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Deflection Limit Calculator

Work out deflection limit instantly with clear inputs, formula shown and shareable results.

About Deflection Limit Calculator

Deflection limits are expressed as a fraction of span because what matters is curvature relative to the finishes, not absolute movement. Span over 250 suits general structures, over 300 where brittle finishes are attached and over 360 for live load deflection beneath plastered ceilings.

Formulas

Allowable deflection

Allowable mm = span mm / limit ratio

Check

Utilisation = actual deflection / allowable deflection

Worked example

With the default values below, Deflection Limit Calculator returns Allowable deflection of 16.67. Change any input above and every figure here is recalculated in your browser.

Inputs used

Spanminimum 0.1 m
6 m
Deflection limitone of Span / 250 — general, Span / 300 — brittle finishes, Span / 360 — live load on plastered ceilings
Span / 360 — live load on plastered ceilings
Calculated deflectionminimum 0 mm
18 mm

Results

Allowable deflection
16.67 mm
Utilisation of the limit
108.0% %
Actual span to deflection ratio
333
Compliance
Exceeds the limit — stiffen the member

Step by step

  1. 1Values used
  2. 2Allowable deflection
  3. 3Check
  4. 4Allowable deflection= 16.67 mm
  5. 5Utilisation of the limit= 108.0 %
  6. 6Actual span to deflection ratio= 333
  7. 7Compliance= Exceeds the limit — stiffen the member

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Deflection criteria and long-term effects must be assessed by a qualified structural engineer to the governing code.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Deflection criteria and long-term effects must be assessed by a qualified structural engineer to the governing code.

Frequently Asked Questions

How is the Deflection Limit figure worked out?

Allowable mm = span mm / limit ratio. Deflection limits are expressed as a fraction of span because what matters is curvature relative to the finishes, not absolute movement.

Should dead load deflection count?

Long-term dead load deflection matters for appearance and for ponding on flat roofs, but limits protecting finishes usually apply to the deflection occurring after the finishes are installed.

What values do I need to enter?

This calculator takes 3 inputs: Span, Deflection limit, Calculated deflection. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

Related Calculators

Deflection Limit Calculator – Calculate Allowable deflection