Shelf spacing is a subtraction that people forget to do. The shelves themselves are solid, so five 18 mm boards in a two-metre opening take ninety millimetres of it before a single thing is stored — and what is left is then divided by the number of openings, not by the number of shelves, because a shelf only creates a usable gap above it. Then there is the real constraint, which is the tallest thing you want to stand up: a run of shelves spaced perfectly evenly is useless if your tallest book is two centimetres too tall for it.
Opening, boards and what goes on them
The boards take height back out of the opening before anything gets stored.
Floor to ceiling, or floor to the underside of whatever is above.
18 mm for most shelving; 25 mm or more for a long span.
The tallest book, folder or box that has to stand up.
Twenty to thirty millimetres to get a book out without scraping.
Result
The boards are not free
Five 18 mm boards take ninety millimetres out of a two-metre opening. That is four and a half percent, which sounds negligible and is not, because it is four and a half percent of the thing that then gets divided. It is the difference between five openings of 382 millimetres and five of 400, and 400 is enough for a shelf of hardbacks where 382 is not.
Thicker boards cost more twice: once in the material and once in the height. A 25 mm board on a long span is usually the right call, because an 18 mm board spanning more than about 800 millimetres sags visibly under a row of books — and the sag is permanent. But it takes a third more height out of the opening for the same number of shelves.
Divide by openings, not shelves
A shelf creates a usable gap above it, so five shelves in a floor-to-ceiling alcove give five openings — and the very top of the alcove, above the highest shelf, is often unusable because it is out of reach. Counting shelves plus one is the mistake, and it gives you a spacing that is slightly too tight on every single opening.
The other half of the calculation is the tallest thing, measured honestly. Books are not all the same height, and a shelf sized for the average is a shelf where the tallest quarter of the collection goes somewhere else. Measure the tallest atlas or the tallest ring binder in the house, add twenty to thirty millimetres to get it out again, and work from that.
Books, files and boxes are not the same height, and a run of identical openings wastes space above everything short and refuses everything tall.
Vary the spacing: tight at the top for paperbacks, a tall opening near the bottom for folios and boxes, and one opening sized for the thing you have that nothing else fits.
Reach, and the top shelf problem
The top opening in a tall run is the one that gets used least, and the reason is reach rather than geometry: a shelf above about 1.9 metres cannot be seen into by most people, and a shelf above about 2.1 cannot be reached safely without a step. Anything stored up there is effectively in storage rather than in use, which is fine if that is what it is for and infuriating if it is not.
The practical fix is to treat the top opening differently rather than to make it the same as the others: make it a shallow one for things you fetch twice a year, and put the everyday things between knee and shoulder height, where they can be seen and reached without thinking about it.
What a shelf can carry
Sag is the failure mode, and it is a function of the span to the fourth power and the thickness to the third. Doubling the span makes a board sag sixteen times as much; going from 18 to 25 mm makes it sag about a third as much. That is why a long run needs either a thicker board or a central support, and why the support is usually cheaper.
A bookshelf full of books is heavy — a metre of shelving loaded with hardbacks weighs around twenty kilograms, and a two-metre run is forty. The fixings into the wall matter as much as the board does, and a shelf bracket into plasterboard with a cavity fixing is not carrying that.
| Stored | Clear height | Span | Board |
|---|---|---|---|
| Paperbacks | 200 mm | 800 mm | 18 mm |
| Hardbacks | 260 mm | 800 mm | 18 mm |
| Folio and art books | 380 mm | 700 mm | 25 mm |
| Lever arch files | 330 mm | 700 mm | 25 mm |
| Box files | 360 mm | 900 mm | 25 mm |
| Vinyl records | 340 mm | 600 mm | 25 mm, heavy |
A worked example
- BoardsFive shelves at 18 mm take 0.09 m out of 2.0 m.
- UsableThat leaves 1.91 m to divide.
- SpacingOver five openings that is 0.382 m each.
- What is neededA 280 mm book plus 30 mm of clearance needs 0.31 m, so there is 0.07 m spare.
- The limitThe same opening would take 6 shelves at that spacing, at 0.318 m each — and no more.
Six shelves instead of five, at 318 millimetres, still clears the tallest book with eight millimetres to spare. That is the value of doing the subtraction properly: one more shelf of storage out of an opening that looked full.
Building and fitting
Cut all the shelves in one batch and check the opening is the same at both ends and in the middle before cutting, because alcoves are rarely square and a shelf cut to the widest point will not go in at the narrowest. Scribe the ends to the wall rather than filling the gap with a strip.
Fix the uprights first, level and plumb, then the shelves onto them. A shelf resting on a batten is stronger and easier to fit than one on visible brackets, and it can be adjusted later. Where the run is going into a stud wall, find the studs and fix into them rather than relying on cavity fixings.
The volume that results is worth checking too: the storage volume figure works out how much the finished run actually holds, which is a different question from how the shelves are spaced. And if the run is being built into a cupboard rather than left open, the furniture fit figure is the one that says whether it fits the wall.
Do
- Subtract the boards before dividing the height.
- Divide by the number of openings, not shelves plus one.
- Size for the tallest item, not the average.
- Reduce the span, or thicken the board, on a long run.
- Fix uprights into studs rather than into plasterboard.
Avoid
- Dividing the full height by the number of shelves.
- Sizing for an average book and discovering the tall ones do not fit.
- Spanning a metre with an 18 mm board and watching it sag.
- Counting the space above the top shelf as usable storage.
- Hanging a loaded run on cavity fixings.
Where the numbers stop being true
This assumes fixed shelves in a simple rectangular opening with nothing else in it. A run with a central upright divides into two shorter spans and needs a different spacing; a run that meets a skirting, a socket or a radiator has less usable height than the measurement suggests.
Nothing here accounts for adjustable shelving, which is the practical answer in most cases and which costs a little height for the holes and the supports. Adjustable is worth it anywhere the contents might change, and the lost height is usually under ten millimetres per opening.
And the spacing is a clear-height figure. A shelf you have to reach over, or one at the very top of a tall run, is effectively smaller than its measurement, because you cannot use the space you cannot comfortably see or reach into.
How the figures are produced
Board height is the number of shelves times the thickness, converted from millimetres. Usable height is the opening less that. Spacing is the usable height divided by the number of shelves, because each shelf creates one opening above it. What is needed is the tallest item plus the clearance, and the maximum number of shelves is the usable height divided by that, rounded down.
Questions people ask
How much space between shelves?
Whatever your tallest item needs plus twenty to thirty millimetres. Around 310 mm for hardbacks, 200 mm for paperbacks.
Do I subtract the shelf thickness?
Yes. Five 18 mm boards take 90 mm out of the opening before anything is stored.
How many shelves fit in a 2 m opening?
Six at 318 mm if your tallest item needs 310 mm. Five at 382 mm, comfortably.
How far can a shelf span?
About 800 mm in 18 mm board, and a metre in 25 mm. Beyond that, add an upright.
Fixed or adjustable?
Adjustable almost always. It costs under ten millimetres per opening and it survives a change of what you keep.
Why is my shelf sagging?
Too long a span for the thickness. Halve the span or go up a thickness.