Reading a beam deflection result: what's actually acceptable?
A computed deflection number on its own doesn't tell you whether a beam is fine — what it's compared against, and why that comparison depends on what the beam actually supports.
Deflection is checked as a serviceability limit state (SLS), which is a genuinely different question from the strength (ultimate limit state) check most of a design focuses on. A beam that comfortably passes its strength check — it won't break — can still fail its deflection check, because a beam that sags visibly, cracks finishes, or feels bouncy underfoot is a real serviceability problem even though it isn't remotely close to collapse. Both checks matter, and passing one says nothing about the other.
A raw deflection number by itself — "this beam deflects 18 mm" — is not yet an answer to "is that okay?". It only becomes meaningful once compared against a limit, and that limit is essentially never a fixed number in millimetres; it's expressed as a fraction of the beam's own span (a span/deflection ratio), because a sag that's barely noticeable on a short beam would be obviously excessive on a long one at the same absolute value.
Why the applicable limit depends on context
Both EN 1990 (via its serviceability annex, with the specific values often set by material codes like EN 1992-1-1 or by project specification) and ACI 318 (Chapter 24) work with this same span-ratio idea, and both distinguish between different deflection cases rather than applying one universal number: total deflection under all loads versus incremental deflection occurring after a brittle finish (like plaster or masonry partitions) is installed, since it's the deflection after installation — not the total — that actually risks cracking that finish. Commonly cited limits in practice include ratios in the range of span/360 for the increment likely to damage brittle elements, and around span/250 for total deflection affecting appearance — but these are illustrative, not universal: the actual limit that applies depends on the code, the National Annex or project specification in force, and specifically what the member supports.
The practical takeaway: the Beam calculator reports the computed deflection itself — a real, calculated number from the beam's stiffness and loading. Deciding whether that number is acceptable is a separate step that depends on context the calculator doesn't have (what the beam supports, which code and limit governs on that specific project) — that judgment call belongs to the engineer, not the tool. RC Beam Design and Steel Beam Design carry this same deflection output into a fuller design workflow (see the related tools below).