Drawer Sizing Math: Box Width, Clearance, and Slide Travel
Drawer Sizing Math: Box Width, Clearance, and Slide Travel - A step into the workshop.
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We've all done it. You measure the opening, you build the box a little narrow, you tell yourself "that'll leave a bit of breathing room," and then you screw the slides on and find out whether "a bit" was right. Sometimes it is. Sometimes the drawer rattles like a paint can, or the slides bind halfway out, or you end up with shims and slotted holes and a bad mood.
The frustrating part is that "leave a little clearance" is the advice everybody gets, and it's nearly useless on its own. Little relative to what? The slide type matters enormously, and the mounting style matters. What I want to do here is explain what the slide makers actually specify, and then be honest about where I can give you hard numbers and where I can't.
Why the clearance question isn't one question
A drawer in my shop is really three dimensions pretending to be one. There's the box itself, which has to fit inside the cabinet opening. There's the travel, meaning how far the slides let you pull the box out. And there's the front, the face you see, which is its own separate geometry.
Those three things get sized independently. Someone with a tape measure in one hand and a coffee going cold on the bench tends to collapse them into a single number, and that's where the trouble starts. The slide type is what dictates the box clearance. The slide's construction is what dictates travel. And the front geometry is its own separate problem.
The four mounting classes
Wikipedia's "Drawer" article is a reasonable starting place for the taxonomy, and it groups slides into four mounting classes. Side-mounted slides are the most common type and are visible when the drawer is open. Under-mounted (bottom-mounted) slides mount under the drawer box at the sides and are concealed when the drawer is open. Center-mounted slides sit near the bottom center, usually hidden, and the article notes they are often limited to lighter loads (with the article itself flagging that clause as uncited, so treat it gently). Wooden slides and wheel or roller runners round out the picture, with the roller type sometimes called European-style.
CabinetParts.com's slide-selection page adds a fifth category in practice: metal drawer systems with steel sides, which sit somewhere between a slide and a complete box. The same page sells Blum, Accuride, and Fulterer slides and warns that quality and cost are always a consideration. That's a fair warning.
What matters for our purposes is that these classes don't all want the same amount of room. They can't, because the geometry of how they attach to a box is different.
What undermount slides really demand
Here's the one sentence from my research that I think every beginning drawer builder should read twice. CabinetParts.com describes undermount slides this way: "Slide is mounted under the drawer box and is not visible when the drawer is extended. Drawer box must be built to exact specifications."
Exact specifications. That's the whole warning in three words. Undermount slides are the type where you cannot fudge it. If your box is the wrong width, the slide can't find its home. The exact numbers for that width come from the slide maker's literature, and you should look them up for whichever specific slide you've bought.
The other thing CabinetParts.com says about undermounts is that they're "popular" when cosmetics matter, which checks out. Wikipedia notes that under-mounted slides of the ball-bearing type can include soft-close, and that "due to its more complex form factor, under-mounted slides usually cost more than side-mounted slides." That's a fair trade for a drawer face that shows no hardware, but it isn't free and it isn't forgiving.
Side-mount slides are the forgiving option
Side-mount slides, by contrast, get described by CabinetParts.com as a "multi-purpose drawer slide," available in weight capacities from 75 lb to 500 lb, in full extension, partial extension, or full extension with overtravel, and "great for dovetail drawer boxes." That's a slide that tolerates a woodworker's ordinary variance.
If you're just getting going, side-mount is where I'd start. Not because it looks better, but because a box that's slightly off is a box you can usually still save. Undermount, by contrast, is the type where an off box becomes scrap or becomes a project for a different afternoon.
European-style nylon wheel slides (the roller type) are described by CabinetParts.com as attaching at the bottom of the box, available in full or partial extension only, in 75-lb and 100-lb ratings, and "ideal for small, lightweight drawers." Center-mount slides are limited to 35-lb or 50-lb ratings per the same page. Both are for light work. Don't overspec a kitchen drawer onto one of these.
Travel: full, partial, and the false back trick
Travel is where a lot of us get surprised. Wikipedia notes that three-section telescoping ball-bearing slides permit full extension, while two-section ball-bearing slides "cannot be fully opened." That's the honest distinction. If you want to see the whole interior of the box at the stop, you need a full-extension slide.
Wikipedia also describes another solution for cabinets without full-extension slides: a short false back or a shortened drawer back, so the interior is visible at the stop even though the box isn't fully out. That trick still works, and I've used it when I wanted a period-appropriate look in a piece that wasn't going to get modern hardware. You lose use of the space behind the false back, so it's usually a small drawer or a shallow one where that loss doesn't hurt.
CabinetParts.com lists "full extension with overtravel" as an option. Beyond that, I can't give you a number. I have no sourced travel figures to quote, so don't ask me for a millimeter column today.
Where the generic advice breaks down
CabinetParts.com calls undermount boxes "exact specifications" work, which is the strongest verified support for the claim that these are the demanding slide type. Wikipedia notes undermount slides are more complex and higher-cost. Neither source gives any numbers. My experience, not a sourced fact, says the arithmetic gets harder when the front sits inside the opening.
I should be straightforward about this. I wanted to build you a sizing table with per-slide clearance numbers and a per-slide box-width formula, and I can't do that honestly from the sources I have. The research for this piece came back thin on dimensional data. Blum's 2024/2025 Catalogue and technical manual places runner systems (MOVENTO, p. 410; TANDEM, p. 430) and box systems (LEGRABOX, p. 196 through METABOX, p. 338) in separate sections, and its product overview page describes TANDEM as the runner system for wood drawers and MOVENTO as the sophisticated one. But the catalogue's dimension pages weren't retrievable to me while I was writing, so no Blum number appears in this piece. When you're ready to build, that manual is where the numbers live.
What to do at the bench instead
Since I can't hand you a table, here's how I'd approach a drawer today, and my own shop practice only, not a sourced rule. Match the slide type to the job first, before you build anything. For a gift like a cherry keepsake box or a set of maple coasters in a small tray, a lightweight roller slide or no slide at all is plenty. For a kitchen or shop drawer that will see daily use, side-mount is the workhorse, and I'd rather overspend on a good side-mount than underspend on an undermount that demands a precision I'll need three tries to hit. For a piece where the hardware has to disappear, undermount is worth the trouble, but buy the slides before you build the box and keep the maker's specification sheet on the bench next to your cutlist.
Then build one drawer first. One. Fit it to its opening, install the slides, pull it out twenty times, and look at it honestly. Any tolerance trouble you find in drawer one shows up in drawers two through ten, and finding it once is cheaper than finding it repeatedly. If drawer one rattles, your clearance number is too generous for that slide. If it drags, the box isn't square or the slide isn't parallel to the cabinet side. If it stops before you can reach the back, you're on a partial-extension slide and you need a different construction choice, not a bigger hammer.
And honestly? Cut your first drawer box slightly generous rather than slightly tight. You can shim a box that's a hair narrow. You cannot un-cut a box that's too wide.
The last thing I'd say is this. The drawer is the part of a piece people touch every day, and it's the part that gets judged first. If you spend a Saturday on the case and five minutes on the drawer, the drawer will tell on you. Get the slide right, get the box made to what that slide wants, and get the front to sit where the piece deserves it to sit. The math is not the hard part. Asking the slide maker first is.