Lumber Spans

Maximum span from species, grade, size, load.

Run the calculator

Example

You enter

Species and grade DF-L_No2
Nominal size 2x10
Total load (psf) 50
Tributary width (in) 16
Deflection limit (L/x) 360

You get

Allowable span 13.88 ft
By bending 13.88 ft
By deflection 15.21 ft
Governing bending

Details, formula, and sources

Allowable simple-beam span: deflection-limited L = sqrt(48 × E × I × Δ_allow / (5 × w × n)); strength-limited from M_allow = Fb × S. F_b and E from AWC NDS-2018 design values; deflection limit L/360 (live) or L/240 (total) per IRC §R301.7.

AWC NDS-2018 (National Design Specification for Wood Construction). IRC 2021 (International Residential Code). §R301.7, R502.3, R802.4.

AWC NDS free at awc.org/codes-standards. Free read-only access at codes.iccsafe.org.

Estimate. AHJ and licensed structural engineer govern.

Field names used by the API: species_grade, nominal_size, total_load_psf, tributary_width_in, deflection_limit, allowable_span_ft, by_bending_ft, by_deflection_ft, governing

Species / grade design values data/construction/lumber-properties.json keyed to species / gradeAWC NDS / lumber-grading agency
Default deflection limit L/360 live, L/240 totalIRC 2021 §R301.7