Work out deflection limit instantly with clear inputs, formula shown and shareable results.
Deflection limits are expressed as a fraction of span because what matters is curvature relative to the finishes, not absolute movement. Span over 250 suits general structures, over 300 where brittle finishes are attached and over 360 for live load deflection beneath plastered ceilings.
Allowable deflection
Allowable mm = span mm / limit ratio
Check
Utilisation = actual deflection / allowable deflection
Preliminary estimate only. Deflection criteria and long-term effects must be assessed by a qualified structural engineer to the governing code.
Allowable mm = span mm / limit ratio. Deflection limits are expressed as a fraction of span because what matters is curvature relative to the finishes, not absolute movement.
Long-term dead load deflection matters for appearance and for ponding on flat roofs, but limits protecting finishes usually apply to the deflection occurring after the finishes are installed.
This calculator takes 3 inputs: Span, Deflection limit, Calculated deflection. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.