How this calculator works
A bookcase is a row of boxes. The calculator first finds how far a shelf of your board can span before it sags, then divides the wall into the fewest equal bays that keep every shelf within that span. For each bay it counts an upright, a top and a bottom panel and the adjustable shelves, adds a plinth and a back, and packs all the parts into sheets.
Worked example: a 12 ft wide, 8 ft high wall, 12 in deep, in 3/4 in plywood, with books on the shelves. A shelf of this board can span 30.4 in before it sags too far, so the wall needs 5 bays, each 27.9 in wide. That is 25 adjustable shelves, 5 sheets for the case and 4 for the back, with 110 shelf pins. Filled with books, the shelves carry about 1,643 lb.
The board matters: in 5/8 in particleboard the same wall needs 8 bays, and with a heavy record collection on the 3/4 in plywood it needs 6.
Assumptions and sources
The assumptions behind your numbers (waste, sizes, rates) are listed in the notes directly under your results, and they update when you change an input.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Bays and sheets by wall width
8 ft high, 12 in deep, 3/4 in plywood, 5 shelves a bay, books, 4×8 sheets.
| Wall width | Bays | Each bay | Case sheets |
|---|---|---|---|
| 6 ft | 3 | 23.0 in | 3 |
| 8 ft | 4 | 23.1 in | 4 |
| 10 ft | 4 | 29.1 in | 5 |
| 12 ft | 5 | 27.9 in | 5 |
| 14 ft | 6 | 27.1 in | 6 |
| 16 ft | 7 | 26.6 in | 7 |
Tips before you buy
- Build the case in bays no wider than the shelf span allows. If a bay is too wide, the shelves sag within a year or two, and the only cure is a middle support.
- Leave the top about 2 in (50 mm) short of the ceiling and cover the gap with a filler strip or a cornice. Floors and ceilings are rarely level, and a scribed filler hides it.
- Fix the case to wall studs or masonry through a rail at the top. A full case of books can weigh several hundred pounds, and a tall unit is a tipping hazard, especially with children or pets about.
- Drill a row of shelf-pin holes with a jig so the holes line up, and make the shelves 1/16 in (about 2 mm) short on each side so they drop in easily.
- Seal and paint the back of MDF and cut edges first: raw MDF swells with damp, and edge banding or filler and paint stops the corners chipping.
Tools & materials for this project
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Frequently asked questions
How do I build a wall-to-wall bookcase?
Divide the wall into bays, build each bay as a box from uprights, a top and a bottom, fix the boxes together and to the wall, then add a back and adjustable shelves on pins. This calculator works out the bays, the sheets and the cut list; the trim and paint go on afterwards.
How wide can a bookshelf be before it sags?
With books on it, a 3/4 in plywood shelf spans about 30 in, MDF a little less, and particleboard less again, if you want it to stay straight. The calculator gives the span for your board and load: 30 in for the defaults.
How many sheets of plywood do I need for a built-in bookcase?
For a 12 ft wide, 8 ft high wall with 5 bays it is 5 sheets for the case plus 4 for the back at the default sizes. It changes with the depth and the number of shelves, so enter your own.
Should the shelves be fixed or adjustable?
Adjustable shelves on pins are handier, and they are what this calculator counts. A fixed shelf at mid-height stiffens the case and the upright, and is worth adding in tall bays.
Does the case need to be fixed to the wall?
Yes. A tall unit full of books can tip over, so fix it to studs or masonry with proper fixings, at the top through a rail and at the base through the plinth. Check what the wall is made of before you choose the fixings.
How far apart should the shelves be?
About 10 to 12 in for paperbacks and 12 to 14 in for hardbacks, so that most books fit with a little room above. Taller bays at the bottom suit large art books.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.