Acceptance limits to ISO 5817
Whether the imperfection you have just measured is acceptable — at quality level B, C or D, for the thickness in front of you.
Short answer
Three quality levels: B stringent, C intermediate, D moderate. Cracks and lack of fusion are not permitted at any of them; every other imperfection has a limit that depends on thickness. The calculator below gives the limit for your thickness and defect.
The three levels
B is stringent, C intermediate, D moderate. The level is not chosen by the welder: it comes from the design, and it appears on the drawing or in the WPS. Pressure equipment and dynamically loaded structures normally call for B; ordinary static structures often accept C.
Most limits are proportional to the thickness with an absolute cap. Undercut at level C, for example, is 0.1·t but never more than 0.5 mm — so on 20 mm plate the cap decides, not the proportion.
| Level | Name | Where it applies |
|---|---|---|
| B | Stringent | Critical load-bearing structures, pressure vessels, EXC3–EXC4 to EN 1090-2 |
| C | Intermediate | The bulk of machinery and structural steelwork, EXC2 |
| D | Moderate | Unloaded and secondary parts, railings, frames |
What is never acceptable
Cracks and lack of fusion are rejectable at every level, with no measurement to argue about. That is the practical reason lack of fusion matters more than a bead that merely looks untidy: an ugly weld can pass, a weld with a crack cannot.
The calculator covers the imperfections that come up most often in visual inspection. Where money or safety depends on the answer, the text of ISO 5817 decides — it has notes, exceptions and a wider list of imperfections than any calculator should pretend to reproduce.
Frequently asked questions
Which quality level applies if the drawing does not name one?
Then it is not specified, and that is a gap in the documentation rather than a decision for the welder. In structural steelwork to EN 1090 the level follows from the execution class: EXC2 normally means C, EXC3 means B. It is not something to guess at.
Why are cracks not permitted at any level?
Because a crack grows under load. A pore is a cavity of fixed size; a crack is a notch with a tip, and the stress at that tip is effectively unbounded. That is why there is no permitted size for it in any quality level.
The measured value sits exactly on the limit.
Then it is acceptable — the standard writes its limits inclusively. But measurement uncertainty counts: if the reading falls within the uncertainty of the limit, you measure again rather than argue.
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