Undermount Drawer Slides vs Side-Mount: How to Choose, Size and Install Them

Undermount drawer slides sit hidden under the drawer box. They run to full extension, usually have soft-close built in, and need a drawer box built to the runner's own dimensions. Side-mount slides screw to the sides of the box. You can see them when the drawer is open, and they need a steady half inch of clearance on each side, but they will fit almost any square box. For new kitchen cabinets, pick undermount. For a new slide on an existing drawer, a roll-out tray or a shop-built box, side-mount is often the easier and cheaper answer.

We sell cabinets for a living and install them as builders, so every number here comes from a real document: Blum's installation sheet for the TANDEM plus BLUMOTION 563H runner (the one under Fabuwood Allure drawers), Accuride's sheet for its 3832EC side-mount slide, and the spec books of the lines we carry. How the soft-close damper works is covered in our soft-close vs self-closing guide. This page is about mounting, sizing and fitting.

Key Takeaways

• Clearance is the real difference. Accuride's 3832EC side-mount needs 1/2″ per side, so the box is built 1-1/16″ narrower than the opening. Blum's 563H undermount wants a box only 13/32″ narrower in total (with 5/8″ sides).
• Length follows depth. A 21″ slide for a 24″-deep kitchen cabinet, an 18″ slide for a 21″-deep vanity.
• Both carry about 100 lb. Blum rates TANDEM at up to 100 lb per drawer. Accuride rates the 3832EC at 100 lb per pair.
• Extension is a separate spec. Fabuwood's roll-outs come as BLUM E (partial extension) and BLUM H (full extension). KCD's Essential Builder Series ships ¾-extension, non-soft-close slides unless you upgrade.
• What we sell: every line with a published slide type puts its main drawers on concealed undermount runners or a metal drawer system. Side-mounts appear only on some roll-out trays.

Undermount vs Side-Mount at a Glance

This comparison uses two widely sold products, Blum's 563H undermount runner and Accuride's 3832EC side-mount slide, rather than a vague average. Other brands differ in detail, but the pattern holds.

The width row matters most. Take an 18″ opening. A side-mount box is 16-15/16″ wide outside, while a 563H box with 5/8″ sides is 17-19/32″. That is about 5/16″ more drawer on each side, and on a stack of three drawers it adds up. You also see the box instead of hardware, which is why undermount runners and dovetailed solid-wood drawers go together on quality spec sheets.

Which Slides Are on Your Drawers Now?

Pull the drawer all the way out. A metal channel on each side of the box is a side-mount. Bare wood sides with a runner visible only from underneath is a concealed undermount. A single center rail or white epoxy channels with plastic wheels are the two budget types, compared in the taxonomy table in our soft-close guide.

A drawer runner is simply a drawer slide: "runner" is Blum's word and the usual UK term, and "glide" is a third name, so undermount drawer glides are the same concealed runners. Fabuwood's spec book says runners, KCD says slides and J&K says glides for the same hardware.

How to take a drawer out (and take the slides apart)

• Undermount (Blum TANDEM): open the drawer fully, squeeze the orange handles on the two locking devices under the front corners, and pull the drawer straight out and up. The runners stay in the cabinet.
• Side-mount: to release the drawer slides, open the drawer fully and press the disconnect lever on each drawer member (on many slides, one lever up and the other down), then pull. That separates the two halves of each slide.
• Putting a side-mount back: push the ball-bearing retainer in each cabinet member fully forward first. Accuride warns that a misplaced retainer makes insertion hard and can itself be damaged.

Full Extension Drawer Slides vs ¾ Extension

That is what "full extension" means: the slide brings the whole box clear of the cabinet face, while a ¾-extension slide leaves the back quarter inside. Our soft-close guide compares extension across slide types in its taxonomy table, so here are just the cases where the same catalog gives you a choice:

• KCD lists its Premier Series drawers as "undermount, full extension, metal soft-close w/ ball bearings". Its Builder Series (sold on our site as Essential Builder) is "undermount, 3/4 extension, non-soft-close", with an "option to upgrade to soft-close, full extension" (KCD spec book, product comparison).
• Fabuwood sells the same roll-out in two versions. ROD-B… BLUM E uses "Blumotion partial extension concealed drawer runners", and ROD-B… BLUM H uses "Blumotion full extension concealed drawer runners" (Allure spec book). At the prices on our site, the full-extension H version costs $45.50 to $52.00 more per tray depending on width: ROD-B18 is $201.50 as BLUM E and $250.25 as BLUM H. Confirm on your quote.

J&K's spec book says "3/4 to Full extension", so ask which a given J&K drawer has.

What Size Drawer Slides Do I Need? Measuring Length and Clearance

Size a slide by its length and the side clearance it needs. Most mistakes come from measuring the old slide instead of the cabinet.

Length: follow the cabinet depth

Blum's full-extension 563H runners come in 9″, 12″, 15″, 18″ and 21″, and Accuride's full-extension 3832EC runs from 12″ to 28″. Buy the longest slide the inside depth allows. KCD's replacement kitchen slide "measures 21″, for use on 24″ deep cabinets", and its vanity slide "measures 18″, for use on 21″ deep cabinets". Blum's minimum inside depths are 21-15/16″ for the 21″ runner, 18-29/32″ for the 18″ and 15-29/32″ for the 15″.

To measure for drawer slides, remove the drawer and measure from the front of the opening to the inside of the cabinet back, less anything that sticks out, such as a pipe or a rear rail. An old slide's closed length tells you what to reorder, but check it against the cabinet.

Width and clearance: follow the slide's sheet

• Side-mount: the box goes 1-1/16″ narrower than the opening. Accuride says "construct drawer 1-1/16″ smaller than cabinet opening" and that slides "may not function properly" with less than 1/2″ per side.
• Undermount (Blum 563H): inside drawer width must equal the opening minus 42 mm (1-21/32″), so outside width is the opening minus 13/32″ with 5/8″ sides. Blum's example: a 21″ opening takes a 20-19/32″ box.
• Height (undermount): leave at least 1/4″ above the box. Blum caps the drawer height at the opening minus 25/32″, with the bottom recessed 1/2″ and a 9/16″ bottom clearance.

Measure the opening at the front and the back. On a framed cabinet the face frame usually makes the front narrower.

Undermount Slides in Face-Frame Cabinets: Rear Brackets and the Frame

In a frameless cabinet the runner screws straight to the side panel. In a face-frame cabinet the stile usually stands proud of the side, so Blum's sheet gives two fixes:

• Blocking. A strip on the side panel, flush with the inside of the frame, carries the runner. Blum's plain 563 runners work only this way.
• Rear bracket. The runner's front screws to the frame and a rear bracket (Blum 295.3750.02) reaches the cabinet back. For the 21″ 563H it suits inside depths of 21-15/16″ to 23-19/32″.

Accuride's side-mount face-frame method is similar: a front screw through the frame and a rear bracket on a 3/4″ × 2″ strip fixed to the cabinet back. Factory cabinets arrive with this done; it is only your job on a retrofit.

How to Install Undermount Drawer Slides, Step by Step

These steps follow Blum's 563H sheet for an overlay front; other runners follow the same order with their own numbers. On a factory cabinet you only need steps 5 and 6.

1. Check the cabinet. Confirm the inside depth meets the runner's minimum and the sides are flat and parallel; add blocking or brackets on a face frame.
2. Size the drawer to the runner. The drawer is as long as the runner, sides are 5/8″ or thinner, the bottom is recessed 1/2″, and width follows the formula above.
3. Prepare the drawer. Cut the rear notch at each back corner, bore a 6 mm hole about 10 mm deep for each rear hook, and pre-drill the locking-device screws under the front corners with a 2.5 mm bit. Blum calls its T65.1600.01 template essential for these holes. Screw on the left and right locking devices.
4. Mount the runners. Set each runner back 1/8″ from the cabinet face, level and matched side to side, with #6 × 5/8″ wood screws in the elongated holes first (±3/16″ of depth adjustment). Once the front sits right, add the screws Blum marks with orange dots; the rated load depends on them.
5. Clip the drawer on. Pull both runners out fully, rest the drawer on them and push it closed. The locking devices engage automatically.
6. Adjust. Align the front with the locking devices and tilt levers (below), then check the soft-close catches the drawer.

For a whole kitchen, drawers come last; see installing kitchen cabinets yourself for everything before this step.

Side-Mount Slides: Installing Them and Where They Still Win

Installing side-mount drawer slides in a cabinet is simpler. Build or check the box 1-1/16″ narrower than the opening, separate each slide, screw the cabinet members level and parallel, and screw the drawer members to the box on the same centerline (for inset fronts, Accuride sets them back the front's thickness plus 1/8″). Slide the drawer in and cycle it a few times; the 3832EC's cam gives 1/8″ of vertical adjustment.

Side-mount slides still win in three places:

• Retrofits. A square box with about 1/2″ of gap each side takes side-mounts with no rebuild.
• Roll-out trays behind doors. Nobody sees the slide, and the tray can be shallow. KCD's roll-out trays YN-ROT15-SC through YN-ROT36-SC come with "sidemount, full extension, soft-close slides". J&K's RD12–RD36 trays use "1 pair of ball bearing soft-close side glides", for 24″-deep cabinets, with a 25″ pantry glide sold separately.
• Heavy loads and odd sizes. Side-mounts come in more lengths (the 3832EC goes to 28″, the 563H stops at 21″) and in heavy-duty versions. Match the rating to the load; our load-rating section explains how ratings are tested.

Soft-close side-mounts are common: the 3832EC's Easy-Close is one. When a listing says "bottom mount", check whether it means a concealed undermount or a bottom-corner epoxy roller, which is rarely damped.

Adding Slides to Old Drawers, and Wooden Runners

Yes, you can add metal slides to old drawers, but the box decides which kind. Check it in this order:

1. Is it square and solid? Measure both diagonals. If they differ by more than about 1/8″ or the joints are loose, fix the box first.
2. How much side gap is there? With about 1/2″ each side, side-mounts bolt on. Old wood-on-wood drawers usually fill the opening, so the box must be narrowed or rebuilt.
3. Could it take an undermount? Only if the sides are 5/8″ or thinner, the bottom is recessed about 1/2″ and the back can be notched. Most older boxes fail one of these.

When it fails, a new dovetailed box built to the runner is often cheaper than fighting the old one. For adding soft-close without new boxes, see our retrofit section.

Wooden runners and hardwood drawer slides. Older and furniture-style drawers often run wood on wood, on a hardwood runner or a dovetail-shaped rail in the drawer side. If they still run straight, keep them. The usual shop fix for a sticky one is to clean the surfaces and rub on paraffin, candle wax or beeswax, avoiding oils that hold dust; a runner worn into a groove can be built up with a glued-on hardwood strip. That is general woodworking practice, not a manufacturer instruction.

Adjusting Slides and Fixing a Drawer That Won't Close

To adjust Blum drawer slides (TANDEM), use the three tool-free settings: side-to-side up to 1/16″ each way on the locking devices, height up to 1/8″ at the front, and tilt up to 3/16″ at the back of each runner. Our drawer-front adjustment guide walks through them alongside door hinges.

When soft-close drawer slides stop working and the drawer stops short or bounces back, check these in order:

• Locking devices not engaged. Take the drawer out and reset it until both sides click.
• Runners out of parallel. Check for a backed-out screw or one runner sitting lower.
• Something behind the drawer. A bag or pipe stops it before the damper takes over.
• Overload. Move the heaviest cast iron elsewhere.
• Front rubbing the frame or the next drawer: adjust height or tilt.

Damper behavior is covered in our soft-close troubleshooting section. Soft-close slide replacement parts: order by the runner's model number (from the runner or your cabinet's spec book), and match its length and extension. KCD sells its own replacement kitchen (21″) and vanity (18″) slides. Fabuwood notes that its warranty is void if drawer slides are modified, so on a newer cabinet, call before you swap hardware.

What Drawer Slides Ship on the Cabinets We Sell

From the manufacturers' spec books and our catalog data. Where a line doesn't state the slide type, we say so rather than guess.

No line that publishes its slide type puts side-mounts on its main drawers; they appear only on KCD and J&K roll-out trays behind doors. The one to question is KCD's Essential Builder Series: price its soft-close full-extension upgrade for pot drawers. Reviews: KCD and J&K. Fabuwood's pull-outs, trash drawers and roll-outs are in our Fabuwood accessories guide. For what Blum's testing and warranty promise, see our Blum review.

Decision Checklist: New Cabinets or New Slides

Drawer bases are where slide quality matters most: browse 3-drawer base cabinets or roll-out drawer cabinets, or read types of base cabinets.

Frequently Asked Questions

Are undermount slides better than side-mount?

For new kitchen drawers, usually: hidden, a wider box, full extension and usually soft-close. A good ball-bearing side-mount carries a similar load (100 lb per pair for the 3832EC) and is easier to fit, so it wins on retrofits and roll-out trays.

How much side clearance do side-mount slides need?

Typically 1/2″ per side. Accuride specifies 1/2″, +1/32″, for the 3832EC and recommends a drawer 1-1/16″ narrower than the opening. With less, the slide may not work properly.

Can you put undermount slides on any drawer?

No. For Blum's 563H the sides must be 5/8″ or thinner, the bottom recessed 1/2″, the back notched and bored, and the box as long as the runner. Most old drawers need rebuilding.

What length slide does a 24-inch-deep cabinet take?

A 21″ slide; Blum's 21″ runner needs 21-15/16″ of inside depth, which a 24″ base provides. A 21″-deep vanity takes an 18″ slide.

How do I remove a drawer with undermount slides?

Open it fully, squeeze the release levers on the two locking devices under the front corners (orange on Blum TANDEM), and lift the drawer up and out. To refit, pull the runners out, set the drawer on them and push it closed.

Can side-mount slides be soft-close?

Yes. Accuride's Easy-Close 3832EC and the slides on KCD's and J&K's roll-out trays are examples. Check the part number, since the same slide is often sold with and without it.

What is a drawer runner?

Another name for a drawer slide, used by Blum and in the UK.

Are Blum slides worth it?

What matters is published specs, adjustment and a model number you can reorder. Blum publishes installation sheets with every clearance, gives TANDEM tool-free height, tilt and side adjustment, and prints the model (563H on Fabuwood Allure) in the spec book.

Get a Quote That Names the Slides

Send us your room measurements and we'll return a free 3D design and an itemized quote within 24–48 hours. Ask us to name the slide brand, type and extension for the line you choose before you order. Start on our design service page, write to us through the contact page, or call 609-900-3358.

Sources for the hardware figures: Blum's TANDEM plus BLUMOTION 563H installation instructions and TANDEM product page, and Accuride's 3832EC quick reference.