Joist Mid-Span Deflection

Mid-span deflection with L/360 and L/240 checks.

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Example

You enter

Uniform load (plf) 50
Span (ft) 12
E (psi) 1600000
I (in⁴) 47.6

You get

Mid-span deflection 0.3063 in
Limit L over 360 (in) 0.4
Limit L over 240 (in) 0.6
Pass L over 360 Yes
Pass L over 240 Yes

Checked against published worked examples from: Jonathan Ochshorn (Cornell University).

Details, formula, and sources

Formula or tableMid-span deflection Δ = 5 × w × L⁴ / (384 × E × I) for uniformly loaded simple span. E and I are entered (take them from the AWC NDS Supplement design values and section properties for the species, grade and size).

Source and editionAWC NDS-2018 (National Design Specification for Wood Construction). IRC 2024 (International Residential Code). §R301.7 (Table R301.7 unchanged from 2021).

Free accessFree at awc.org. Free read-only access at codes.iccsafe.org.

What governsEstimate. AHJ and licensed structural engineer govern.

API field namesuniform_load_plf, span_ft, E_psi, I_in4, deflection_in, limit_L_over_360_in, limit_L_over_240_in, pass_L_over_360, pass_L_over_240

Section properties S4S actual dimensions per AWC NDS SupplementAWC NDS-2018