LAY OUT THE OPENINGS BEFORE THE CUTS

Shelf spacing calculator

Find equal clear openings for a bookcase or open cabinet. Enter the outside case height, actual panel thickness and number of internal shelves.

How equal shelf spacing is calculated

With top and bottom panels of the same actual thickness as the shelves, clear opening = [outside height − (shelf count + 2) × panel thickness] ÷ (shelf count + 1).

The top surface of the bottom panel is one panel thickness above the outside bottom datum. Add the first clear opening to locate shelf 1’s underside. Each next underside is one shelf thickness plus one clear opening higher.

Worked example: a 48-inch bookcase

Use a 48-inch outside height, actual 3/4-inch panels and three internal shelves. The inside height is 46 1/2 inches. Subtract 2 1/4 inches for the three shelves; 44 1/4 inches remains for four clear openings. Each opening is 11 1/16 inches.

Measured from the outside bottom of the side panel, shelf undersides are 11 13/16, 23 5/8 and 35 7/16 inches. These are calculated layout marks. A round support pin’s center sits below the shelf underside; the offset depends on its actual shape.

Which shelf connection will you use?

The opening calculation does not choose the joint. Removable shelves, connector-pin shelves and dado shelves need different cut lengths and machining. For recessed shelves, the groove lower edge can align with the shelf underside, while groove width and depth remain separate settings.

Create a case plan →

What this calculator assumes

• Top and bottom panels sit between full-height sides, flush with the case ends.
• All horizontal panels use one entered thickness; openings are equal and shelves are level.
• Unequal panel thicknesses, face frames, plinths, edging and inset top/bottom positions need a different layout.
• This is a geometry calculation, not a check of span, loads, support strength or anti-tip installation.

Next: calculate shelf length for dado joints, check actual plywood thickness, or plan bookshelf materials.