Nominal Length (NL) vs Installation Length: Measuring and Understanding Drawer Runners Correctly

The nominal length (NL) of a drawer runner defines the standard dimension specification of the extension and generally corresponds to the exact depth of the drawer box. The installation length, on the other hand, refers to the minimum space required inside the cabinet (internal cabinet depth). For smooth installation, the installation length must always be a few millimetres greater than the nominal length.

What is the difference exactly?

When planning furniture and installing drawer runners, confusing nominal length and installation length frequently leads to construction errors.

The nominal length is the standard dimension under which a runner is classified. It determines how long the drawer box itself can be from the back panel to the inside edge of the front panel. The installation length (or installation dimension) is the absolute minimum depth that the cabinet must have inside so that the runner, including all mechanical components (such as soft-close dampers or release catches), physically fits inside.

You can imagine this principle like a shoe and its matching shoebox: the box (the cabinet depth) must necessarily be slightly larger than the shoe (the nominal length) so that the contents fit without pressure and the lid can close.

Dimensions, standards and typical values

Drawer runners in the furniture industry are mostly manufactured in a 50 mm grid. This often correlates with System 32, the standardised drilling line pattern in cabinet making.

Classic nominal lengths start with very short versions such as drawer runners 250 mm for bathroom furniture or narrow console tables. For standard kitchen base units and chests of drawers, drawer runners 400 mm or drawer runners 500 mm are mostly used. Depending on the manufacturer and runner type, the difference to the installation length is usually 3 mm to 5 mm.

Example: A runner with a nominal length of 500 mm often requires an internal cabinet depth (installation length) of at least 503 mm.

Runner types and their influence on installation length

Additional mechanical functions in the drawer system alter the space requirement inside the cabinet:

• Full extension vs single extension: A full extension drawer runner can be pulled out 100 %. The mechanism (often a telescopic system) is more complex, which on some models requires a slightly deeper installation length than simple single-extension runners.
• Damping: With soft-close drawer runners with damping, the damper cylinder is usually integrated in the rear section of the runner. Here, the manufacturer specifications for the installation length must be observed precisely, as the runner will otherwise collide with the cabinet back panel.
• Handleless opening: Push-to-open drawer runners require a trigger clearance of 2 mm to 3 mm at the front. The drawer must therefore be able to be pressed slightly inwards. This tolerance must strictly be added to the standard installation length.
• Load capacity: A solid heavy duty drawer runner (e.g. for loads from 45 kg) features thicker material gauges and wider cages for the ball bearings, which likewise influences installation tolerances.
• Runner type: A simple roller drawer runner has different tolerances compared to a concealed undermount runner. Undermount runners often require additional installation length for the rear locking hooks.

Application & selection: measuring correctly in 3 steps

To determine the correct nominal length for a piece of furniture, fitters proceed according to the following scheme:

1. Measure internal cabinet depth: Determine the inside dimension of the cabinet from the back panel to the front edge (excluding the overlaying front panel) in mm.
2. Deduct tolerance: Deduct a flat 5 mm from the measured value to ensure sufficient clearance for installation and any mechanisms.
3. Select nominal length: Choose the next smaller available standard nominal length. If the internal dimension is 480 mm, for example, deducting a 5 mm tolerance (475 mm) makes a runner with a 450 mm nominal length the professional choice.

Related terms

• Internal cabinet depth: The exact clear space inside a piece of furniture.
• Extension loss: The distance that remains inside the cabinet on a single-extension runner and cannot be pulled clear.
• System 32: Industry standard for hole drilling in furniture construction with a hole spacing of 32 mm.