Deflection Limit Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
Enter Values
How to use this calculator
- Enter the span length L in millimetres.
- Enter the deflection ratio denominator — the N in L/N (e.g. 360 for L/360).
- Optionally enter a measured or calculated deflection to see pass/fail and utilisation.
How it works
The allowable deflection is simply the span divided by the ratio: allowable = L ÷ N. So L/360 means the member may deflect at most 1/360 of its span.
If you enter an actual deflection, the tool reports the utilisation (actual ÷ allowable × 100%), a pass/fail against the limit, and the remaining margin. This is a serviceability check (how much it sags), not a strength or capacity check.
Worked example
6 m span at L/300. Allowable deflection = 6000 ÷ 300 = 20.0 mm. A measured 12 mm is 60% utilisation — pass. A 4.8 m span at L/360 allows 13.33 mm.
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.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.