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Why drawer boxes are smaller than the opening
A drawer box must be smaller than the cabinet opening it slides into, because the slides need room. Side-mount slides typically need about half an inch of clearance on each side, so the box comes out about one inch narrower than the opening. Height and depth need clearance too, for the slide mechanism and for the drawer to clear the frame. Get the box too big and it jams; get it too small and it rattles and looks wrong.
This calculator turns an opening size into a box size. Enter the cabinet opening width, height and depth, and it returns the box width, height and depth using the common allowances: one inch off the width for two side-mount slides, three quarters of an inch off the height, and one inch off the depth for slide clearance. Always confirm the allowance your own slides need.
How to use the calculator
- Measure the opening width, height and depth in inches, inside the cabinet frame.
- Enter the three numbers and read the box size.
- Check the box size against your slide maker’s own clearance table before cutting.
A worked example
An opening 18 inches wide, 6 inches high and 20 inches deep gives a box 17 inches wide, 5.25 inches high and 19 inches deep. The width lost one inch to the two slides, the height lost three quarters of an inch for clearance top and bottom, and the depth lost one inch so the slides can close fully. A box built to the full opening size would not fit at all, and a box built to these numbers slides in with room to breathe.
Drawer front size is a separate job. The front usually covers the opening with a small overlap, so it is bigger than the box and often bigger than the opening. Do not confuse the two: the box is the working part that must clear the slides, and the front is the visible part that must cover the gap. Measure them separately and cut them separately.
Tips for drawers that slide well
- Build the box square. A box that is out of square by even 1/16 inch binds on one side.
- Use the slide length that matches your box depth, not the longest slide you own.
- Fix slides to solid cabinet sides, not to thin panels that flex under load.
- Test the empty box in the opening before you fit the front, because adjusting an empty box is easy and adjusting a finished drawer is not.
Common mistakes
The most common mistake is building the box to the opening size and discovering the slides do not fit. The second most common mistake is using the wrong slide clearance: undermount slides and side-mount slides need different gaps, and heavy-duty slides need more than light ones. A third mistake is forgetting depth: a box deeper than the slide travel stops the drawer from closing, and a box much shorter wastes storage.
Remember that this tool is an educational estimator using common allowances. Your slides’ own instructions govern the real clearances, and they vary by brand and type. Use this calculator to get close fast, then verify against the slide maker’s table before you cut.
Drawer bottoms deserve thought too. A thin bottom sags under heavy contents, so use a bottom thick enough for the load, set into grooves in the box sides rather than just nailed underneath. Grooves carry the weight along the whole side instead of at a few nail points, and they keep the box square during glue-up. Glue the joints, square the box on a flat surface, and let it dry fully before fitting the slides: a box that dries out of square will never slide well.
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