Coating Thickness Finder — ASTM A123 / ISO 1461
Every galvanizing spec starts with one question: what coating thickness does the standard actually require for this steel? Select the product, enter the thickness, and get the requirement from both major standards — including the local vs. mean distinction that causes most inspection disputes.
For assemblies, the requirement applies per component category and thickness — not to the assembly as a whole.
Specimen/lot acceptance math (how many measurements, averaging rules) is a separate calculation per the standards' sampling clauses — planned as a member tool.
How the requirement is determined
Both standards scale the required coating thickness with the steel thickness, because heavier steel develops thicker coatings naturally in the bath — longer immersion and more iron available for the zinc–iron reaction. Where they differ is the answer format:
- ASTM A123 answers in coating grades — a grade number equal to the minimum average coating thickness in µm for the material category and thickness range (Grade 75 = 75 µm minimum average).
- ISO 1461 answers with two numbers — a local minimum (the lowest value allowed at any single point) and a mean minimum (the required average across the article), each with a g/m² coating-mass equivalent.
Worked example
An 8 mm structural beam:
- ASTM A123: Grade 100 (100 µm minimum average) — structural shapes, ≥6.4 mm range.
- ISO 1461: 70 µm local / 85 µm mean (505 / 610 g/m²) — steel > 6 mm.
Same beam, two standards, two different numbers — which is why export fabricators check both before quoting.
Members get the full specifier workflow
- Specification Summary PDF — requirement + service life, one page, ready for the project file
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Questions
What's the difference between local and mean coating thickness in ISO 1461?
Local is the lowest coating thickness permitted at any single reference area on the article; mean is the required average across the reference areas. The mean requirement is always higher — and confusing the two is the most common source of ISO 1461 inspection disputes.
Are ASTM A123 and ISO 1461 requirements the same?
No — close, but not identical. They use different thickness ranges and different concepts (ASTM grades vs ISO local + mean minimums). The tool shows both side by side for the same input, and notes where they differ.
What coating grade does ASTM A123 require for structural steel?
It depends on the steel thickness — from Grade 45 for very thin material up to Grade 100 (100 µm minimum average) for structural shapes 6.4 mm and thicker, per ASTM A123/A123M-24 Table 1.
Does a thicker coating mean better protection?
Yes — service life is proportional to zinc thickness for a given environment. Take your result to the Galvanizing Lifetime Estimator to see the years to first maintenance.