What this calculator does
A beam can be strong enough in bending yet still feel “bouncy” or crack plaster if it deflects too far. This tool uses the classic simply-supported uniform load formula δ = 5wL⁴ / (384EI) with E = 210,000 N/mm² and typical section second moments of area to give a planning estimate of mid-span deflection.
How to use it
- Choose the section — each option includes an approximate Iₓ value.
- Enter clear span — distance between supports in metres.
- Enter uniform load — line load in kN/m (floor/roof load + self-weight if needed).
- Pick a limit — L/360 for many domestic floors, L/250 for general beams.
Worked example
A 203×133×25 over a 4.0 m span with 12 kN/m UDL typically shows several millimetres of mid-span deflection. Compare that with L/360 ≈ 11.1 mm. If the estimate is close to the limit, discuss a deeper section or reduced span with your engineer — online tools are only a first pass.
Disclaimer: Simplified estimate only. Point loads, cantilevers,
partial fixity, shear deflection and lateral-torsional buckling are not modelled.
Building Control requires engineer calculations for structural work.