Undermount Drawer Slides: Measure Once, Fit Correctly

Undermount drawer slides sit beneath the drawer box, so the hardware stays out of sight when you open it. Choose the exact runner and its matching locking devices before you size the box: the manufacturer's drawing, not the cabinet's outside width, controls the fit.

This guide takes you from an empty opening to a checked installation. It uses one documented Blum TANDEM example to explain width calculations. It is not a universal cut list or a ranking of slide brands. The workshop photos are AI-generated editorial images, not CarpDraw results or installation evidence.

Start with the hardware and the real opening

A drawer can look right from the front and still be wrong underneath. A flush bottom, an unsuitable box width or an obstruction at the back can stop a concealed runner from fitting. Buying the slide first makes these constraints visible before you cut expensive parts.

For a new cabinet, I would order one complete hardware set and build one test drawer before making the rest. For an existing cabinet, I would remove one drawer and audit the opening before ordering anything. A label saying "soft close" tells you how closing is controlled. It does not tell you the required box geometry.

Your situation	First check	Decision before cutting
New cabinet and new drawer	Exact hardware drawing and actual sheet thickness	Approve one sample box
Existing cabinet, replacement drawer	Clear opening at the front and rear	Confirm runners have a straight mounting path
Existing drawer you want to keep	Inside width, side thickness and bottom construction	Confirm compatibility before modifying it
Drawer sticks or sits unevenly	Squareness, parallel mounting and full engagement	Correct the cause before changing adjustment settings

These are planning checks, not standard dimensions. Keep the cabinet's overall size separate from the opening used by the hardware. Our kitchen cabinet dimension guide explains the overall envelope; it cannot supply the measured opening inside your particular cabinet.

Make a hardware record

Write down the runner model, nominal length, locking-device model, permitted mounting arrangement and instructions revision. Keep the left and right parts paired. Include any required brackets or support blocks in the order rather than assuming they come in the box.

Check the maker's load rating for the exact installation. Count the drawer itself as well as its contents when the rating is for total moving weight. A deep drawer is not automatically a heavy-duty drawer. The bottom, joints, cabinet and fixing screws also need to carry the intended load.

If you cannot identify the existing hardware, take clear photographs of its markings and underside. Ask the hardware supplier to confirm compatibility. Guessing from the color of a release lever is not a reliable specification.

Measure the box from the correct reference faces

Measure the cabinet where the runners will actually sit. Record opening width at the front and rear, available depth to the nearest obstruction, opening height, drawer side thickness and the underside construction. Check both cabinet diagonals. An opening that narrows toward the back needs attention before you choose a box width.

A tape measure is useful for the cabinet. Use a caliper for side thickness, particularly with plywood. Nominal sheet thickness is a purchasing description, not a measurement of your finished drawer. See our plywood thickness guide before basing a cut list on a label.

A model-specific width calculation

The Blum TANDEM 563H/563 installation instructions, page 2, specify inside drawer width as opening width minus 42 mm. They call for measuring the actual side thickness to calculate outside width. This rule belongs to that documented system; do not apply it to an unidentified runner.

The example below assumes an opening of 500 mm and drawer sides exactly 16 mm thick. These are illustrative inputs, not measurements taken from the photos.

Width calculation	Millimeters	Centimeters	Inches, rounded
Illustrative clear opening	500 mm	50.0 cm	19.685 in
Model-specific deduction for inside width	42 mm	4.2 cm	1.654 in
Calculated inside drawer width: 500 - 42	458 mm	45.8 cm	18.031 in
Assumed thickness of each side	16 mm	1.6 cm	0.630 in
Calculated outside width: 458 + 16 + 16	490 mm	49.0 cm	19.291 in
Total opening minus outside width	10 mm	1.0 cm	0.394 in

The important distinction is inside width versus outside width. Subtracting 42 mm from the opening and calling that the outside box width would make this example 32 mm (3.2 cm, about 1.260 in) too narrow. Write the reference faces beside every dimension so the shop cannot misread them.

Keep the maker's original units for fabrication. The inch values above are rounded conversions for reading, not replacement tolerances. Do not round several intermediate values and then add them together.

Keep depth, height and bottom details separate

Do not use the width diagram as a full construction detail. It shows width arithmetic only. Read the exact model's side view and rear preparation drawing for length, vertical clearances, bottom recess, notches and hook holes. Obtain the matching current sheet through the manufacturer's runner instruction page.

In the cited 563H/563 sheet, the 563H5330B row lists a 533 mm drawer length (53.3 cm, nominal 21 in), a 548 mm runner length (54.8 cm, about 21.575 in), and a 557 mm minimum inside cabinet depth (55.7 cm, about 21.929 in). These are three different dimensions. Confirm the application drawing and front arrangement before using that row.

Also mark where the cabinet back, pipes, electrical fittings or projecting screws reduce depth. A convenient outside cabinet measurement cannot prove usable space inside it. Dry-position the parts and inspect their whole travel path.

Install one drawer before repeating the job

Undermount drawer slides installation works best as a controlled sequence: prepare the box, locate the runners, test engagement, then finish the fixing pattern. Do not use the adjustment features to hide a box built to the wrong dimensions.

Clear the work area and support the cabinet securely. Wear eye protection for drilling and machining. Use the specified screws and pilot holes; check their length against the material so they cannot break through a finished face. Drill with a stop where a hole must not pass through.

When the first fit is tight

Stop and locate the contact point. A rubbed edge or repeated scrape is more useful evidence than tightening every screw harder. Check for an overlong screw, trapped chip, crooked support, bowed side or incorrect bottom detail.

Measure the opening again at the depth where movement stops. If the drawer runs freely near the front but binds farther in, adjusting the visible front alone will not fix a narrowing path. Remove the drawer and correct the mounting or construction issue.

A supplier can help interpret the hardware drawing, but send measurements with the photos. "It sticks" is hard to diagnose. "It moves freely until the last part of travel, and the rear opening is narrower" gives the maker something concrete to check.

Test the fit and leave a useful workshop brief

A working sample is more valuable than a tidy front gap on an empty drawer. Test opening, closing, retention and access to the release points. Check that neighboring fronts do not collide and that the contents cannot catch above the box.

Mistakes that cost another drawer

• Using outside cabinet width as opening width: the resulting box can be too wide before the slide is even installed. Record the clear internal reference instead.
• Confusing inside and outside drawer width: side thickness is lost from the calculation. Label both dimensions on the section drawing.
• Copying a rear notch from a photograph: its apparent size changes with perspective. Use the matching machining template or a verified dimensioned drawing.
• Mixing runner and locking-device families: parts may appear to fit without retaining the drawer correctly. Check part numbers as a set.
• Leaving only temporary fixing screws: a successful trial movement does not complete the installation. Finish the required fixing pattern before loading.
• Testing only when empty: contents can reveal rubbing, poor retention or weak box construction. Test within the specified load and correct any movement in the fixings.

The brief a second person can build from

Prepare a front section, a side section and a rear preparation detail. Name the model, list the measured opening and actual material thickness, and mark every unverified value. Attach the manufacturer drawing rather than redrawing a hole pattern from memory.

Keep your measurements in one table with a date and revision. If the workshop changes the side stock or front construction, recheck the affected hardware dimensions before releasing the next drawer. That is where a casual sketch becomes slow: information gets split between a photo, a notebook and the hardware sheet.

Explore CarpDraw as a furniture-documentation project, but keep this installation's measured brief and manufacturer sheet as the source of truth. The live drawing workflow is not currently available for production-ready output, and a photo cannot verify the concealed hardware dimensions. Do not delay a working manual brief while waiting for an automated drawing.