How Do I Convert Coating Weight Between g/m2, oz/ft2 and AS Grades?
How Do I Convert Coating Weight Between g/m2, oz/ft2 and AS Grades?
Direct answer: 1 oz/ft2 equals 305.15 g/m2. The conversion is easy; the trap is that some figures describe one surface and some describe both. ASTM G designations and EN Z designations both state the total coating mass on both surfaces combined, which is exactly why G90 and Z275 describe the same coating.
| Designation | System | What the number means | Example |
|---|---|---|---|
| g/m2 both sides | Metric, general | Total mass on both surfaces combined | Z275 = 275 g/m2 total |
| g/m2 per side | Metric, occasionally quoted | One surface only - double it to compare | 137 g/m2 per side = Z275 |
| oz/ft2 | ASTM imperial | Total both surfaces combined | G90 = 0.90 oz/ft2 |
| Z grade | EN 10346 | g/m2 total both sides | Z275, Z350, Z450 |
| G grade | ASTM A653 | oz/ft2 total both sides | G60, G90, G115 |
| AS grade | Aluminized (Type 2) | g/m2 total both sides | AS80, AS120, AS150 |
The single most expensive mistake
Reading a per-side figure as a total. A quotation that says 275 g/m2 per side is offering double the coating of Z275, and a buyer who reads it as Z275 will either overpay or, in the other direction, accept half the corrosion protection they specified.
Whenever a coating weight appears without units and without a side convention, stop and confirm. It is the one number in a steel quotation that is routinely ambiguous.
Quick reference table
Common ASTM G grades converted to total g/m2: G30 = 90, G40 = 120, G60 = 180, G90 = 275, G115 = 350, G140 = 427. Reading across a quotation, the pairing to remember is G90 with Z275 and G115 with Z350, because those two pairs appear in most export specifications.
Why the grade ladder is not linear in cost
Coating cost does not scale purely with mass - line speed falls as coating weight rises, and heavy coatings need more zinc in the pot and more care at the edges. Moving from Z275 to Z350 costs more per tonne than the extra 75 g/m2 suggests, which is why specification reviews sometimes find that a lighter coating with a better substrate is the cheaper path to the same service life.
