What size beam do I need? Calculate beam size requirements based on span, load, and wood species. Determine the right beam for headers, floor beams, ridge beams, and deck beams using simplified span tables.
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Last updated March 2026 by our expert review team
Material cost per linear foot installed. Steel includes fabrication. Prices vary by region, supplier, and beam dimensions.
Ehsan Ghazanfari
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FISE-certified structural engineer with 11+ years designing bridges, retaining walls, and foundations. MSc from Aalto University.
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How we verify our calculatorsAlways consult a licensed structural engineer for load-bearing beams. This calculator provides estimates based on simplified span tables, not a full engineering analysis.
Use our rafter calculator to determine roof loads that transfer to ridge beams and headers.
Plan your framing layout with our framing calculator for studs and plates around beam pockets.
LVL beams can span roughly 50% further than dimensional lumber of the same depth. They cost more upfront but eliminate the need for mid-span posts.
Doubled beams must be through-bolted every 24 inches with 1/2-inch carriage bolts. Nailing alone does not meet code for structural beams.
Enter the beam span (distance between supports) and tributary width (how wide an area the beam supports on each side).
Select the load type (floor, roof, deck, or header) and beam material. Each combination has different load requirements and allowable stresses.
Get a recommended beam size, bending moment, deflection check, and cost estimate. The calculator matches your loads to the smallest beam that meets the required section modulus.
Formulas
Linear Load (plf) = Total Load (psf) x Tributary Width (ft)
Max Bending Moment (ft-lbs) = (w x L²) / 8
Required Section Modulus (in³) = (M x 12) / Fb
How do I size a beam for a given span?
Calculate the total load per linear foot (tributary width times total load in psf), then find the required section modulus using the bending moment formula. Match that to a beam from span tables or this calculator.
What are span tables?
Span tables are published by lumber associations and the IRC. They list maximum allowable spans for specific beam sizes, wood species, and load conditions. This calculator uses simplified versions of those tables.
LVL vs dimensional lumber beams?
LVL (Laminated Veneer Lumber) is engineered wood with consistent strength and no knots. It handles longer spans and heavier loads than solid lumber. Use LVL when spans exceed 10-12 feet.
What is deflection and why does it matter?
Deflection is how much a beam bends under load. Building codes limit deflection to L/360 for floors (span divided by 360) and L/240 for roofs. Excessive deflection causes bouncy floors and cracked finishes.
How do I calculate load on a beam?
Multiply the total load (dead + live, in psf) by the tributary width (the distance the beam supports on each side). This gives pounds per linear foot (plf) on the beam.
When should I use a steel beam?
Steel beams handle very long spans (over 20 feet) and heavy point loads that wood cannot support. They require an engineer for sizing and a crane or crew for installation.
How do I size a header over a window or door?
Headers carry the load above the opening. For standard residential headers up to 6 feet wide, doubled 2x10s or 2x12s usually work. Wider openings need LVL or engineered headers.
Do I need posts under beam ends?
Yes. Every beam needs adequate bearing at each end, typically a post or column sized to carry the beam reaction force down to the foundation. Minimum bearing length is 1.5 inches on wood, 3 inches on masonry.
A common basement or crawlspace beam supporting first-floor joists. Doubled 2x10s handle the 400 plf load with room to spare.
A vaulted ceiling ridge beam carrying rafters from both sides. LVL is the go-to choice for ridge beams over 12 feet because it handles the span without mid-point support.
A wide patio door or garage entry header. The narrower tributary width keeps loads manageable for dimensional lumber at this span.
A deck beam supporting joists on one side. Doubled 2x12s are the standard for residential deck beams up to about 12 feet between posts.
Undersizing the beam for the actual load
A beam that is too small will sag, bounce, or crack finishes over time. Always calculate with the full dead load plus live load for the actual tributary area, not just a guess.
Not using proper posts or supports at beam ends
A correctly sized beam is useless without adequate bearing. Each end needs a post or column rated for the full reaction force, sitting on a footing that reaches below the frost line.
Ignoring deflection limits
A beam can be strong enough to carry the load but still deflect too much. Code requires L/360 for floors and L/240 for roofs. Excessive deflection causes cracked drywall and bouncy floors.
Using the wrong lumber grade
Not all 2x12s are equal. A #2 grade Douglas Fir has an Fb of 1,350 psi while SPF #2 is only 1,000 psi. Using the wrong grade in your calculation leads to an undersized beam.
Not accounting for point loads
This calculator assumes uniform loading. If another beam or post sits on your beam (a point load), the bending moment increases dramatically at that spot. Point loads require separate engineering analysis.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.