Beam Deflection Calculator
Maximum deflection of a simply supported or cantilever beam, with the L/360 check.
What is Beam Deflection?
Deflection is the serviceability side of beam design: a beam can be strong enough and still sag far enough to crack the plaster above it or feel springy underfoot. It depends on the load, the span and the section's stiffness (E·I) — so it needs two numbers that strength does not: the material's modulus E and the second moment of area I.
How it works
The standard cases give δ = P·L³ ÷ (48·E·I) for a simply supported beam with a central point load, δ = 5·w·L⁴ ÷ (384·E·I) under a uniform load, and δ = P·L³ ÷ (3·E·I) for a tip-loaded cantilever. The result is reported in millimetres and compared against L/360, the common deflection limit for a floor — a ratio rather than a distance, because it scales with the span. Doubling the span multiplies deflection by eight, which is why span dominates every rule of thumb in carpentry.
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Examples
Realistic sample
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Common mistakes
Expecting a server round-trip
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