Concrete Cover Thickness Lookup
Minimum cover from exposure class and member type, with every code correction applied.
Inputs
Result
Formula
| Lookup | c = Table 8.2.1 (exposure class × member type) |
| Grade correction | add 5 mm when concrete is C25 or below |
| Life correction | ×1.4 for a 100-year design life, rounded up to 5 mm |
| Precast | 5 mm reduction allowed, floors 15 / beams 20 |
| Bar check | c not less than the bar diameter d |
| Foundation | ≥40 mm with blinding, ≥70 mm without |
Class 2a, beam, C30, 50-year life, cast in situ, 20 mm bars: table gives 25 mm; no grade or life correction; the 20 mm bar does not govern, so cover is 25 mm.
FAQ
Is cover measured to the stirrup or to the main bar?
To the outermost bar. Where stirrups exist the outermost bar is the stirrup, so cover runs from the stirrup surface to the concrete face and the main bar sits one stirrup diameter deeper. Spacers must be set to the stirrup, which is the most commonly mis-set item on site.
Why add 5 mm for C25 and below?
Lower-grade concrete is less dense and more permeable, so carbonation reaches the bar sooner and the passive layer breaks down. The extra 5 mm compensates. It is also why C20 is rarely used outdoors today - not for lack of strength but for lack of durability.
Is this table enough for coastal or chemical plant structures?
No. This table stops at class 3b. Class 4 (seawater) and class 5 (aggressive substances) are covered by separate codes and usually need coatings, corrosion inhibitors or sulfate-resisting cement as well. Chemical plants, treatment works and marine structures need the specific codes.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.