How to use the uniform load beam deflection calculator
Floor and roof loads spread evenly along a member are modeled as uniform loads, and deflection under them is usually what decides the member size. Enter the support condition, the uniform load, the span, the rectangular section, and the material to get the moment of inertia, the maximum deflection, and a check against the L/360 limit.
Each support condition has its own closed-form deflection. Simply supported gives d = 5wL⁴/(384EI). A beam fixed at one end and pinned at the other gives d = wL⁴/(185EI) at about 0.42 of the span. Fixed at both ends gives d = wL⁴/(384EI), and a cantilever gives d = wL⁴/(8EI) at the free end.
Deflection grows with the fourth power of span and falls with the moment of inertia, which for a rectangle is I = bh³/12 and therefore cubic in depth. Shortening the span or deepening the section changes the answer far more than switching materials does.
Real connections are never perfectly rigid, and concrete members keep deflecting through creep and cracking well after the load is applied. Treat this as an elastic first-pass check, then confirm vibration serviceability, finish damage limits, and the governing code criterion with the structural engineer of record.
Frequently asked questions
A simply supported beam deflects 5wL⁴/(384EI) while a fixed-fixed beam deflects wL⁴/(384EI), exactly one fifth as much. Real connections rarely provide full fixity, so the gain is usually smaller.
It is the live load deflection limit for floor members supporting plaster or gypsum ceilings in IBC Table 1604.3. Other members use L/240 or the tighter L/480 depending on the finish.
Deflection scales with span to the fourth power and inversely with the cube of section depth. Shortening the span or deepening the beam beats widening it or switching to a stiffer material.