Deflection Limit Check (IBC / ASCE 7)
Compares a computed beam deflection with the code limit for the member and the load. Pick the limit from IBC Table 1604.3 [1] (L/120 to L/360 for roofs, floors, walls and partitions), the ACI 318 L/480 limit for concrete supporting fragile finishes [3], or enter your own n. The page gives the allowable deflection, the utilization and a pass or fail. It also lists the common limits L/120 to L/480 for the same span, so you can see how much margin a stricter finish would leave. Designers use it after a deflection calculation to show the member meets the code.
The check
δallow = L/n
utilization = δ/δallow
L is the span between supports and n the limit’s denominator. δ is the deflection you computed for the load named in the limit, for example the live load alone for a floor at L/360. The member passes when the utilization is 1 or less. The page compares the magnitude of δ, so its sign does not matter.
For a cantilever, both IBC Table 1604.3 and ACI CODE-318-25 Table 24.2.2 take L as twice the cantilever’s length. Enter that doubled length as the span.
Limits and common mistakes
- Match the deflection to the load in the limit. A live-load limit uses the live-load deflection only, not the total.
- The IBC limits are minimums for serviceability. Vibration, ponding on flat roofs and the needs of glazing or equipment may call for a stricter limit.
- For wood installed at under 16 % moisture and used dry, a footnote to IBC Table 1604.3 lets the deflection from L + 0.5D stand in for L + D.
- The ACI L/480 limit applies to the deflection that occurs after the fragile elements are attached, which includes long-term creep and shrinkage of the concrete.
- The page checks a deflection you supply; it does not compute one. Get δ from a beam formula or analysis.
Worked example
Floor joists spanning 5.2 m deflect 11.8 mm under the design live load. Check them against the IBC floor limit.
- Enter Span 5.2 m and Computed Deflection (δ) 11.8 mm.
- Choose Applicable Limit Live load — L/360, under Floor members.
- Press Solve. The Solution shows:
L/360 allowable: 14.4444 mm
L/480 allowable: 10.8333 mm
Selected limit (L/360): 14.4444 mm
Utilization δ / allowable: 0.816923
Result: OK
Check: 5,200 mm / 360 = 14.44 mm, and 11.8 / 14.44 = 0.817. The L/480 row, 10.8333 mm, shows that the same joists would not meet that stricter limit.