Wood Beam Span Calculator
Check a wood beam for your span and load: a single 2x or 4x, or 2x lumber nailed together in two to four plies. Pick the lumber, species and grade, say whether it's indoors or on an outdoor deck, and see whether it works and the longest span it can carry.
Works: a 2-2x10 Southern Pine No.2 over 8 ft uses 66% of its capacity, limited by bending.
| Check | Used | |
|---|---|---|
| Bending | 66% | OK |
| Shear | 33% | OK |
| Live load deflection (span/360) | 28% | OK |
| Total load deflection (span/240) | 24% | OK |
Longest span for this load: 9′ 10″. The beam weighs 7 lb/ft, which the checks add to the load.
IRC and NDS: like the deck beam tables in the International Residential Code (IRC R507.5), a deck beam of two plies takes a 10% bending increase and three or more plies 15%. The National Design Specification (NDS) gives that increase only to joists and other members spaced 24 in or closer, so an NDS check of the same beam allows a shorter span. The beam designer follows NDS and checks lumber dry: it doesn't apply the wet service or built-up beam factors, so its results will differ from these.
Simply supported, with allowable stress design (ASD) under the National Design Specification (NDS) for wood. Loads are per foot of beam, without the beam's own weight. The button opens this beam in the free beam designer, where you can add point loads, overhangs and more spans.
What load to enter
A beam carries the floor or deck halfway to the next support on each side. Multiply that width by the load per square foot. A deck is usually designed for 10 psf dead load (the framing and boards) and 40 psf live load (people and furniture); a house floor for 15 psf dead and 40 psf live. Add any wall, roof or hot tub that bears on the beam.
Deck: 10 ft joists from the house put 5 ft on the beam.
Floor: 12 ft joists on each side put 12 ft on the beam.
Deck: joists spanning 10 ft from the house to the beam put 5 ft of deck on it: 50 lb/ft dead and 200 lb/ft live. With posts 8 ft apart, a 2-2x10 Southern Pine No.2 works, using 66% of its capacity in bending.
Floor: joists spanning 12 ft on each side put 12 ft of floor on the beam: 180 lb/ft dead and 480 lb/ft live. Over 8 ft, a 3-2x12 Douglas Fir-Larch No.2 works, using 76% of its capacity in bending. Open this example in the beam designer.
Deck beam span table
The longest distance between posts for common deck beams, by how far the joists span to the beam. The deck is 10 psf dead and 40 psf live, the joists bear on one side of the beam, and the lumber is treated and wet, as on an uncovered deck.
The table's two spans: joists from the ledger to the beam, and posts along the beam.
With a 2 ft overhang, 10 ft joists put 7 ft on the beam: look up 14 ft joists.
| Beam | 6 ft joists | 8 ft joists | 10 ft joists | 12 ft joists | 14 ft joists | 16 ft joists |
|---|---|---|---|---|---|---|
| 4x6 | 8′ 8″ | 7′ 6″ | 6′ 9″ | 6′ 2″ | 5′ 9″ | 5′ 4″ |
| 2-2x8 | 10′ 8″ | 9′ 3″ | 8′ 4″ | 7′ 7″ | 7′ 0″ | 6′ 7″ |
| 3-2x8 | 13′ 0″ | 11′ 7″ | 10′ 4″ | 9′ 6″ | 8′ 10″ | 8′ 3″ |
| 4x8 | 11′ 6″ | 10′ 0″ | 9′ 0″ | 8′ 2″ | 7′ 7″ | 7′ 1″ |
| 2-2x10 | 12′ 7″ | 11′ 0″ | 9′ 10″ | 9′ 0″ | 8′ 4″ | 7′ 10″ |
| 3-2x10 | 15′ 7″ | 13′ 8″ | 12′ 3″ | 11′ 3″ | 10′ 5″ | 9′ 9″ |
| 4x10 | 13′ 7″ | 11′ 10″ | 10′ 7″ | 9′ 8″ | 9′ 0″ | 8′ 5″ |
| 2-2x12 | 14′ 9″ | 12′ 11″ | 11′ 7″ | 10′ 7″ | 9′ 10″ | 9′ 2″ |
| 3-2x12 | 18′ 3″ | 16′ 0″ | 14′ 4″ | 13′ 2″ | 12′ 3″ | 11′ 6″ |
| 4x12 | 15′ 11″ | 13′ 10″ | 12′ 6″ | 11′ 5″ | 10′ 7″ | 9′ 11″ |
| Beam | 6 ft joists | 8 ft joists | 10 ft joists | 12 ft joists | 14 ft joists | 16 ft joists |
|---|---|---|---|---|---|---|
| 4x6 | 7′ 9″ | 6′ 9″ | 6′ 0″ | 5′ 6″ | 5′ 1″ | 4′ 9″ |
| 2-2x8 | 10′ 4″ | 9′ 0″ | 8′ 0″ | 7′ 4″ | 6′ 10″ | 6′ 4″ |
| 3-2x8 | 12′ 10″ | 11′ 2″ | 10′ 0″ | 9′ 2″ | 8′ 6″ | 7′ 11″ |
| 4x8 | 10′ 2″ | 8′ 10″ | 7′ 11″ | 7′ 3″ | 6′ 9″ | 6′ 3″ |
| 2-2x10 | 12′ 6″ | 10′ 11″ | 9′ 10″ | 8′ 11″ | 8′ 4″ | 7′ 9″ |
| 3-2x10 | 15′ 7″ | 13′ 7″ | 12′ 2″ | 11′ 2″ | 10′ 4″ | 9′ 8″ |
| 4x10 | 13′ 5″ | 11′ 9″ | 10′ 6″ | 9′ 7″ | 8′ 11″ | 8′ 4″ |
| 2-2x12 | 14′ 6″ | 12′ 7″ | 11′ 4″ | 10′ 4″ | 9′ 7″ | 9′ 0″ |
| 3-2x12 | 17′ 11″ | 15′ 8″ | 14′ 1″ | 12′ 11″ | 12′ 0″ | 11′ 3″ |
| 4x12 | 15′ 7″ | 13′ 7″ | 12′ 2″ | 11′ 2″ | 10′ 4″ | 9′ 9″ |
- If the joists overhang the beam, add twice the overhang to the joist span you look up: the beam carries all of the overhang and half of the span behind it. If joists bear on both sides, add the two spans.
- Live load deflection is limited to span/360 and total load deflection to span/240. The beam is simply supported between posts and braced by the joists.
- The Douglas Fir-Larch table assumes incised lumber; the Southern Pine table doesn't.
- IRC and NDS: like the deck beam tables in the International Residential Code (IRC R507.5), a deck beam of two plies takes a 10% bending increase and three or more plies 15%. The National Design Specification (NDS) gives that increase only to joists and other members spaced 24 in or closer, so an NDS check of the same beam allows a shorter span.
What the calculator checks
- Bending against the allowable stress Fb′: the species and grade's Fb times the size factor (CF, from 1.3 for a 2x6 down to 1.0 for a 2x12; Southern Pine's values already include it).
- Shear against 2/3 Fv′ times the beam's area.
- Deflection: live load to span/360 and total load, including the beam's weight, to span/240.
An uncovered deck beam stays wet, so the wet service factor (CM) lowers its stiffness by 10%, its shear strength by 3%, and its bending strength by 15% where Fb CF is over 1,150 psi. Treated Douglas Fir, Hem-Fir and Spruce-Pine-Fir is also incised (slit so the treatment soaks in), which takes another 20% off bending and shear and 5% off stiffness (Ci). Together they drop a 2-2x10 Douglas Fir-Larch No.2's allowable moment from 3,529 ft-lb indoors to 2,824 ft-lb on a deck, before the built-up beam increase below.
On a deck, a beam of two or more plies takes the increase the deck beam tables in the International Residential Code (IRC) use: 10% in bending for two plies and 15% for three or more. Indoors the calculator follows the National Design Specification (NDS), which gives that increase only to joists and similar members spaced 24 in or closer, so a built-up beam doesn't get it.
The calculator assumes the plies are nailed or screwed so they bend together, and that the joists or decking brace the top of the beam. It doesn't check bearing on the posts or size the connections. The beam designer follows NDS and checks lumber dry: it doesn't apply the wet service, incising or built-up beam factors yet.
Wood beam span questions
How far can a 4x6 beam span?
As a deck beam under 8 ft joists, a 4x6 Douglas Fir-Larch No.2 spans 6′ 9″ between posts; under 6 ft joists, 7′ 9″. Enter your own span and load above for anything else.
How far can a double 2x10 span?
As a deck beam under 10 ft joists, a 2-2x10 Southern Pine No.2 spans 9′ 10″ between posts. A third ply carries half as much again.
Does this match the deck code tables?
Closely, for most beams, usually within a couple of inches. The deck beam table in the International Residential Code (IRC), Table R507.5, looks beams up by an effective joist span that allows for joists overhanging the beam; without an overhang it's two-thirds of the joist span. This table uses the joist span itself. If your building department asks for the code table, use it; for a deck that falls outside it, a calculation like this one is what an engineer or plan reviewer will look for.
What if no lumber beam works?
Bolt a steel plate between the plies (a flitch beam), use an engineered wood beam (glulam or LVL in the beam designer), or a steel beam.