Can I replace a single-extension runner with a full-extension runner later on? Yes, this is generally straightforward, provided the new runner requires the exact same installation clearance (e.g. 12.7 mm for ball bearing runners) as the old hardware. The nominal length must also match the drawer depth.

What runner length do I need for a 450 mm deep drawer? You require fittings with a nominal length of exactly 450 mm. The internal cabinet depth must be at least 453 mm to 455 mm. Suitable models can be found in the drawer runners 450 mm section.

Why does my soft-close drawer fail to shut properly over the last few centimetres? This usually indicates that the activator pin on the damping cylinder is no longer engaging correctly with the runner mechanism. Common causes include dirt in the mechanism, a runner profile slightly bent from overloading, or misaligned runners that are not installed perfectly parallel inside the carcass.

When is a runner classified as heavy-duty? In hardware engineering, runners with a dynamic load rating starting from approximately 45 kg are considered reinforced. Dedicated heavy-duty classification typically begins with models capable of supporting 80 kg, 130 kg, or up to 250 kg per pair. These feature thicker steel profiles and more robust ball cages.