What shelf sag is
Every shelf bends a little under load. Sag is how far the middle drops below the ends. It grows with the cube of the span, so a shelf twice as long sags eight times as much, and it shrinks with the cube of the thickness, so going from 3/4" to 1" stock cuts it by more than half.
The wood matters too. Stiff species like hard maple, birch and Douglas fir resist bending; MDF and particle board are about a quarter as stiff as solid wood, which is why a long MDF shelf full of books ends up looking like a smile.
How much sag is too much
Woodworkers measure sag in inches per foot, the way the Sagulator does: the sag divided by half the span in feet. At about 1/32" (0.03") per foot, most people can see the bow from across the room, so that's the usual limit, and this calculator calls anything over it excessive.
The target is lower, 0.02" per foot, because wood creeps. A shelf that carries the same load for years sags about 50% more than it did on the first day. A shelf at 0.02" today ends up near the limit, not past it. Between the two, it's marginal: fine for a while, likely to show later.
Shelf span chart
The longest span for each material that stays within the 0.02" per foot target, worked out with the same math as the calculator. Shelf span charts disagree with each other because most don't say what they assume. This one assumes:
- A shelf 3/4" thick and 10" deep, with no edge band.
- Fastened at both ends (screwed, glued or in dadoes). A shelf resting on pins sags five times as much.
- The load spread evenly along the shelf.
- Sag of no more than 0.02" per foot, the Sagulator's target, with spans in 1/2" steps.
| Load per foot of shelf | |||
|---|---|---|---|
| Material | Light 10 lb/ft | Books 20 lb/ft | Heavy 40 lb/ft |
| White Oak | 62" | 49" | 39" |
| Red Oak | 62 1/2" | 49 1/2" | 39" |
| Hard Maple | 62 1/2" | 49 1/2" | 39 1/2" |
| Soft Maple | 60" | 47 1/2" | 37 1/2" |
| Cherry | 58 1/2" | 46 1/2" | 36 1/2" |
| Walnut | 60 1/2" | 48" | 38" |
| Ash | 61 1/2" | 48 1/2" | 38 1/2" |
| Birch | 64 1/2" | 51" | 40 1/2" |
| Beech | 63 1/2" | 50" | 40" |
| Poplar | 55 1/2" | 44" | 35" |
| Douglas Fir | 64" | 50 1/2" | 40" |
| Southern Yellow Pine | 62" | 49 1/2" | 39" |
| White Pine | 55" | 43 1/2" | 34 1/2" |
| Spruce | 57 1/2" | 45 1/2" | 36" |
| Cedar | 54" | 43" | 34" |
| Redwood | 56 1/2" | 44 1/2" | 35 1/2" |
| Hemlock | 55 1/2" | 44" | 35" |
| Cypress | 54 1/2" | 43" | 34" |
| Birch Plywood | 59 1/2" | 47" | 37 1/2" |
| Oak Plywood | 59" | 47" | 37" |
| Pine Plywood | 54 1/2" | 43" | 34" |
| Baltic Birch Plywood | 59 1/2" | 47" | 37 1/2" |
| MDF | 37 1/2" | 30" | 23 1/2" |
| HDF | 40 1/2" | 32" | 25 1/2" |
| Particle Board M-1 | 36" | 28 1/2" | 22 1/2" |
| Particle Board M-2 | 37 1/2" | 30" | 23 1/2" |
| Particle Board M-3 | 39" | 31" | 24 1/2" |
| OSB | 36" | 28 1/2" | 22 1/2" |
For any other size, wood, load or support, or with an edge band, use the calculator at the top of the page.
Fastened or resting
A shelf screwed, glued or set in dadoes at both ends is held flat at the ends and sags a fifth as much as the same shelf sitting loose on shelf pins or cleats under an even load. Adjustable shelves rest, so they need to be stiffer.
What helps
- Thicker stock. Thickness is cubed, so a little goes a long way: 1" instead of 3/4" makes a shelf about 2.4 times as stiff.
- An edge band or face frame. A strip of hardwood glued on edge along the front acts like a small beam and carries part of the load. A 3/4" × 1 1/2" band on a 48" shelf of 3/4" oak cuts its sag by about a third, enough to take it from marginal to acceptable. Turn on edge banding above to try it.
- A shorter span. Span is cubed too. A center divider or an extra support halves the span and cuts the sag to an eighth or less.
- A stiffer material. Plywood instead of MDF, or a hardwood instead of pine.