Wood Beam Span Calculator
Estimate the maximum span for a wood beam, from its section size and the load it carries.
Reviewed by the ToolNestr Editorial Team — July 2026
How the wood beam span calculator works
A beam's real-world safe span is set by whichever of two separate limits is more restrictive: strength (whether it would break) or deflection (whether it sags too much for comfort and finishes). This calculator runs both checks and reports the shorter, governing span, because a beam is only as good as its weaker limit — passing strength alone doesn't mean the beam is adequate if it fails deflection at that same span.
The load on the beam is found from the tributary width — the strip of floor or roof that funnels its weight onto this beam, generally half the distance to the next parallel support on either side — multiplied by the load per square foot (accounting for both the structure's own dead weight and the live load it's designed to carry). That gives a load per linear foot of beam, which feeds into both the strength and deflection formulas.
This differs from a standard joist span table, which assumes a fixed, typical residential load and joist spacing. Here you enter your own load and beam section directly, which is useful for sizing a girder or beam outside a joist table's scope — but it remains a simplified single-load-case estimate. A real load-bearing beam, especially a girder carrying multiple joists or a significant point load, needs to be sized from the full NDS span tables or an engineer's calculation for the actual species, grade, load combination, and local code requirements.
Bending and deflection formulas follow the American Wood Council's National Design Specification (NDS) for Wood Construction and its associated Span Tables.
Enter the section
Beam width and depth in inches.
Describe the load
Tributary width and total load in psf.
Read the governing span
The shorter of the strength and deflection limits.
The formula explained
Load per foot
w = tributary width × psf. 12 ft trib, 50 psf: 12 × 50 = 600 lb/ft.
Strength-limited span
From Mₓ = Fb·S and M = wL²/8, solved for L. A 3.5×9.25 in beam, 900 psi, 600 lb/ft: ≈ 7.1 ft.
Deflection-limited span (L/360)
From 5wL⁴/(384EI) = L/360, solved for L. Same beam, E = 1.6M psi: ≈ 9.7 ft.
Worked example
A 3.5×9.25 in beam (a 4×10), 12 ft tributary width, 50 psf load, Fb=900 psi, E=1.6M psi.
Real-world context
A garage door header is a classic case where strength governs before deflection becomes noticeable. A doubled 2×10 header (net 3 in × 9.25 in) carrying a 4 ft tributary width at 40 psf sees 160 lb/ft, giving a strength-limited span of about 12.7 ft against a deflection limit of 14.3 ft — the beam would snap its allowable bending stress before it ever sagged past L/360, which is why headers over wide garage openings often need to jump to an LVL or a steel flitch plate rather than a bigger solid-sawn section.
Deck beams tell a similar story at a smaller scale. A 5.5×5.5 in beam (a rough-sawn 6×6) picking up an 8 ft tributary width at 60 psf (40 psf live plus dead load) carries 480 lb/ft, which works out to a strength-limited span of about 5.9 ft versus a deflection limit near 7.2 ft — a reminder that post spacing on a deck is usually set by the beam's bending capacity, not by how bouncy it feels underfoot.
Basement girders carrying multiple floor joists are where the stakes get higher. A doubled 2×12 girder (3 in × 11.25 in) using a higher-grade lumber (Fb = 1350 psi, E = 1.7 million psi) with a 14 ft tributary width at 55 psf sees 770 lb/ft, landing at roughly 8.6 ft of strength-limited span versus 10.5 ft for deflection. That 2 ft gap between the two limits is exactly the margin an engineer is checking when they decide whether a girder needs a mid-span post or a switch to engineered lumber.
Common misconceptions
"A bigger beam is always better, so oversizing never hurts." It costs you elsewhere. A beam sized well past what strength and deflection require adds unnecessary weight, material cost, and often forces awkward header heights or post sizes — matching the section to the actual governing limit, with a reasonable safety margin, is the more efficient design.
"If the beam doesn't sag, it's fine." Deflection is only one of two checks. A beam can look perfectly straight and still be overstressed in bending — visible sag usually shows up long after the fibers are already past a safe stress level. Both the strength and deflection limits need to be checked, not just the one you can see.
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Frequently asked questions
How is a wood beam span estimated?
This tool combines a bending-strength check and a deflection check (at L/360) for a simply supported beam under uniform load, and reports the shorter (governing) span of the two. Beams often are limited by deflection before they reach their bending strength limit.
What is tributary width?
Tributary width is the width of floor or roof area that transfers its load onto this particular beam — essentially half the distance to the next parallel beam or wall on each side. A wider tributary width means more load per foot of beam, which shortens its safe span.
Why does this differ from a floor joist span table?
Joist span tables assume a fixed, standard residential live load (typically 40 psf) and standard joist spacing. This calculator lets you enter your own load and beam section directly, useful for girders and beams outside a standard joist table's scope, but it is still a simplified single-case estimate, not a full beam design.
Should I use this to size a real structural beam?
No. Use it to compare options and build intuition about how span, section size, and load interact. Any load-bearing wood beam — especially a girder carrying multiple joists — needs sizing from the actual NDS span tables or an engineer's calculation for your species, grade, load combination, and local code.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.