Bookshelf spacing calculator: how far apart should shelves be?

How the spacing works

The gap between two shelves, the opening, has to be taller than the tallest book that will stand in it, plus a little headroom so you can tip the book out. The calculator adds 20 mm (¾ in), which is our assumption; set less if your shelves are fixed and every centimetre counts, more if you like to reach in easily. Then it works out how many openings of that height fit in the bookcase. Each opening has a shelf board above it except the top one, so with n openings of height h, shelves of thickness t and an inside height H, n × h + (n − 1) × t must not exceed H.

The result shows how many shelves fit if every opening suits one book type, and a mixed plan: one opening for the tallest books, usually at the bottom, and the rest sized for the next tallest type, which also takes everything shorter.

Shelf height for paperbacks, hardbacks and art books

Book sizes vary too much for a single right answer, so the form starts with our stated assumptions, not measured data: paperbacks 200 mm tall, hardbacks 240 mm and large-format or art books 310 mm. Before you drill or move shelf pins, stand your tallest few books on a table and measure them, then type those heights in. A ruler held against the spine is enough.

If a book type doesn’t fit the current opening, the result says so and draws it poking through the shelf above. Don’t force such books in at an angle. The Library of Congress says books too large to stand upright can be stored flat, fully supported by the shelf underneath, and NEDCC suggests piles of only two or three. It also says to shelve books of similar size together so each cover is supported by its neighbours, which is another reason to give the tall ones their own shelf.

How deep should a bookshelf be?

At least as deep as your deepest books, so nothing overhangs the front edge, where spines catch the light and get knocked. With our assumed sizes that means about 130 mm (5.1 in) for paperbacks, 160 mm (6.3 in) for hardbacks and 240 mm (9.4 in) for art books, so a 250 to 300 mm (10 to 12 in) shelf takes all three. Much deeper is not better: the UK National Archives’ book care leaflet says it is best not to push books right to the back of the shelf, so air can circulate, and a very deep shelf invites a second row hidden behind the first.

A worked example

The example is a bookcase 1,900 mm (74.8 in) tall inside with 19 mm shelves. Paperback openings of 220 mm fit eight shelves (8 × 220 + 7 × 19 = 1,893 mm), hardback openings of 260 mm fit six, and art-book openings of 330 mm fit five. The mixed plan puts one 330 mm opening at the bottom and five 260 mm openings above it, six shelves in all, using 1,725 mm and leaving room to adjust. The current 280 mm opening is too low for the 310 mm art books, and the 280 mm depth clears the 240 mm deepest book with 40 mm to spare.

How many books fit, as a bonus

The result also counts books side by side: the inside width, less a share kept free for new books, divided by the average spine thickness. In the example that is 21 books on a 760 mm shelf and 105 per bookcase. The 20% kept free comes from library planning: the Wisconsin Department of Public Instruction’s Public Library Space Needs (4th edition, June 2025) says a shelf’s working capacity ranges between 65 and 80 percent of its length. NEDCC adds that books should not be shelved so tightly that they are damaged when removed. If counting is what you mainly need, by type and for a whole collection, Shelf Sonnet’s bookshelf calculator is built for exactly that.

Common mistakes

• Spacing for the average book. Size each opening for the tallest book that will live there, not the typical one.
• Forgetting the boards. Every shelf takes its own thickness out of the bookcase’s height; five 19 mm shelves use almost 100 mm.
• No headroom. A book that fits with a millimetre to spare can only be pulled out by the top of its spine, which NEDCC says not to do.
• Putting the heavy art books up high. Tall shelves at the bottom keep the weight low; check a long span with the bookshelf sag calculator.
• Measuring the outside. Use the inside height and width between the panels.

Moving the whole library first? Calm Doorway’s moving box calculator estimates the boxes.