Push-to-Open Full Extension Runners: Calculating Minimum Installation Depth and Nominal Length

The nominal length of a full extension runner indicates the exact length of the closed slide, while the minimum installation depth defines the required space inside the cabinet carcass and is generally 3 mm to 5 mm longer on handleless systems.

Implementing handleless furniture fronts demands the highest precision in the workshop and on site. When drawers need to open purely by pressing the front, the mechanism must be matched precisely to the carcass dimensions and the weight of the drawer. An error in calculating the installation depth inevitably causes the release mechanism to jam or the front to protrude. This guide provides joiners and fitters with the technical parameters to determine the appropriate runner length and properly prepare the installation of handleless drawer runner systems.

Key Buying Criteria: Dimensions, Load Capacity, and Mechanics

When selecting the right fittings for drawers, various technical specifications interlock. The mere cabinet depth is not sufficient as a reference value.

Nominal Length (NL) and Internal Carcass Depth

The nominal length is the most critical measurement when purchasing. It describes the length of the runner in its fully closed state. To determine the maximum nominal length for a project, the internal carcass depth (clear dimension from the front edge to the back panel) is measured.

For standard systems with an ejector mechanism, the nominal length must always be less than the internal carcass depth. The range generally covers lengths from 250 mm - 800 mm. For common bathroom furniture, a drawer runners 400 mm size is often used, whilst kitchen base units are typically fitted with drawer runners 500 mm.

Calculating Minimum Installation Depth (MID)

The minimum installation depth is the absolute minimum space required inside the cabinet so that the runner, including all mechanical components at the rear end, fits and operates smoothly. For push-to-open systems, the following basic rule applies: Minimum installation depth = nominal length + trigger travel + tolerance.

The trigger travel (the distance the drawer must be pressed inwards to unlock the spring) is usually 2.5 mm to 3 mm. Added to this are component-related overhangs of the runner at the rear end. In practice, the fitter therefore calculates a minimum installation depth that is exactly 3 mm to 5 mm greater than the nominal length.

Load Capacity and Dynamic Load

Load capacity is specified in kilograms (kg) and refers to the dynamic load – meaning the weight of the drawer including its contents in motion. For standard residential applications, 30 kg to 40 kg is sufficient. As soon as extremely deep drawers are planned, such as for tools or larder pull-outs, the leverage forces increase dramatically. Here, heavy duty drawer runners are used, which can carry 80 kg to over 130 kg depending on the design. This is especially mandatory for extreme lengths such as a 1200mm full extension runner to prevent the drawer from sagging when fully extended.

Types and Options at a Glance

Not every runner behaves the same way. The construction type determines the running characteristics and suitability for specific furniture projects.

Full Extension vs Partial Extension

A full extension allows the drawer to be pulled out of the carcass to 100% of its nominal length. The user has unrestricted access to the entire contents right up to the back panel. Those who opt for full extension drawer runners generally fit three-part telescopic slides. A partial extension, on the other hand, stops at around 75% of the nominal length. Part of the drawer remains inside the carcass, which makes accessing the rear area difficult in very deep cabinets.

Guide Types: Ball Bearing, Roller, and Undermount

• Ball bearing slides: The runners glide on hardened steel balls. They offer high lateral stability and are usually fitted to the side of the drawer (side mounting).
• Roller runners: A simple, proven design where plastic rollers run in powder-coated metal profiles. Roller drawer runners are robust, but have slightly more lateral play compared to ball bearing slides.
• Undermount slides: In this configuration, the runner is fitted invisibly beneath the drawer base. The drawer sides remain timber, which is visually more sophisticated. Undermount drawer runners are the standard in high-end cabinetry.

Push-to-Open vs Soft-Close (Damping)

This is one of the most critical technical distinctions in furniture construction. Push-to-open (also known as tip-on) is a purely mechanical ejector. Pressing the front releases a spring that pushes the drawer a short distance out of the carcass. This system is mandatory for handleless fronts. Corresponding push to open drawer runners require momentum to close until the mechanism re-engages.

Soft-close (damping) is the exact opposite. Here, an activator catches the drawer during the final third of the closing travel, pulls it in actively (self-closing), and cushions it gently via a hydraulic damper. A standard soft-close system always requires a handle on the front for opening. Both systems are mechanically incompatible: a drawer cannot be simultaneously pushed out by a spring and pulled in by a damper. Highly complex (and costly) combination fittings exist on the market, but fitters generally choose one of the two functions.

Which Runner for Which Application?

The choice of runner depends largely on the intended use, carcass depth, and desired level of convenience.

• For standard kitchen base units (handleless): With a typical carcass depth of 560 mm, the choice falls on a nominal length of 500 mm. A full extension runner with an ejection mechanism is ideal here to provide complete access to pots and pans.
• For deep workshop or office furniture: If the carcass measures 650 mm, the joiner chooses drawer runners 600 mm. For tool drawers, high load capacity (from 45 kg) should strictly be ensured.
• For campervan conversions and under-stair storage: Extreme depths are often reached here. Heavy-duty full extension runners with lengths from 800 mm to 1200 mm are required. For vehicles, a robust catch or lock when closed must also be considered.
• For narrow bathroom vanity units: Space is often limited by waste traps and pipework. Short nominal lengths of 300 mm or 350 mm are used here.

Installation Advice for Handleless Fronts

Installing a push system is like tensioning a spring – the trigger travel must be calculated precisely, otherwise the entire mechanism will jam.

1. Allow for a front gap: To allow the front to be pressed in to trigger ejection, it must not sit flush against the carcass edges when closed. A front gap of strictly 2.5 mm to 3 mm must be left. Many undermount runners feature depth adjustment to fine-tune this gap precisely.
2. Use System 32: Most modern runners are aligned with System 32 (32 mm hole spacing, 37 mm distance to the front edge). The screw positions on the carcass runner coincide with these hole centres.
3. Synchronisation for wide fronts: If a handleless drawer is wider than 600 mm, a synchronisation rod should be fitted. This connects the left and right runners. If the user presses the front at the outer edge, the rod transfers the force to the opposite side so that both ejectors release simultaneously. Without synchronisation, the drawer would twist and jam.

FAQ – Frequently Asked Questions on Calculation and Fitting

How do I calculate the minimum installation depth for push-to-open? The formula is: nominal length of the runner + trigger travel of the mechanism (approx. 3 mm to 5 mm). For a runner with a 500 mm nominal length, the interior of the carcass must therefore be at least 503 mm to 505 mm deep, excluding the back panel thickness.

Can I combine push-to-open with soft-close? As standard, both mechanical principles are mutually exclusive. Push-to-open ejects the drawer, whereas soft-close pulls it closed. While specialised mechatronic systems or high-end mechanical combination fittings exist, with standard drawer runners one must choose between the two functions.

What exactly does full extension mean? A full extension ensures that the drawer can be pulled out of the carcass to 100% of its nominal length. The back panel of the drawer sits flush with the front edge of the cabinet carcass when fully extended.

What size do I need for a 600 mm deep cabinet? If the external depth of the cabinet is 600 mm, deduct the thickness of the front (e.g. 19 mm) and the back panel (e.g. 8 mm). The internal carcass depth is then around 573 mm. The maximum possible nominal length for the runner in this case is 550 mm.

Why does my handleless drawer fail to trigger? It is usually due to a missing front gap. If the drawer front rests directly against the carcass, the 2 mm to 3 mm inward travel required to unlock the release mechanism inside the runner is missing. The depth adjustment on the runner must be re-adjusted.

Which screws should I use for installation? For fixing the carcass runners into the system hole line (System 32), Euro screws (Ø 6 mm to 6.3 mm) are recommended. Alternatively, for pre-drilled holes without a system grid, traditional countersunk woodscrews (Ø 3.5 mm to 4 mm) can be used.