Pick what goes on the shelf and what the shelf is made of. You get how deep and how tall each shelf must be, which board is wide enough, how far it sags under the load, and the longest span that keeps the sag out of sight.
Your shelf
A 30″ shelf of ¾″ pine holds hardcovers: fine today, a visible sag in time0.06″ sag today (0.022″ a foot) · span up to 28¾″ keeps it under 0.02″ a foot
Quick answer: a bookshelf must be as deep as your books are wide: 6″ for a 6 × 9″ paperback, 7″ for a 7 × 10″ hardcover, 9″ for 9 × 11″ books and 12⅜″ for records, so a 1×8 board (7¼″) holds the first two and a 1×10 (9¼″) the large books. Each opening must be taller than the tallest book. A shelf looks straight while it sags less than 1/32″ a foot; Fine Woodworking designs for 0.02″ a foot, which keeps a ¾″ pine shelf of hardcovers to about 28¾″ between supports, and ¾″ MDF to 21¾″.
How to use the bookshelf calculator
- Pick what the shelf holds: paperbacks, hardcovers, large books or vinyl records. If you know your deepest and tallest one, type them; if you weighed a foot of them, type that too.
- Describe the shelf: what it is made of, the clear span between its supports, its thickness and depth, and whether it rests on shelf pins or is fixed into the sides.
- You get the depth and opening the books need, the stock board that is wide enough, the load, the sag today and after a few years, and the longest span that keeps the sag out of sight, drawn from the front to scale.
Hanging a shelf on a wall? The wall anchor weight calculator works out what the screws and anchors carry. Filling cubes? The cube storage bin size finder sizes the bins.
The rules, and where they come from
How deep. A book stands spine out, so its width is the shelf depth it needs. The most common paperback in the U.S. is 6 × 9″ (Amazon KDP), and KDP prints hardcovers from 5½ × 8½″ to 8¼ × 11″. A 12″ LP’s jacket is 12⅜″ square (Coral Graphics’ jacket template). NEDCC, the conservation center: “Shelving that is too shallow forces books to extend beyond the edges of the shelf.”
Which board. Lumber is sold by its nominal size: a 1-inch board is ¾″ thick, and 6, 8, 10, 12, 14 and 16-inch boards are 5½, 7¼, 9¼, 11¼, 13¼ and 15¼″ wide (the USDA’s Wood Handbook, Table 6–6).
When sag shows. Fine Woodworking (Jeff Miller, “Building Bookshelves to Last”): “our eyes won’t notice sag if it’s less than 1⁄32 in. (0.031 in.) per foot … design shelves to limit any initial sag to no more than 0.02 in. per foot under a load of full-size books.” The calculator measures that per foot of span: 0.06″ on a 3′ shelf.
It keeps sagging. Wood creeps under a steady load. The Wood Handbook: “after several years the additional deformation caused by creep may approximately equal the initial, instantaneous elastic deformation”, so the calculator doubles today’s sag for the years ahead.
How much it sags. A shelf loaded evenly along its length sags 5wL⁴/384EI resting on pins and wL⁴/384EI fixed at both ends (the beam formulas WoodBin’s Sagulator uses). E is the wood’s stiffness from the Wood Handbook at 12 % moisture: pine 1.24, poplar 1.58, red oak 1.82, white oak 1.78, maple 1.83, cherry 1.49 and walnut 1.68 million psi, Douglas-fir plywood 1.08, and the mean of the mills it tested for MDF (0.53) and particleboard (0.48).
What books weigh. WoodBin weighed a foot of each: paperbacks 6 lb, small hardcovers 20 lb, large 9 × 11″ hardcovers 32 lb, magazines 40 lb. A sound archivist on the ARSCList weighed LPs: “20 LPs, 3 inches thick, weight 11 pounds. So around 44 pounds per foot.”
Shelf depth and opening for each
| On the shelf | Deepest | Board | Opening over | A foot weighs |
|---|---|---|---|---|
| Paperbacks | 6″ | 1×8 (7¼″) | 9″ | 6 lb |
| Hardcovers | 7″ | 1×8 (7¼″) | 10″ | 20 lb |
| Large books | 9″ | 1×10 (9¼″) | 11″ | 32 lb |
| Vinyl records | 12⅜″ | 1×14 (13¼″) | 12⅜″ | 44 lb |
Hardcovers here are KDP’s 7 × 10″; measure your own tallest book and type it in.
Longest span for a ¾″ shelf on pins
| Shelf | Paperbacks | Hardcovers | Large books | Records |
|---|---|---|---|---|
| Pine | 43¼″ | 28¾″ | 26¾″ | 27″ |
| Poplar | 46¾″ | 31¼″ | 29″ | 29½″ |
| Red oak | 49″ | 32¾″ | 30½″ | 30¾″ |
| Maple | 49″ | 32¾″ | 30½″ | 30¾″ |
| Plywood | 41¼″ | 27½″ | 25½″ | 25¾″ |
| MDF | 32½″ | 21¾″ | 20¼″ | 20½″ |
| Particleboard | 31¼″ | 21″ | 19½″ | 19¾″ |
Each on the board its load needs (above), at 0.02″ a foot. Deeper boards span farther; a shelf fixed into the sides spans about 1.7 times as far on paper.
What IKEA’s bookcases measure
| Bookcase | Width | Depth | Height | Max load a shelf |
|---|---|---|---|---|
| BILLY | 31½″ | 11″ | 79½″ | 66 lb |
| BILLY, narrow | 15¾″ | 11″ | 79½″ | 31 lb |
| HEMNES | 35⅜″ | 14⅝″ | 77½″ | 66 lb |
| BRIMNES | 23⅝″ | 13¾″ | 74¾″ | 40 lb |
| IDANÄS | 31⅞″ | 15⅜″ | 83⅛″ | 44 lb |
| KALLAX, 2 × 4 | 30⅛″ | 15⅜″ | 57⅝″ | 29 lb |
From IKEA US’s measurement tabs. IDANÄS lists 13¾″ between its shelves.
Worked examples
A 30″ shelf of ¾″ pine holds hardcovers: fine today, a visible sag in time. It sags 0.06″ (0.022″ a foot), about 0.11″ after a few years; 28¾″ between supports, or a ¾ × 1½″ pine strip along the front, keeps it under 0.02″ a foot.
A 31½″ shelf of ¾″ MDF will sag visibly under hardcovers: 0.16″ today and about 0.31″ later. MDF’s 0.53 million psi is under a third of red oak’s 1.82. A poplar strip on edge along the front cuts it to 0.05″.
A 36″ shelf of 1″ red oak holds hardcovers with no visible sag: 0.03″. A thicker shelf is the strongest fix: twice the thickness sags an eighth as much.
A 13″ shelf of ¾″ plywood holds records with no visible sag: a short span shrugs off 44 lb a foot. Records need 12⅜″ of depth and an opening over 12⅜″.
Common mistakes
- Buying the board by its name. A 1×10 is 9¼″ wide, not 10″; a 9″-deep book needs every bit of it.
- Judging sag the day you load the shelf. It keeps creeping for years; build for the sag you will have, not the one you see.
- Long MDF or particleboard shelves. At 0.53 and 0.48 million psi they are less than half as stiff as pine (1.24), so they need short spans or a solid-wood front edge.
- Standing large books upright. NEDCC: “House very large or heavy volumes lying flat … stacks should be 2-3 volumes high.”
- Ignoring the maker’s rating. A bookcase you buy has a load per shelf; IKEA prints it on every measurement tab.
More on bookshelves at Dreamy Home Style
- Full wall bookshelf ideas
- IKEA BILLY bookcase built-in designs
- Built-in shelves in the living room
- Moody home library designs
- Bookshelf ideas that rethink your space
Planning the room around it? Draw it to scale in the free 2D floor planner.
Bookshelf questions
What is the standard bookshelf depth?
IKEA’s BILLY is 11″ deep and its HEMNES 14⅝″. What the shelf needs is your books’ width: 6″ for a 6 × 9″ paperback, 7″ for a 7 × 10″ hardcover, 12⅜″ for records.
How deep should a bookshelf be?
At least as deep as the widest book on it, so nothing sticks out past the front. A 1×8 board (7¼″) holds 6 × 9″ paperbacks and 7 × 10″ hardcovers; a 1×10 (9¼″) holds 9 × 11″ books.
What are standard bookcase dimensions?
IKEA’s BILLY is 31½″ wide, 11″ deep and 79½″ tall (15¾″ wide too); HEMNES is 35⅜ × 14⅝ × 77½″ and IDANÄS 31⅞ × 15⅜ × 83⅛″.
What is the standard bookshelf height?
The tall IKEA bookcases above stand 74¾″ (BRIMNES) to 83⅛″ (IDANÄS), with BILLY at 79½″ and a 41¾″ low BILLY. Each opening inside must be taller than your tallest book.
How far apart should shelves be?
More than your tallest book: over 9″ for 6 × 9″ paperbacks, over 10″ for 7 × 10″ hardcovers, over 12⅜″ for records. IDANÄS spaces its shelves 13¾″ apart.
How far can a shelf span without sagging?
At Fine Woodworking’s 0.02″ a foot, a ¾″ pine shelf of hardcovers spans about 28¾″ on pins, red oak 32¾″, plywood 27½″ and MDF 21¾″.
How much weight can a bookshelf hold?
IKEA rates its 31½″ BILLY shelf for 66 lb and its 15¾″ one for 31 lb. A foot of hardcovers weighs about 20 lb, so a full 30″ shelf carries about 50 lb.