The thickness tax
People divide an opening by the number of spaces they want and stop there. The board itself is the part that gets forgotten. Four shelves of three-quarter stock are three inches of solid material standing in the opening, and in a 72 inch case that is four percent of the height gone before anything is stored. Put eight shelves in the same case and it is six inches, a whole small bay.
| Shelves in a 72 in opening | Board thickness total (3/4 stock) | Clear per bay, equal spacing |
|---|---|---|
| 3 | 2.25 in | 17.44 in |
| 4 | 3 in | 13.80 in |
| 5 | 3.75 in | 11.38 in |
| 6 | 4.5 in | 9.64 in |
| 8 | 6 in | 7.33 in |
Those clear heights are the raw division, before any pin system rounds them. The jump from four shelves to five costs nearly two and a half inches of clear height per bay, which is the difference between a bay that takes a stack of folded sweaters and one that does not. Decide what has to fit first and let the shelf count fall out of it, which is what the second mode on this calculator does.
Pin holes round everything down
An adjustable shelf can only sit where there is a hole. If the rows are drilled at 1.25 inch centres and the arithmetic wants a bay of 13.8 inches, the shelf goes at 12.5 and the extra 1.3 inches has to go somewhere else in the case. Do that in five bays and you have collected six and a half inches at the top, which reads as a design decision if you plan it and as a mistake if you do not.
The 32 millimetre system, which most European hardware and most line-boring jigs are built around, works out to 1.2598 inches. Imperial rows at 1 or 2 inches are common on shop-made cases. A 2 inch row loses up to two inches per bay in the worst case, which is why fine adjustment matters more on short bays than on tall ones: two inches out of a 20 inch bay is nothing, and two inches out of an 8 inch bay is a quarter of it.
One taller bay is usually the right answer
Almost every real storage problem has one item that breaks the pattern. A cereal box, a stand mixer, a set of ring binders, a pair of boots. Sizing every bay to that item wastes height everywhere else; sizing every bay to the average leaves the tall item lying on its side. The calculator handles this by taking one or more bays out at the height the tall item needs and dividing the rest evenly among what is left.
The clearance over the tall item is worth thinking about rather than defaulting. An inch is enough to lift something straight up and out. Anything you have to tip to remove needs more, and anything you slide out flat needs almost none. Half an inch is fine for trays and books, and it is not fine for a jar you take out one-handed.
Where this stops
Spacing is geometry. Whether the shelf holds up is not, and this page deliberately says nothing about it. A 30 inch span of three-quarter melamine under a row of hardbacks will bow visibly within a year, and no amount of correct spacing changes that. The shelf sag calculator takes the span, the material, the thickness and a load figure of your own and predicts the deflection against a sag limit you set. Run it for the widest bay before cutting anything.
For the material itself, the plywood sheet calculator works out how many sheets a set of shelves takes and the cut list optimizer handles nesting parts along stock lengths. If the shelves are going in a closet, the reach-in closet layout calculator gives you the shelf run above the rod and the pantry shelf capacity calculator converts a shelf into a count of what actually stands on it.
A tall case loaded with heavy things is a tipping hazard, and it is worst when the heavy things are high and a child can climb the front. Heavy items belong on the bottom bays, and a full-height case belongs fastened back to the structure using whatever the case maker or the anchor maker specifies. This calculator does not rate a fixing.
Questions people ask
How far apart should bookshelves be?
It depends on what is going on them, which is why this calculator asks rather than answers. Mass market paperbacks stand around 7 to 8 inches, trade paperbacks 9, most hardbacks 9 to 10, and art and reference books run past 12. Add an inch to lift a book out straight rather than tipping it, and that is your clear bay height. Feed it into the second mode and the calculator tells you the most shelves that clear it, and what the bays actually come out at.
Why do my shelves not divide evenly?
Two reasons, and they compound. The shelves themselves occupy height, so an opening divided by the bay count is always wrong by the total board thickness. Then, if the shelves sit on pins, each bay rounds down to the nearest hole and the accumulated remainder has to end up somewhere, usually at the top. The calculator reports both losses separately so you can see which one is costing you.
What is 32 mm shelf pin spacing in inches?
32 millimetres is 1.2598 inches. It is the spacing that most European cabinet hardware and most commercial line-boring machines are built around, which is why shelf pins, drawer slides and hinge plates from those systems all land on the same grid. Shop-made cases often use a round 1 inch or 2 inch row instead, drilled from a shop jig. The finer the row, the less you lose to rounding on each bay, and the more holes you drill.
Should I make the bottom bay taller?
It is the usual choice, for two unrelated reasons. Heavy and bulky things belong low, both because you are not lifting them overhead and because a loaded case is more stable with the weight at the bottom. And the bottom bay is the one you see into at an angle rather than straight on, so a little extra height makes it usable. The calculator lets you assign any number of bays as tall bays; set it to one and the rest divide evenly around it.
Does shelf thickness affect how much the shelf sags?
Enormously, and not in proportion. Stiffness goes with the cube of thickness, so a 1 inch shelf is well over twice as stiff as a 3/4 inch one of the same material. Span works the other way and harder still: deflection goes with the fourth power of the span, so a shelf 20 percent longer sags about twice as much. That is why an extra support in the middle of a wide bay does more than any thickness change you can afford. The arithmetic is in the shelf sag calculator; this page only tells you how many shelves fit and how tall each bay is.