Nominal Length (NL) vs Installation Depth: How to Measure Drawer Runners Correctly

The nominal length (NL) of a drawer runner describes its actual length when closed, whereas the installation depth defines the minimum space required inside the cabinet carcass.

When planning or repairing furniture, selecting the correct fittings is essential for optimal function. An error of just a millimetre inevitably leads to drawers failing to close completely or drawer fronts catching on the carcass. This guide provides fitters and joiners with the technical parameters to measure drawer runners precisely, specify the correct load capacity, and choose the ideal runner system for any project.

Key Purchasing Criteria When Measuring

The foundation of every drawer installation is the accurate determination of dimensions. The hardware industry uses standardised terms that must be clearly distinguished from one another.

Nominal Length (NL) and Drawer Depth

The nominal length is the most critical dimension when purchasing. It indicates the length of the runner in an unmounted, fully closed state. As a rule, the nominal length corresponds to the external length of the drawer box (from the back panel to the inside edge of the front panel). Runners are available in standardised increments. Typical standard sizes used in cabinet making include drawer runners 450 mm or drawer runners 500 mm.

Internal Carcass Depth (Installation Depth)

The internal carcass depth is the usable space inside the cabinet from the front edge to the rear panel. The golden rule for selection is: the internal carcass depth must always be at least 3 mm to 5 mm greater than the nominal length of the runner. If the carcass interior is exactly 500 mm deep, you can fit a runner with a maximum nominal length of 495 mm, which in practice requires selecting the next smaller standard size.

Installation Width (Side Clearance)

Every type of runner requires clearance between the carcass interior and the drawer exterior. For classic ball-bearing runners (telescopic slides), this dimension is standard at 12.7 mm per side. The drawer box must therefore be constructed 25.4 mm narrower overall than the clear internal width of the carcass. Deviations cause excessive friction and damage the runner mechanism.

Extension Types and Load Capacity

The choice of extension determines how far the drawer box can be pulled out and what weight it can support during daily use.

Full Extension vs Single Extension

A full extension allows the drawer to be pulled 100% out of the carcass. The extension length matches the nominal length exactly. This offers complete access right to the rearmost corner. A single extension (or partial extension), by contrast, only extends to approximately 75%. The rear portion of the drawer remains inside the carcass. For kitchens and workshops, drawer runners full extension are the technical standard.

Load Capacity and Heavy-Duty Applications

The total weight of the drawer including its contents must not exceed the stated load capacity of the runners. Ratings in kilograms (kg) always refer to a pair of runners.

• Standard runners: 25 kg to 35 kg load capacity (sufficient for clothing, cutlery).
• Medium load: 45 kg to 50 kg (for pans, files).
• For machinery, tools, or larder pull-outs, heavy-duty drawer runners must be fitted, which are often rated from 90 kg to over 130 kg.

Overview of Runner Types

The runner design determines running smoothness, lateral stability, and the mounting position.

• Ball-bearing runner: Telescopic slides mounted to the side of the drawer box or fitted into a 17 mm deep groove. They utilise steel ball bearings for extremely precise, play-free movement.
• Undermount runner: These runners are installed invisibly beneath the drawer base. They provide a very clean look, though they require specific routing and pre-drilled holes in the drawer box.
• Roller runner: A straightforward system where plastic wheels run inside an epoxy-coated metal track. Fitting is done along the bottom edge of the drawer. For simple furniture, drawer runners roller runner provide a proven, practical solution.

Opening and Closing Mechanisms

Integrated features enhance convenience. It is technically essential not to confuse these two mechanisms, as they operate on opposite physical principles.

Soft-Close Damping

A hydraulic or pneumatic damper absorbs the momentum of the closing drawer. Over the final few centimetres, a spring mechanism engages to pull the drawer smoothly and silently into its closed position. If you require this comfort, choose drawer runners with damping or drawer runners with self-closing mechanism.

Push-to-Open (Tip-On)

This mechanism is mandatory for handleless furniture fronts. Pressing the front panel compresses an internal spring, which then pushes the drawer out from the carcass. Drawer runners push to open generally cannot be combined with soft-close damping, as the damper would absorb the energy needed to trigger the release spring.

Which System for Which Application?

Selecting the appropriate hardware system depends on your specific application.

• For kitchen cabinets (pots, provisions): Choose a full extension undermount runner with soft-close. The load capacity should be at least 40 kg.
• For workshop trollies and vehicle conversions: Use ball-bearing heavy-duty runners without soft-close, but equipped with a locking mechanism (lock-in/lock-out) so drawers remain closed while in transit.
• For contemporary living room units (handleless): Undermount runners with a push-to-open mechanism are required here.
• For basic desk pedestals: A side-mounted ball-bearing runner or roller runner with single extension fully meets the technical requirements.

Installation Guidelines for Assembly

Installation demands absolute parallelism. The drawer must run like a train on its tracks—if the carcass is out of square by even a single millimetre, the mechanism will bind.

1. Use System 32: Modern cabinet carcasses feature pre-drilled hole patterns spaced at 32 mm intervals. The mounting holes on most runners are designed around this System 32. The hole diameter is typically 5 mm.
2. Height positioning: Mark the bottom edge of the drawer front and add the required bottom clearance. Transfer this line onto the carcass interior using a marking gauge.
3. Screw fixing: For chipboard, use dedicated Euro screws (typically 6.3 mm diameter) or standard wood screws (3.5 mm to 4 mm diameter) with PZ or TX drive heads. Countersunk screw heads must sit fully flush inside the runner track to avoid obstructing the carriage.

Frequently Asked Questions (FAQ)

How do I measure a faulty drawer runner correctly? Measure the old runner in its fully closed position from the front edge to the rear edge. This measurement in millimetres is the nominal length. Alternatively, measure the external depth of the wooden drawer box.

Can I replace a 450 mm runner with a 500 mm runner? Only if the internal carcass depth of the cabinet is at least 503 mm to 505 mm. If the cabinet interior is shallower, the drawer front will protrude or the runner will strike the back panel.

What is the difference between self-closing and soft-close? A self-closing runner pulls the drawer shut over the final few centimetres using spring tension alone. A soft-close system incorporates an additional damper (often a small fluid cylinder) that decelerates this motion, preventing the front from slamming noisily against the carcass.

How much clearance do I need to leave on the sides for ball-bearing runners? The standard clearance for side-mounted telescopic slides is 12.7 mm (half an inch) per side. The drawer box must therefore be exactly 25.4 mm narrower than the clear internal opening of the carcass.

Finding the Right Drawer Runners

To finalise your installation, determine the required nominal length based on your carcass dimensions. For compact bedside tables or bathroom vanities, select drawer runners 300 mm. Standard chests of drawers typically require drawer runners 400 mm, while deep wardrobes or workbenches can accommodate lengths from 500 mm up to 800 mm. Always ensure you select the appropriate opening mechanism and calculate the combined weight generously.