Bookshelf spacing calculator: how many shelves, and where
Enter the bookcase's inside height and width and the books going on it. This works out how many shelves fit, where each one goes measured from the bottom, and whether a shelf that wide will sag under them.
Top of the bottom to underside of the top
The shelf's span
= 0.75"
At the bottom
Advanced settings10" books + 1" · 13" oversize · 11 1/4" deep · Hardwood plywood · 25 lb/ft
To tip a book out by its spine
= 11.25"
Paperbacks 20, hardcovers 25, art books 35 — rules of thumb
Fit
5shelves
Openings
5 at 11 7/8"
Oversize
1 at 14 7/8"
Sag, loaded
0.037"
Marks from the inside bottom
- Shelf 1Underside; its top is at 15 5/8"
- 14 7/8"
- Shelf 2Underside; its top is at 28 1/4"
- 27 1/2"
- Shelf 3Underside; its top is at 40 7/8"
- 40 1/8"
- Shelf 4Underside; its top is at 53 1/2"
- 52 3/4"
- Shelf 5Underside; its top is at 66 1/8"
- 65 3/8"
Each mark is the shelf's underside — the lower edge of a dado, or the pin line — measured from the top of the bottom panel, every one from the same end so rounding never compounds up the case. Openings, bottom first: 14 7/8", 11 7/8", 11 7/8", 11 7/8", 11 7/8", 11 7/8", which with 5 shelves at 3/4" add back to 78". The marks run the full height of the case, so they go on with a tape rather than a printed template; for a run of identical openings, a spacer block cut to 11 7/8" sets every dado the same. For adjustable shelves, the shelf-pin calculator lays out the holes instead.
Will a 30" shelf sag?
3/4" Hardwood plywood, 11 1/4" deep, loaded at 25 lb a foot and resting on its ends — the same beam the shelf sag calculator solves, where you can add a stiffening strip to the front edge.
All 2 checks passShow the numbers
- Sag within span ÷ 360 — passes0.037" of sag against an allowance of 0.083" — that is span ÷ 810.
- Under 1/32" of sag per foot of span — passes0.015" per foot. Past about 0.031" the curve is visible against a straight line above or below it — which is how anyone notices a sagging shelf.
- Sag within span ÷ 360 — passes0.037" of sag against an allowance of 0.083" — that is span ÷ 810.
- Under 1/32" of sag per foot of span — passes0.015" per foot. Past about 0.031" the curve is visible against a straight line above or below it — which is how anyone notices a sagging shelf.
Neither limit is law about a bookshelf: span ÷ 360 is borrowed from the IRC's floor deflection limit, and a thirty-second of an inch per foot is the woodworker's rule for when a sag becomes visible.
Worked example: a 30" bookcase
A bookcase 78" high inside, for hardcovers up to 10" with one oversize opening at the bottom, takes 5 shelves of 3/4".
Shared out evenly, the openings come to 14 7/8", 11 7/8", 11 7/8", 11 7/8", 11 7/8" and 11 7/8", and the shelf undersides go at 14 7/8", 27 1/2", 40 1/8", 52 3/4" and 65 3/8" from the inside bottom.
A 30" span of 3/4" hardwood plywood loaded at 25 lb a foot sags 0.037". It passes all 2 of the sag checks the calculator applies.
- Shelves
- 5
- Openings
- 5 at 11 7/8"
- Shelf 1
- 14 7/8"
- Shelf 2
- 27 1/2"
- Shelf 3
- 40 1/8"
- Shelf 4
- 52 3/4"
- Shelf 5
- 65 3/8"
These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.
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About bookshelf spacing
- How far apart should bookshelves be?
- Far enough for the tallest book plus about 1" to get a finger over its spine. Paperbacks up to 8 1/2" want 9 1/2"; hardcovers up to 10" want 11"; oversize up to 13" want 14". Put any taller openings at the bottom, where the case is strongest and the heavy books do least harm, and share whatever height is left over evenly between the rest.
- How many shelves fit in a 6 foot bookcase?
- A 72" case with a 3/4" top and bottom is 70 1/2" inside. For hardcovers up to 10" with an inch of finger room, that fits 6 openings of 11 1/8" — 5 shelves of 3/4". Make the bottom opening tall enough for 13" art books and it fits 4 shelves instead. Mostly paperbacks fit more.
- How wide can a bookshelf be before it sags?
- Loaded with hardcovers at 25 lb a foot and resting on its ends, a 3/4" × 11 1/4" shelf stays inside span ÷ 360 up to about 39 5/16" in hardwood plywood, and only about 25 5/16" in particleboard, which is a fraction as stiff. Past that, a strip glued on edge along the front, a thicker shelf or a centre divider are the fixes — the span is the thing that matters most, because sag grows with its fourth power.
- Should bookshelves be adjustable or fixed?
- Fixed shelves in dadoes stiffen the whole case — each one ties the sides together and stops the case racking — and they sag a little less because the ends are held. Adjustable shelves on pins let the spacing follow the books you actually own. A common compromise is one fixed shelf near the middle for strength and pins for the rest; the shelf-pin calculator lays those holes out.
You might also need
- Shelf sag and spanHow far a shelf bends under books, how far it can span, and what a strip on the front edge buys you.
- Shelf-pin hole layoutHole count and marks down a cabinet end panel, on the 32mm system or a plain 1" pitch.
- Cabinet boxesA carcass cut list from the finished outside dimensions, face-frame or frameless.
- Desk and workbench height calculatorSeated and standing desk, chair seat, monitor and workbench heights from your elbow height or your height, with the sources named.