Zinc Coating Unit Converter
ASTM talks in mils and oz/ft², ISO in µm and g/m² — and the mill certificate might use any of them. Type a value in any field and the other three update live, on the ISO 1461 Annex D density basis.
“mils” is not millimetres. A mil is one thousandth of an inch (1 mil = 25.4 µm = 0.025 mm). A millimetre is one thousandth of a metre (1 mm = 1,000 µm ≈ 40 mils). Type into either field above and watch the other — galvanized coatings live around 45–200 µm, which is 0.045–0.2 mm.
Advanced — density basis
The math
Watch the two that get confused: a mil is a thousandth of an inch; a millimetre is a thousandth of a metre. They differ by a factor of about 40. Coating specs never use mm — ASTM works in mils and oz/ft², ISO in µm and g/m² — but the mm field is there because steel thickness is quoted in mm all day long, and the habit carries over.
The only judgement call is the density. Pure zinc is 7.14 g/cm³, but a hot-dip galvanized coating includes zinc-iron alloy layers, so ISO 1461:2022 Annex D uses a nominal 7.2 g/cm³ for converting between coating thickness and coating mass. This tool defaults to 7.2; flip to 7.14 under Advanced when you need the pure-zinc basis.
One caution for spec work: ASTM A123's own Table 2 lists mils/oz-ft² equivalents with the standard's own rounding — when a contract cites that table, use the standard's printed values, not raw math.
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Questions
How do you convert µm to g/m² for a zinc coating?
Multiply the thickness in micrometres by 7.2 — the nominal coating density in g/cm³ per ISO 1461:2022 Annex D. An 85 µm coating is therefore ≈ 612 g/m². On the pure-zinc basis the factor is 7.14.
How many µm is 1 oz/ft²?
About 43 µm — the common ASTM shorthand is 1 oz/ft² ≈ 1.7 mils ≈ 43.2 µm. Computed exactly, 1 oz/ft² = 305.15 g/m², which converts to 42–43 µm depending on the density basis.
Why 7.2 vs 7.14 g/cm³?
7.14 is pure zinc. A galvanized coating also contains zinc-iron alloy layers, so ISO 1461 Annex D uses a nominal 7.2 for thickness↔mass conversion. Default here is 7.2; the pure-zinc basis sits under Advanced.
Is a mil the same as a millimetre?
No — and they're easy to mix up. A mil is one thousandth of an inch: 1 mil = 25.4 µm. A millimetre is one thousandth of a metre: 1 mm = 1,000 µm ≈ 40 mils. Galvanized coatings run roughly 45–200 µm (about 1.8–8 mils), so a coating thickness quoted in mm is almost always an error.