How far can a beam span without support?

Maximum span depends on beam size, species, grade, and load. A doubled 2x10 Douglas Fir beam can typically span 8-10 feet under normal residential loads. LVL beams of the same depth can span significantly farther.

What size beam do I need?

Beam size is determined by the span, tributary load width, species, and grade. Use IRC span tables or an engineering calculation to match the beam depth and ply count to your specific load and span requirements.

How do I calculate beam span?

Look up the allowable span in IRC Table R602.7 or R507.5 based on the beam size, lumber species, and the tributary load width it supports. For loads beyond table limits, consult a structural engineer.

What is the maximum span for a 4x8 beam?

A solid 4x8 beam in Douglas Fir #2 can span approximately 7-8 feet supporting a 12-foot tributary load width. The exact span varies by species, grade, and whether the beam carries floor, roof, or deck loads.

Beam Span Calculator

Estimate maximum allowable beam spans based on simplified IRC residential span tables. Select lumber species, beam size, load type, and tributary width to size your beam correctly.

Want an engineering-accurate answer? This tool gives a quick estimate from simplified span tables. For a member that's checked against bending, shear, AND deflection at your exact span, spacing, and load, use our Wood Beam, Joist & Rafter Designer — it computes the actual NDS allowables.

Lumber Species / Material

Beam Size

Load Type

Number of Plies

Tributary Width (ft)

Distance between beams (typically 4-8 ft)

Results

Max Allowable Span

13.4

feet

Load per Linear Foot

300

lbs/ft

Total Load (PSF)

psf (live + dead)

Max Span by Beam Size (2-ply, 6 ft tributary)

Beam Size	Floor	Roof	Deck
2x8	10.6 ft	13.4 ft	9.9 ft
2x10	13.4 ft	17.0 ft	12.7 ft
2x12	16.3 ft	20.5 ft	15.6 ft
3-1/8x9-1/4 LVL	17.0 ft	21.2 ft	16.3 ft
3-1/2x9-1/4 LVL	18.4 ft	22.6 ft	17.7 ft
3-1/2x11-7/8 LVL	22.6 ft	28.3 ft	21.9 ft
3-1/2x14 LVL	26.9 ft	33.2 ft	26.2 ft

Did this help?

How to Use This Calculator

Choose the lumber species and grade, beam size, number of plies (laminations), and the load type (floor, roof, or deck). Enter the tributary width — the distance between beams that defines how much load each beam carries. The calculator shows the maximum allowable span and the load per linear foot.

Formula Used

Max Span = Base Table Span x sqrt(6 / Tributary Width) x sqrt(Plies). Load per linear foot = Total PSF x Tributary Width. Base spans derived from IRC residential beam span tables for common lumber species and grades.

Wood Beam & Joist Designer (NDS)Wall Framing Calculator Stair Calculator

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Disclaimer

This calculator is for estimation purposes only. Results may vary based on local conditions, materials, and building codes. Always consult a licensed professional before making decisions based on these calculations. MyContractorTools is not responsible for any errors or omissions.