Wood Beam Span Calculator
Find out how far a wood beam can span. Pick the build-up and depth, the tributary floor width it carries, the live and dead loads, and the species — get the maximum span (bending or deflection, whichever governs), and enter your span for a pass/fail check, with by-depth and by-build-up comparison tables and a downloadable PDF.
| Beam | Max span | Capacity used | Status |
|---|---|---|---|
| Enter a beam to compare | |||
| Build-up | Max span | Capacity used | Status |
|---|---|---|---|
| Enter a beam to compare | |||
How to use the wood beam span calculator
Four quick steps turn a built-up beam and its load into a maximum span — limited by bending or L/360 deflection, whichever governs — with a pass/fail check and by-depth and by-build-up comparison tables.
Pick the build-up
The number of 2× plies and the depth of each member.
Set the tributary width
The width of floor the beam carries — half of each adjacent bay.
Enter the loads
The live and dead loads, and the species of the lumber.
Read results & save a PDF
Get the max span, and enter your span for a pass/fail — then download a report.
The formulas behind the span
The load comes from the tributary width and the pressures; the maximum span is set by bending capacity or the live-load deflection limit, whichever is reached first.
▶ Worked example
A triple 2×12 (4.5 in × 11.25 in) in Douglas Fir-Larch #2, carrying an 8 ft tributary width at 40 psf live + 10 psf dead.
Line load w = 8 × (40 + 10) = 400 plf; section modulus S = 4.5 × 11.25² ÷ 6 = 94.9 in³.
Bending and L/360 deflection are both checked; bending governs here.
Max span 11.9 ft (bending) · at a 10 ft span, 70.2% used · PASS
A double 2×12 would reach about 9.7 ft; a quad about 13.8 ft.
Wood beam span quick reference
The span formulas, the maximum span by depth, and how the build-up (ply count) changes the reach.
| Quantity | Formula |
|---|---|
| Beam width | plies × 1.5 in |
| Line load | tributary width × (live + dead) |
| Section modulus | b × d² ÷ 6 |
| Max span (bending) | √(8 × Fb·cf × S ÷ w) |
| Max span (deflection) | ∛(384 E·I ÷ (360 × 5 wₗ)) |
| Governing span | the smaller of the two |
| Beam | Width × depth | Max span | Governs |
|---|---|---|---|
| (3) 2×8 | 4.5″ × 7.25″ | 8.4 ft | Bending |
| (3) 2×10 | 4.5″ × 9.25″ | 10.3 ft | Bending |
| (3) 2×12 | 4.5″ × 11.25″ | 11.9 ft | Bending |
| (3) 2×14 | 4.5″ × 13.25″ | 13.3 ft | Bending |
| Build-up | Width | Max span | Governs |
|---|---|---|---|
| (2) 2×12 | 3.0″ | 9.7 ft | Bending |
| (3) 2×12 | 4.5″ | 11.9 ft | Bending |
| (4) 2×12 | 6.0″ | 13.8 ft | Bending |
Species shifts these a little: a 3-ply 2×12 at this load spans about 11.8 ft in SPF or Hem-Fir and about 12.6 ft in Southern Pine. Depth is the strongest driver of span, then ply count, then species.
Everything the calculator returns
One calculation gives the full span picture — the maximum reach, the governing limit, and a check of your span.
Beam span at a glance
The limits built into the check, and the default result.
Built for the way you work
Whether you are framing a girder, planning a renovation, or drawing the plans, the span check starts from the same inputs.
Need a built-up beam size that reaches the span and carries the load, with a by-depth table to see the margin each option leaves.
- Set the tributary width right
- Compare depths and plies
- Hand the PDF to the inspector
Putting in a beam to open up a space and want to know the size and how far it can safely span before pulling a permit.
- Start at 40 psf live, 10 dead
- Go deeper before wider
- Permit structural work
Specifying a girder on a drawing and want a quick span check before the code tables or an engineer finalizes it.
- Test depths and loads fast
- Note where sawn runs out
- Flag long spans for LVL
Size a beam that reaches
Seven things that keep a beam span estimate sound.
Depth beats width
Bending capacity rises with the square of depth and stiffness with the cube. A deeper beam almost always outspans a wider one.
Get the tributary width right
A beam carries half of each adjacent bay. Using the full joist length instead of half doubles the load and badly undersizes the span.
No repetitive bonus
A single beam gets no repetitive-member factor like joists do. The plies share load but the strength is not boosted.
Watch deflection on long spans
A beam can pass bending yet fail the L/360 deflection limit. Longer or heavier spans often hit deflection first.
Reach for engineered lumber
When a quad 2× still won’t span it, an LVL or glulam of the same depth carries much more. Switch rather than stacking plies.
Mind bearing and connections
The span check assumes solid bearing at each end. Posts, columns and the beam-to-support connection need their own check.
Point loads change everything
This is a uniform-load check. A post or beam landing on the span is a point load that needs a separate analysis.
Frequently asked questions
Common questions about wood beam spans, tributary loads, build-up and when to use engineered lumber.
A wood beam’s span depends on its size and build-up, the load it carries, and the species. The calculator works it out as a simple beam: the maximum span is the point where the bending stress reaches the allowable, or the live-load deflection reaches L/360, whichever comes first.
The load comes from the tributary floor width the beam carries times the total of the live and dead loads. A deeper beam or more plies spans farther; a heavier load or wider tributary shortens it.
For example, a triple 2×12 in Douglas Fir-Larch carrying an 8 foot tributary width at 40 psf live plus 10 psf dead can span about 11.9 feet, governed by bending. This calculator gives the maximum span and checks any span you enter.
The load on a beam comes from the area of floor it supports, expressed as a tributary width. A beam carries half of the joist span on each side of it, so the tributary width is half of one adjacent bay plus half of the other. Multiply that width by the total load — live plus dead — to get the pounds per foot on the beam.
For example, a beam between two 8 foot bays carries 4 feet from each side, an 8 foot tributary width; at 40 psf live plus 10 psf dead that is 8 times 50, or 400 pounds per foot.
The tributary width, not the joist length, is what matters. This calculator takes the tributary width and the loads and works out the line load for you.
A built-up beam is made by nailing or bolting two or more 2x members together side by side, so its width is the number of plies times 1.5 inches — a triple 2×12 is 4.5 inches wide. A single beam is one solid piece, such as a timber or an engineered member.
Built-up beams are common for residential girders because they are made from ordinary dimensional lumber and assembled on site. The plies share the load, so the capacity is roughly the sum of the members.
For the span check, a built-up beam of a given depth and ply count behaves like a solid rectangle of that width and depth. This calculator sizes built-up 2x beams from the ply count and depth you choose.
Yes, but less than adding depth. Each extra ply adds 1.5 inches of width, and the bending and deflection capacity rise in direct proportion to width — so going from two plies to three increases capacity by half, and the span by a smaller amount.
Depth is far more effective, because capacity rises with the square of the depth for bending and the cube for deflection. A deeper beam of fewer plies usually outspans a shallower beam of more plies.
For example, a 2×12 in Douglas Fir-Larch at an 8 foot tributary goes from about 9.7 feet as a double to 11.9 feet as a triple. This calculator shows the span for 2, 3 and 4 plies so you can compare.
L/360 is the standard live-load deflection limit for floor beams: the beam may sag no more than its span divided by 360 under the live load. For a 12 foot span that is about 0.4 inch.
The limit is about stiffness and comfort, not strength. A beam that is strong enough not to break can still feel bouncy or crack finishes if it flexes too much, so the code caps the deflection to keep floors solid.
Longer or more heavily loaded beams sometimes reach the deflection limit before the bending limit. This calculator checks both the bending capacity and the L/360 deflection and reports whichever governs the span.
It depends on the load it carries, but for a common case it is around 12 feet. A triple 2×12 — three 2x12s making a 4.5 inch by 11.25 inch beam — in Douglas Fir-Larch, carrying an 8 foot tributary width at 40 psf live plus 10 psf dead, spans about 11.9 feet, governed by bending.
A lighter load or a narrower tributary width lets it span farther; a heavier load shortens it. A stronger species such as Southern Pine gains a little more.
So a triple 2×12 commonly spans roughly 10 to 13 feet as a floor girder, depending on the load. This calculator gives the exact figure for your tributary width, loads and species.
When a built-up dimensional beam runs out of span or gets impractically wide. LVL, LSL and glulam are stronger and stiffer than sawn lumber and come in deeper sections, so they span farther in the same depth and carry heavier loads without needing four or more plies.
Engineered beams are also more dimensionally stable and come in long, consistent lengths, which matters for a long girder. The trade-off is cost and that they usually need a supplier’s span tables or software.
If a triple or quad 2x beam will not reach your span, an engineered beam is the usual next step. This calculator sizes built-up sawn beams; for an engineered beam, use the manufacturer’s data or an engineer.
No. This calculator is a planning aid that estimates a beam’s span from a simplified uniform load and a simple-beam check. It assumes a uniform load, a simply supported single span, adequate bearing and connections, and no point loads, cantilevers or notching.
Real projects are governed by the span tables in the building code or a supplier’s literature, or by an engineered design, which account for load combinations, point loads, bearing and lateral support that a quick estimate does not.
Use this to get a sense of the size and compare options, then confirm the final beam, bearing and connections against the current code and your building official or a licensed engineer.
Disclaimer: US units (feet, psf, inches). Built-up sawn 2× beams, width = plies × 1.5 in. Line load w = tributary width × (live + dead). Maximum span is the smaller of the bending limit (from Fb × size factor × S) and the live-load L/360 deflection limit, as a simply supported single span. No repetitive-member factor is applied (single beam). It assumes a uniform load and adequate bearing and connections, and does not handle point loads, cantilevers, notching, lateral support or engineered lumber. A planning aid only — confirm species values, bearing, connections and the final beam against the current code span tables or a licensed engineer.