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Structural Calculators

Beam Deflection Calculator

Maximum deflection of a simply supported or cantilever beam, checked against the span/250 limit.

Enter values

Results update as you type.

Result

Maximum deflection

1.667 mm

Span / deflection
2,400
higher is stiffer
Span / 250 limit
16 mm
Check
Passes the usual span/250 serviceability limit
Stiffness EI
16,000 kN·m²

Indicative only — a qualified engineer must carry out the final design check.

About the Beam Deflection Calculator

Maximum deflection of a simply supported or cantilever beam, checked against the span/250 limit.

Also known as: beam sag calculator, deflection formula, joist deflection, beam bending calculator.

How to use this calculator

  1. Enter beam and load, span or cantilever length, modulus of elasticity, second moment of area and the other fields.
  2. The result updates instantly as you type — press Calculate (or Enter) at any time.
  3. Review the breakdown shown under the main result.
  4. Use Copy result or Share to save the answer or send a link with your inputs.

Formula

Simply supported UDL: δ = 5wL⁴ / 384EI · Cantilever point: δ = PL³ / 3EI

Worked example

Using the values pre-filled in the calculator above:

  • Beam and load: Simply supported, distributed
  • Span or cantilever length = 4 m
  • Distributed load = 8 kN/m
  • Modulus of elasticity = 200 GPa
  • Second moment of area = 8,000 cm⁴

the calculator returns:

  • Maximum deflection: 1.667 mm
  • Span / deflection: 2,400
  • Span / 250 limit: 16 mm
  • Check: Passes the usual span/250 serviceability limit