What does L/360 mean?

Carpentry & FramingIRC Structural Deflection

Beam & Joist Deflection Calculator

Calculate vertical deflection for simply supported wood beams and floor joists under uniform loads to verify building code stiffness limits.

Carpentry & Framing Field StandardsIRC Structural Deflection

Engineering formulations calibrated to IPC, UPC & NEC specificationsSub-Zero Cold Climate Tolerances

Quick Summary & Key Answer

Under International Residential Code (IRC Table R502.3.1), standard residential floors must not deflect more than L/360 under live loads (span length divided by 360). For a 15-foot joist span (180 inches), maximum allowable center deflection is 180 / 360 = 0.50 inches. For tile and stone flooring, stiffness must meet L/480.

L/360 is the standard floor deflection limit.
L/480 is required beneath stone or ceramic tile to prevent cracked grout.
Boring holes in joists reduces Moment of Inertia (I), increasing deflection.

Interactive Calculator

Adjust inputs for real-time results

Clear Span of Beam / Joistfeet

feet

Uniform Load on Beamlbs / linear ft

lbs / linear ft

Wood Beam / Joist Size (Douglas Fir-Larch)

Deflection Limit Standard

Calculated OutputFAILS (L/308 < L/360)

Calculated Mid-Span Deflection

0.546"inches

Allowable Deflection Limit: 0.467" (L/360)
Moment of Inertia (I): 98.9 in⁴
Modulus of Elasticity (E): 1,600,000 psi
Tile Floor Compatibility: No (Risk of cracked tile grout)

Note: Formula: Δ = (5·w·L⁴) / (384·E·I). Plumbers notching or boring floor joists must maintain structural deflection standards.

Plumbing Holes & Beam Deflection

Holes bored in the middle third of a joist span reduce Moment of Inertia significantly, causing bouncy floors.

Frequently Asked Questions

Common questions regarding this calculation and code compliance

L/360 means allowable deflection is the clear span in inches divided by 360. A 12-foot span (144 inches) allows a maximum deflection of 0.40 inches.