The load figure is the one nobody has
Deflection arithmetic is well understood and the formula is not the problem. Span, thickness, depth and modulus are all things you can look up or measure with a tape. The pounds per running foot is the input that stops people, because it is the only one that depends on what you personally own, and the published tables for it are averages of collections that are not yours.
Weighing is the whole answer and it takes five minutes. Take a foot of shelf off, put it on a bathroom scale, note the number, put it back. That single measurement is more reliable than any table, and it captures the things a table cannot: that your paperbacks are old and yellowed and light, that half your shelf is ring binders, that the bottom row is atlases.
Why the same shelf carries very different loads
Media formats sit at very different densities per running foot, and the surprises run in the direction of more weight rather than less. The calculator ships no table of them, because the spread within any one category is wider than the gap between categories and a default would be a guess dressed up as data. Weigh yours. A collection with a heavy section — the reference shelf, the run of bound journals, the box files — is worth weighing separately from the rest, because that is the section that decides the span.
What is worth internalising is the shape of the arithmetic rather than the numbers. Load per foot times span gives the shelf. Shelf times levels gives the bay. Bay times bays gives the run, and the run divided by the strip of floor under it gives a pressure figure that is usually two or three times what people guess. A wall of full shelving is a large sustained load in a small footprint.
Per shelf, per bracket, per upright
The same total splits four ways and each split answers a different question. Per shelf is what the board and its supports carry, and it is the number that goes into a deflection calculation. Per bracket is that divided by however many supports the shelf actually has, which is two on most domestic shelving and three or four on wider spans. Per upright is the whole run divided by one more than the bay count, because a run of bays shares its verticals. Per square foot of floor is the total over the footprint.
The per-upright figure comes with a caveat the calculator states in the output and is worth repeating: it assumes even loading, and shelving is never evenly loaded. One bay of atlases and reference sets can carry twice what the bay next to it carries, and the upright between them takes the difference. If one section of the run is obviously heavier, work that section on its own rather than averaging the whole wall.
What is reported and what is not
This page reports pounds. It does not report whether they are acceptable pounds, and the field that asks for a limit is there so your own figure sits beside the load rather than so the page can grade it. That distinction is not pedantry. A bracket rating might be per pair rather than per bracket, quoted at a shorter arm depth than yours, and conditional on a specific fastener into specific backing. A shelving standard is rated for a load distribution that a real shelf may not match. None of that is visible in a single number, and a calculator that turned it into a pass or a fail would be inventing certainty.
Three hazards to name plainly, without a procedure. Loaded shelving tips, and full library shelving is far heavier than people expect, so anchoring back to the structure is a real question for the maker of the fixing and for the building. A floor carrying a room of paper or records is a structural question for an engineer, not for a spreadsheet. And a shelf that passes on paper can still bow over years under a load it is already carrying, which is creep, and it is what the shelf sag calculator models with its long-term multiplier.
For the capacity side of the same problem, the book shelving calculator converts a collection into linear feet and bays. For a wall of cantilever arms holding lumber, the lumber rack capacity calculator does the same split by arm and upright. For a shop wall of tools and bins, the garage shelving planner works on depth against the vehicles.
Questions people ask
How much does a foot of books weigh?
It depends enough that the calculator asks you to weigh it rather than assuming. The variation is driven by the same thing that drives books per foot — format — and it goes the other way: the thick heavy volumes that give you few books per foot also give you the most pounds per foot. Take a foot off the shelf and put it on a bathroom scale. That number, from your own collection, beats any published average, and it is the input every deflection calculation actually needs.
Do I include the weight of the shelf board itself?
Yes, and the calculator has a field for it, because the board rides on the same brackets and pins as the contents. On a light shelf under a heavy load it is a rounding error. On a thick hardwood or a long span of sheet goods it is not: a 32 inch shelf of one inch hardwood can be eight or ten pounds on its own, which matters when the bracket figure you are looking at is quoted in the seventies.
Is the per-upright figure the load on each anchor?
No, and the difference matters. Per upright is the total divided by the number of uprights, which tells you what one vertical carries if the run is loaded evenly. How that gets to the wall depends on how many anchors are in that upright and how they share the load between pull-out and shear, which is a fixing question rather than an arithmetic one. Treat the per-upright number as the total each vertical is passing on, and take the fixing question to whoever makes the fixing.
Why does the calculator not tell me if the shelf is strong enough?
Because it cannot see the shelf, the bracket, the fastener or the wall, and every one of those decides the answer. What it can do honestly is report the load and put your own limit figure next to it as a percentage. If that percentage is uncomfortable, the useful next steps are a shorter span, a middle support, or a thicker board, and the deflection side of it is worked out in the shelf sag calculator. A calculator that issued a verdict here would be guessing on your behalf about things it was never told.
What about the floor under a home library?
It is a real question and it is an engineer's question. The calculator gives you pounds per square foot over the footprint of the run, which is a useful thing to hand to somebody, and it is deliberately silent on whether that number is fine. Concentrated loads along a wall, over a basement, on an upper floor, or in an older building are all situations where the honest advice is to ask somebody who can look at the structure rather than to look up a table.