Full Extension vs Partial Extension: When Is the Upgrade for Drawers Worth It?

A full extension runner allows the drawer to be pulled out of the cabinet by 100%, whereas a partial extension runner stops at around 75%.

When building drawers or workshop units, choosing the appropriate runner system is essential. The decision between a partial extension and a full extension runner significantly influences everyday convenience, usable storage area, and the static load capacity of the furniture piece. This technical guide breaks down the mechanical differences and indicates which fitting is the correct technical choice for specific constructions.

What is a partial extension runner?

A partial extension runner is a drawer slide where the drawer remains partially inside the cabinet carcass when fully opened. The travel distance is shorter than the nominal length of the runner. Typically, the loss of extension is around 20% to 25%. For instance, with a runner length of 400 mm, only around 300 mm can be pulled out.

Mechanically, this is usually achieved via a simple roller runner (with plastic rollers) or a basic, two-piece ball bearing slide. These systems consist of a carcass profile and a drawer profile. They are mechanically straightforward and feature lower installation tolerances.

What is a full extension runner?

A full extension runner allows the drawer to be pulled out from the cabinet over its entire depth. The travel distance corresponds exactly to the nominal length of the runner. This is achieved using a three-piece telescopic system: alongside the carcass profile and the drawer profile, there is an intermediate profile that bridges the additional distance.

This design is found almost exclusively as a ball bearing slide or as an undermount runner fitted beneath the drawer base. Due to the more complex mechanism, full extension drawer runners are built more solidly and can statically support higher lever forces.

Technical differences in detail

To make the right choice for your project, the mechanical characteristics of both systems must be compared.

• Accessibility and ergonomics: Much like an iceberg, around a quarter of the drawer remains hidden with a partial extension runner. Items in the rear section are harder to reach. A full extension runner provides unrestricted top-down access to all stored contents.
• Load capacity and stability: Partial extension runners are generally designed for standard loads of 15 kg to 25 kg. Thanks to the three-piece profile, full extension runners can hold significantly more weight. For tool chests or deep pan drawers, fitters rely on heavy-duty drawer runners that handle weight capacities of 45 kg, 80 kg, or even over 100 kg.
• Installation dimensions: Side-mounted ball bearing slides typically require an installation clearance of 12.7 mm (per side) between the drawer side and the internal cabinet wall, regardless of whether they are partial or full extension. However, the profile height of a full extension runner is often larger (e.g. 45 mm instead of 35 mm or 27 mm), which must be considered during the vertical planning grid of the drawers.
• Mechanical complexity: A partial extension runner can usually be removed simply by lifting the drawer. With a ball bearing full extension runner, a small disconnect lever on the drawer profile typically needs to be pressed to separate the drawer from the runner.

Which system is suitable for which application?

The decision depends on the drawer depth, the intended contents, and the project budget.

When a partial extension is sufficient

The partial extension runner proves its strengths in shallow carcass depths and light loads. In a small bedside table fitted with drawer runners in 250 mm length, the loss of a few centimetres of travel is hardly noticeable. For simple office furniture, light chests of drawers, or projects with strict budget constraints, a classic drawer runner with roller mechanism is often entirely sufficient.

When a full extension is strictly necessary

As soon as the drawer becomes deeper or carries heavy items, a full extension runner is the professional solution. In kitchens, drawer runners in 500 mm or drawer runners in 600 mm are standard. If a partial extension were fitted here, accessing rear pots or provisions would be difficult. The same applies to workshop trolleys or deep underbed storage boxes. For office filing cabinets with suspended files, full extension is also essential to access the rearmost folders.

Additional features: Soft-close and handleless opening

Regardless of the extension length, modern runner systems can be equipped with mechanical add-on features. It is important not to confuse these two mechanisms, as they serve opposite purposes:

1. Soft-Close: This is a damping mechanism. When closing the drawer, an integrated spring and damper mechanism catches the profile over the last few centimetres, decelerating it and pulling it smoothly and quietly into the carcass. Drawer runners with soft-close damping are standard in modern high-spec furniture construction.
2. Push-to-Open: This mechanism is used for handleless front panels. Pressing on the front releases a tensioned spring that pushes the drawer outward by a short distance. Drawer runners with push-to-open require precise front gap adjustment (usually 2 mm to 3 mm) to ensure sufficient trigger travel.

Mounting types: Side-mounted vs Undermount

While standard roller and ball bearing slides are mounted to the sides of the drawer box, joiners in premium interior fit-outs increasingly rely on concealed fittings. Undermount drawer runners are fitted, as the name suggests, beneath the drawer bottom.

The advantage: when opening the drawer, no metal hardware is visible at the sides, displaying only the drawer box material (timber or decorative board). Undermount runners are available in both partial and full extension configurations. However, they require a specific drawer construction (the side walls must extend downwards past the drawer base to accommodate the runner mechanism).

Frequently Asked Questions (FAQ)

Can I easily replace a partial extension runner with a full extension runner?

Yes, in most cases this is possible provided the side installation clearance matches. If the old runner required a 12.7 mm clearance per side, a telescopic ball bearing runner with the same clearance can be retrofitted. It is important that the cabinet hole pattern (often drilled using the System 32 grid) aligns with the mounting points of the new runner.

Which runner length should I choose?

The nominal length of the runner is determined by the depth of the drawer box (external drawer dimension). If the drawer is 400 mm deep, you will need drawer runners in 400 mm. The runner must never exceed the internal clear depth of the cabinet carcass.

What is the difference between nominal length and extension length?

Nominal length describes the measurement of the runner when fully closed. The extension length defines the distance the drawer travels out of the cabinet. With full extension, both measurements are identical. With partial extension, the extension length is shorter than the nominal length.

Are there full extension runners that pull out further than 100%?

Yes, these are known as over-extension runners. With these, the travel distance is several centimetres longer than the nominal length of the runner. This is particularly useful when the drawer is installed beneath an overhanging worktop and needs complete, unobstructed access from above.