Deflection Checker Calculator

The formula δ = 5wL⁴/(384EI) raises span length to the fourth power, so doubling L multiplies deflection by 2⁴ = 16, all else held equal. This is why span is by far the most sensitive input in this calculator — a modest increase in beam span can push a passing design well over the allowable limit, while the same percentage change in load, E, or I only scales deflection linearly.

L/360 is the common choice for a floor supporting plaster or drywall, since those brittle finishes crack at relatively small amounts of sag. L/240 is more permissive and fits a roof or a floor without brittle finishes, while the tighter L/480 and L/600 limits apply to ceilings and vibration- or deflection-sensitive equipment where even small movement is a problem. This calculator simply divides your span by whichever divisor you pick to get the allowable deflection.

No — the underlying formula assumes a simply-supported beam with a single support at each end carrying a load spread evenly along its full length. A cantilever, a continuous multi-span beam, or a beam under a point load instead of a uniform load all follow different deflection formulas, so plugging those cases into this calculator will give an inaccurate result.

Since moment of inertia (I) sits in the denominator and often varies with the cube or higher power of a section's depth depending on the shape, increasing beam depth is usually far more effective than swapping to a marginally stiffer material. Reducing the span (adding an intermediate support) has the largest effect of all, since deflection scales with L⁴, but if the span is fixed, increasing I by choosing a deeper section is typically the practical next step.