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Deflection Limit Calculator

Calculate allowable beam deflection limits using L/240, L/360 and other span ratios for common structural design checks, free in your browser.

100% FreeRuns in your browserIntermediate
How to use this calculator

Inputs

Allowable deflection L/N with your own N, the actual deflection checked by magnitude.

L/N

From your project specification or standard — this tool has no default.

Sign shows direction (e.g. − for sag); the check uses the magnitude.

More options
Your rules

Entering it applies it: the smaller of L/N and this cap governs.

Enter the values and press Calculate.

Or load an example to see a worked result.

Your deflection check
Schematic
Your deflection checkSchematic beam with a deflection limit line.allowable L/Nactualspan Lundeflected beam (displacement exaggerated)
Calculate to draw your deflection against your limit.

Batch from CSV

One row per calculation, with a header row. Values use the units currently selected in the form above. Up to 10,000 rows; nothing is uploaded.

How the method works

Span ratio limit and actual deflection
Illustrative method
Span ratio limit and actual deflectionA simply supported beam with an exaggerated deflected shape reaching 60 % of the depth of a dashed allowable-deflection line.allowable L/300 = 20 mm12 mm (60 %)L = 6,000 mmundeflected beam (displacement exaggerated)
This is a displacement limit, not a strength calculation: L/N = 6,000 mm ÷ 300 = 20 mm allowable; an actual 12 mm deflection uses 12 ÷ 20 = 60 % of it, leaving 8 mm. N comes from your specification.

Fixed example values for illustration — not your result.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the span length L in millimetres and the ratio denominator N from your specification (e.g. 360 for L/360).
  2. If your specification also caps deflection at a fixed value, enter that absolute limit — the smaller of the two governs.
  3. Optionally enter a measured or calculated deflection (signed if you like) to see pass/fail, utilisation and margin.
  4. Label the load case and record your rule source. Under More options add an absolute cap (the smaller limit governs) and further limits as label, N lines to compare them side by side; the engineering note exports as CSV and the batch CSV run checks many cases.

How it works

Allowable deflection = L ÷ N, so L/360 means the member may deflect at most 1/360 of its span. With an absolute limit too, the governing allowable is the smaller of the two.

Utilisation = |actual| ÷ allowable × 100% and margin = allowable − |actual|; the limit itself passes. A negative entry is checked by its size and its direction is shown separately.

This is a serviceability check (how much it moves), not a strength or capacity check. Camber, creep and load combinations are outside this algebra.

Worked example

6 m span at L/300. Allowable = 6000 ÷ 300 = 20 mm. A measured 12 mm (or −12 mm) is 60% utilisation with 8 mm to spare; 20 mm exactly passes and 20.001 mm fails. With a 10 mm absolute cap, 10 mm governs and 12 mm fails.

Common mistakes

  • Mixing units — enter the span in millimetres so the allowable comes out in millimetres.
  • Treating this as a strength check. A member can be within its deflection limit and still be overstressed, or vice versa.
  • Picking a ratio without checking the right value for the member and finish (e.g. brittle finishes often need a tighter limit).

Frequently asked questions

What does L/360 mean?

The deflection should not exceed 1/360 of the span. For a 6000 mm span that's 16.7 mm. Smaller ratios like L/250 are slacker; larger ratios like L/500 are stricter.

How do I calculate allowable deflection?

Divide the span by the ratio denominator: allowable = span ÷ N. The tool does it instantly and also checks a measured value.

Which deflection limit should I use?

It depends on the member, the load case and the finish it supports — codes give different limits for total vs live load and for brittle finishes. Always check the limit required by your project and code.

Is this a structural design check?

No. It only checks serviceability deflection against a ratio. It does not compute the actual deflection from loads, nor strength, buckling or connections — use a full design process and a qualified engineer.

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Cite this tool

Zerdly. (2026). Deflection Limit Calculator. Retrieved from https://www.zerdly.com/tools/deflection-limit-calculator/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.