How Far Apart Should Shelf Standards Be?

The short answer is 24 in or 36 in. The longer answer is why two numbers, why those two, and why nobody should sell you a longer shelf.

Shelf standards sit at the edges of the shelf, so the distance between them is the shelf width. On a Desh wall that is 24 in or 36 in — the two widths the Shelves and Hangtracks are made in. Adjacent bays share the standard between them, so a run of n bays takes n + 1 standards: three bays, four standards.

That is the whole answer, and you can stop there. What follows is why the number is 24 or 36 and not 32, or 40, or "whatever you like" — because the reasoning is what tells you whether the answer is any good.

There are three constraints, and they point at different numbers. Spacing is where they are made to agree.

1. Two widths, and almost every run is buildable

The first constraint is arithmetic, and it is the reason there are two widths rather than one.

Two feet and three feet. Keep both in the catalogue and you can build any whole-foot run except one:

It does not stop at 12. Every whole-foot length above 1 ft is buildable, forever, out of two parts.

The property doing the work is that 2 and 3 are coprime — they share no factor — and that they are the smallest pair that is. Coprime alone is not enough: 3 and 4 are also coprime, and you cannot build 5 ft out of them. Nor is it enough to have a 2 in the pair: 2 and 5 are coprime, and you cannot build 3 ft.

For any two coprime widths, the largest length you cannot build has a name — the Frobenius number — and a formula: multiply the two widths together, then subtract both of them. For 3 and 4 that is 12 − 3 − 4 = 5. For 2 and 5 it is 10 − 2 − 5 = 3. For 2 and 3 it is 6 − 2 − 3 = 1, the smallest that expression can ever get. So one foot is the only length you cannot make — no pair of widths does better — and one foot is a length nobody wants a shelf to be.

Competition mathematicians know the same result by a better name. McDonald's once sold Chicken McNuggets in boxes of 6, 9 and 20, and the largest order you could not assemble exactly out of those boxes was 43 — which is why this is taught to teenagers as the Chicken McNugget theorem.

It is a modest piece of middle-school number theory, and it may be the most useful thing most of us ever do with prime numbers. It is also worth stating plainly that a manufacturer who sells four widths has not solved this better than a manufacturer who sells two. They have solved it worse, with four times the tooling and four times the inventory, and passed the cost to you.

A rule of thumb for your own wall. Use as many 3 ft bays as fit. If you are left with 1 ft, drop one 3 ft bay and use two 2 ft bays instead. That produces the fewest parts every time — a 12 ft wall as four 36 in bays needs five standards; the same wall in 24 in bays needs seven.

Bay widths add along the track, which is how the tracks are meant to be combined: a 96 in wall takes four 24 in Hangtracks, or two 36 in and one 24 in. Measure the wall first. Walls are rarely a whole number of feet, and it is better to know that before you order than after.

2. The ceiling on spacing is sag, not strength

The second constraint is the one that decides how far apart standards are allowed to be, and it is not the one most people assume.

Shelves almost never fail. They sag. A loaded shelf that has quietly bowed in the middle is holding every book that was put on it and has lost nothing but its line — and the line is the entire reason you chose the shelf. It is the most common defect in domestic shelving and it is nearly universal in bookcases: look at the background of any academic's video call and count the shelves with a curve in them. Nothing there is failing. All of it looks wrong.

For a shelf packed with books at the same density, deflection rises with the fourth power of the span. Going from a 24 in bay to a 36 in bay is one and a half times the span and about five times the sag. There are two ways to answer it. Add thickness, which costs money and weight on the largest piece of steel in the box. Or put the material where bending actually resists it.

The Desh– shelf takes the second route. Along both the front edge and the back edge, the steel is folded into a closed hem that forms a thin edge and then a flange. The result is a shelf that reads thin but behaves thick, with no perceptible sag.

3. We could go longer. There are four reasons not to

Honestly stated: the section would carry more span than 36 in. We stopped there anyway, and every reason is a reason that benefits the person buying it.

A long run does not need a long shelf. The slender cantilever is drawn so that nothing projects above or below the shelf line at a join, so shelves placed end to end read as one continuous line across the wall. If a 12 ft run of 36 in shelves looks like a 12 ft shelf, a 48 in shelf buys appearance you already have.

Long parts ship badly. Past 37 in a shelf stops being a parcel and becomes oversized freight — more expensive to move, and handled by fewer people, more times, with more opportunity to arrive dented. A dent in matte white steel is not a cosmetic footnote; it is the part. Every shelf we make stays under that line. The Standard cannot — at 85 in nothing would — so it ships as two 42.5 in halves: still over 37 in, but paying the smallest version of that penalty rather than the largest.

Long parts lock the layout. A 48 in shelf can only ever be 48 in. Two 24s can be 48 in today and two separate 24 in bays in a different room later. Length bought in large units is length you cannot rearrange, and rearranging is the thing the whole system is for.

A standard is also a position. Every standard is a place you are allowed to put a shelf edge, hang a shorter row, or start a second column at a different height. Spanning further means fewer standards on the wall, which means fewer places the wall can change later.

4. The spacing has to suit the parts that are not shelves

The last constraint is the one that would be invisible if we had not committed to it early.

A shelving system that stays useful for a decade eventually stops being only shelves. A desk surface. A drawer. A basket for clothing. A cabinet. Each of those is a real object with its own ergonomics, and each has widths at which it works.

We are not promising those parts here. The point is narrower and more useful: 24 in and 36 in were chosen partly because they are the widths those objects want anyway, so the geometry does not have to be re-drawn if any of them arrives.

Spacing is the one decision in a modular system you cannot revise. Everything else here moves without tools. The distance between standards is set the day the shelf width is set, and every part that ever hangs on the wall inherits it.

A footnote, because the coincidence is too good to leave out. Before Desh– I was Coca-Cola's principal engineer for the McDonald's account. Nothing in those years predicted that the nuggets would turn up again here, as the tidiest way to explain why a shelf is 24 in or 36 in.

The parts this post describes: the Shelf Standards at $49, shipped as two usable 42.5 in halves, and the Shelves in 24 in and 36 in widths from $84.99. They hang from the Hangtrack, or arrive sized as a Starter Kit. Why the system is priced the way it is: The price of an idea.