How far can a 3/4 inch shelf span before it sags?
A 3/4" hardwood plywood shelf holding books runs to about 39" between supports before the sag shows. Maximum spans by material, thickness and load.
About 39 5/16" between supports, for 3/4" hardwood plywood carrying 25 lb of books per foot.
At a 36" span that shelf sags 1/16" — a ratio of L/469, comfortably inside the L/360 limit.
How that is worked out
- Sag goes with the *cube* of the span. Doubling the distance between supports does not double the sag, it multiplies it by eight — which is why the answer changes so fast as a shelf gets longer.
- It goes inversely with the cube of the thickness too, for the same reason. Doubling a shelf's thickness cuts its sag to an eighth.
- The limit used here is L/360, which is the IRC's floor deflection limit borrowed as a yardstick — it is not law about a bookshelf. The woodworker's rule is tighter: about 1/16" of sag per foot of span, which is roughly where the eye starts to notice.
Maximum span carrying 25 lb/ft, 11 1/4" deep, ends simply supported
| Material | 3/4" thick | 1" thick | 1 1/2" thick |
|---|---|---|---|
| MDF | 27 1/4" | 36 5/16" | 54 1/2" |
| Melamine-faced particleboard | 25 5/16" | 33 3/4" | 50 5/8" |
| Particleboard | 25 5/16" | 33 3/4" | 50 5/8" |
| Baltic birch plywood | 40 3/16" | 53 9/16" | 80 5/16" |
| Hardwood plywood | 39 5/16" | 52 7/16" | 78 5/8" |
| OSB | 31 7/8" | 42 1/2" | 63 3/4" |
| Softwood plywood (CDX) | 37 1/2" | 50" | 74 15/16" |
| MDO plywood | 38 7/16" | 51 1/4" | 76 13/16" |
| Hardboard (Masonite) | 30 1/2" | 40 5/8" | 61" |
| Walnut (solid) | 40 13/16" | 54 7/16" | 81 5/8" |
| White oak (solid) | 41 5/8" | 55 1/2" | 83 1/4" |
| Red oak (solid) | 41 15/16" | 55 7/8" | 83 7/8" |
| Hard maple (solid) | 42" | 56" | 84" |
| Soft maple (solid) | 40 1/2" | 54" | 81" |
| Cherry (solid) | 39 1/4" | 52 5/16" | 78 7/16" |
| Poplar (solid) | 40" | 53 5/16" | 80" |
| Ash (solid) | 41 5/16" | 55 1/16" | 82 5/8" |
| Hickory (solid) | 44 3/8" | 59 3/16" | 88 13/16" |
| Mahogany (solid) | 39 5/16" | 52 7/16" | 78 5/8" |
| Sapele (solid) | 41 3/8" | 55 3/16" | 82 3/4" |
| White pine (solid) | 36 7/8" | 49 3/16" | 73 13/16" |
| Douglas fir (solid) | 42 7/8" | 57 3/16" | 85 13/16" |
| Cedar (solid) | 35 9/16" | 47 7/16" | 71 1/8" |
Worth knowing
- Load matters as much as span. These assume books at 25 lb per running foot; a shelf of LPs is over twice that, and it changes the answer more than swapping the material would.
- Fixing both ends properly — housed into dadoes rather than sitting on pins — deflects about a fifth as much as the simply supported case above. A 3/4" shelf that is marginal on pins is comfortable in dadoes.
- A stiffening lip does more than thickness does, for less material: doubling the thickness to 1 1/2" helps, but a strip of wood glued to the front edge puts material where the bending stress is highest and is the standard fix for a shelf that is too long.
- Panel moduli are nominal and vary by mill more than wood varies by tree. The ANSI minimum for particleboard is well under what a good sheet makes, so treat the panel rows as a guide and the solid wood rows as firmer.
Work out your own numbers
- Shelf sag and span
How far a shelf bends under books, how far it can span, and what a strip on the front edge buys you.
- Closet shelving
Shelf and rod heights for a single-hang, double-hang or shelf-only closet, with the shelf's sag checked.
- Shelf-pin hole layout
Hole count and marks down a cabinet end panel, on the 32mm system or a plain 1" pitch.
Related questions
- How far can a 3/4 inch plywood shelf span?
- A bit over 39" carrying books, before the sag becomes visible. Solid hardwood of the same thickness goes a little further and particleboard a good deal less. The honest answer depends on what is going on it — the same shelf holding LPs rather than paperbacks loses most of a foot.
- How do I stop a shelf from sagging?
- In order of how much they buy you per unit of effort: add a support in the middle, glue a stiffening lip along the front edge, house the ends in dadoes instead of resting them on pins, or make the shelf thicker. The lip is the one people underuse — a 1 1/2" strip on the front edge of a 3/4" shelf stiffens it far more than taking the shelf itself to an inch would.
- Is a sagging shelf dangerous?
- Usually not — it is a stiffness problem rather than a strength one, and shelves that sag visibly are typically nowhere near breaking. What it does do is get worse: wood creeps under sustained load, so a shelf that sags a little when loaded keeps going over years and does not come back when you unload it.