Drawer Clearance Calculator

Frequently Asked Questions

How do I size a drawer box for side-mount slides?

Box width = opening width − total slide clearance, which is typically 1 in (1/2 in per side) for standard ball-bearing slides. A 16 in opening yields a 15 in drawer box width.

How much clearance do undermount slides need?

Undermount slides usually require the box to be about 5/16 in narrower per side and a specific bottom and back setback - follow the maker's spec sheet exactly, since tolerances are tight (often ±1/32 in).

What about drawer height and depth?

Leave clearance at top and bottom (commonly 1/2–1 in total) so the drawer clears the face frame, and make the box length match the slide length, typically 1 in shorter than cabinet depth.

Why does over- or under-sizing matter?

Too wide and the drawer binds or won't insert; too narrow and it racks and rattles. Matching the slide's exact clearance spec is what gives smooth, full-extension travel.

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Estimates for informational purposes only.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

A drawer box has to be smaller than the cabinet opening in every direction so the slides have room to work and the box never binds. This calculator sizes the box from the opening width and height and the cabinet depth, subtracting the right clearances. The biggest one is the side clearance for the runners: classic ball-bearing side-mount slides need a total of ½ inch - ¼ inch on each side - so the box width is the opening minus ½ inch. Undermount and centre-mount systems use different gaps, so you pick the slide type. The box height is the opening minus a top clearance so the drawer clears the rail and the box front, and the box depth is the cabinet's usable depth minus a back-set so the box does not hit the cabinet back or sit proud of the face frame. Work in inches or centimetres; the tool returns the three inside-the-opening box dimensions ready to cut.

Formula / Method

The width rule for the dominant case is box width = opening width − total side clearance, where side-mount slides take ½ inch total (the manufacturer's near-universal spec), undermounts roughly &frac516; inch, and wooden centre runners about &frac316; inch. The height rule is box height = opening height − top clearance, typically ½–¾ inch so the box rides under the rail and the drawer front overlays or insets cleanly. The depth rule is box depth = cabinet depth − back set, with the back-set (often around 1 inch) keeping the box short of the cabinet back and any face-frame return. Centimetre inputs convert by ÷2.54 internally and back for display. Every clearance must be non-negative, all openings positive, and the clearances cannot exceed the opening, or the tool returns a friendly message instead of a negative box size.

Worked Example

A face-frame opening is 15 inch wide × 6 inch tall, the cabinet is 21 inch deep, and you are using standard side-mount slides (½ inch total) with a 1 inch back-set and a ¾ inch height clearance. Box width = 15 − 0.5 = 14.5 inch - exactly the slide spec, ¼ inch each side. Box height = 6 − 0.75 = 5.25 inch. Box depth = 21 − 1 = 20 inch, so you would buy a 20 inch slide (the common stock length). Cut your sides and bottom to suit a 14½ × 5¼ × 20 inch box. Switch to undermount slides and the width grows to 15 − 0.3125 = 14.6875 inch, which is why undermounts are matched to a specific opening-minus formula in their instructions.

Common Mistakes

Forgetting the ½ inch total side clearance for side-mount slides and building a box that will not fit.

Measuring the cabinet outside instead of the clear opening between the face-frame edges or carcass sides.

Sizing depth to the full cabinet depth, so the box jams against the back or the drawer front cannot close.

Using a side-mount clearance with undermount slides (or vice versa) - each system has its own gap spec.

Ignoring the drawer-front style; inset fronts need extra reveal that affects the height and overlay math.

Tips & FAQ

Why exactly ½ inch for side-mount? The slides are ½ inch thick when extended - ¼ inch per side - so the opening-minus-½ rule is the manufacturer standard for nearly every ball-bearing slide. What back-set should I use? Enough to clear the cabinet back, wiring, or plumbing - 1 inch is common, but match the slide length you intend to buy (slides come in 12, 14, 16, 18, 20, 22, 24 inch). How much top clearance? ½–¾ inch lets the box clear the rail and gives room for the drawer-front fasteners; check your slide's instructions. Inset vs. overlay fronts? The box math is the same; the front sizing differs - overlay fronts are larger than the opening, inset fronts smaller with an even reveal. Undermount specifics? Undermounts demand a precise box width and a notch for the locking devices - always follow the brand's sizing chart. What about drawer-bottom dado? Account for the groove depth when cutting the bottom panel. Treat these box dimensions as a starting point and confirm against your exact slide's installation sheet before cutting.

Drawer Clearance Calculator

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