The zinc coating decides how long galvanized steel lasts, yet each standard writes it in different units. This guide explains how galvanized coating thickness is specified, how Z275 compares with G90, and which coating weight suits your project.
How Galvanized Coating Thickness Is Specified
Galvanized sheet and coil are not ordered by thickness in microns. They are ordered by coating mass, which is the weight of zinc per unit area.
Two systems are common:
- EN 10346 (Europe and much of Asia): The “Z” number is the total zinc mass on both sides in g/m². Z275 means at least 275 g/m² in total.
- ASTM A653 (North America): The “G” number is the total zinc mass on both sides in oz/ft², multiplied by 100. G90 means at least 0.90 oz/ft² in total.
Both numbers are a triple-spot minimum. The tester takes three samples across the width and averages them. Each single spot may be slightly lower, within a separate single-spot limit.
The most common mistake is reading Z275 as 275 g/m² on each side. It is the total. Each side carries roughly half.
Converting g/m² to Microns
Zinc has a density of about 7.14 g/cm³. So 7.14 g/m² of zinc equals about 1 µm of thickness on one side.
To estimate thickness per side:
Thickness per side (µm) ≈ coating mass (g/m², both sides) ÷ 2 ÷ 7.14
For Z275: 275 ÷ 2 ÷ 7.14 ≈ 19 µm. EN 10346 lists a typical value of 20 µm per side for Z275.
Galvanized Coating Thickness Chart
The table shows common EN coating designations with their typical thickness per side and the nearest ASTM A653 class.
| EN 10346 | Total mass, min (g/m², both sides) | Typical thickness per side (µm) | Nearest ASTM A653 class |
|---|---|---|---|
| Z100 | 100 | 7 | G30 (≈ 92 g/m²) |
| Z140 | 140 | 10 | G40 (≈ 122 g/m²) |
| Z180 | 180 | 13 | G60 (≈ 183 g/m²) |
| Z200 | 200 | 14 | — |
| Z225 | 225 | 16 | — |
| Z275 | 275 | 20 | G90 (≈ 275 g/m²) |
| Z350 | 350 | 25 | G115 (≈ 351 g/m²) |
| Z450 | 450 | 32 | — |
| Z600 | 600 | 42 | G185 (≈ 564 g/m²) |
To convert an ASTM class to g/m², multiply oz/ft² by about 305. For example, 0.90 oz/ft² × 305 ≈ 275 g/m².
Z275 vs G90: Are They the Same?
For most projects, yes. G90 equals about 275 g/m², so Z275 and G90 carry practically the same amount of zinc.
The differences are in the paperwork, not the zinc:
- Standard and steel grade: Z275 is ordered under EN 10346 with grades such as DX51D or S250GD. G90 is ordered under ASTM A653 with designations such as CS Type B or SS Grade 33.
- Single-spot limit: Each standard sets its own single-spot minimum.
- Test method and reporting: The mill certificate states results in the units of the ordered standard.
So if your drawing says G90 and your supplier offers Z275, the zinc protection is equivalent. Still, ask for the material to be certified to the standard your customer requires. An inspector checks the standard on the certificate, not the chemistry of the zinc.
How Long Does a Galvanized Coating Last?
Zinc corrodes slowly and evenly, so coating life is roughly proportional to thickness. ISO 9223 gives typical first-year zinc corrosion rates for each environment.
| ISO 9223 category | Typical environment | Zinc loss per year (µm) |
|---|---|---|
| C2 – Low | Dry inland, rural areas | 0.1–0.7 |
| C3 – Medium | Urban areas, mild coastal | 0.7–2.1 |
| C4 – High | Industrial areas, coastal | 2.1–4.2 |
| C5 – Very high | Severe industrial, salt spray | 4.2–8.4 |
A simple estimate: divide the thickness per side by the yearly loss.
For example, Z275 has about 20 µm per side. In a C3 urban environment, that gives roughly 10 to 28 years before the first maintenance. In a C4 coastal zone, the same coating may need attention after 5 to 10 years.
Treat these figures as a planning range, not a guarantee. Standing water, crevices and contact with wet timber or soil shorten coating life a lot. Good drainage often adds more years than a heavier coating.
Which Coating Weight Should You Choose?
Match the coating to the environment and to how the steel will be processed. More zinc is not always better.
| Application | Common coating range | Comment |
|---|---|---|
| Indoor appliances, electrical cabinets, interior ducting | Z60–Z120 | Dry service; lighter coatings form and weld more easily |
| Base metal for PPGI roofing and cladding | Z80–Z150 | Paint adds a second barrier on top of the zinc |
| Light-gauge framing, purlins, outdoor sheet | Z180–Z275 | Z275 / G90 is the usual choice for exposed construction |
| Unpainted roofing, agricultural buildings | Z275 and above | Long exposure to rain, ammonia and humidity |
| Coastal and heavy industrial sites | Z350–Z600 | Consider heavier coatings or a painted system |
Heavier coatings have practical limits. Thick zinc can crack or flake on very tight bends. It also creates more fumes and electrode wear during spot welding. If your parts are deep-drawn or spot-welded in volume, ask whether a lighter coating plus paint gives you a better result.
Sheet for indoor use is sometimes electro-galvanized steel sheet instead. Its zinc layer is only a few microns thick, but very even and smooth for painting. It is not intended for unpainted outdoor use.
Does Spangle Affect Coating Thickness?
No. Spangle is the crystal pattern on the zinc surface. It changes appearance, not coating mass.
Spangled galvanised steel shows the classic visible flower pattern. Zero spangle galvanized steel has a smooth, uniform surface that suits painting and visible parts. Both can carry the same coating mass, such as Z275. We supply Z275 and structural grades in zero spangle as well.
What About Galvanized Pipe & Tube?
Pipe is coated differently from sheet. Hot-dip galvanized pipe is usually dipped after forming, so zinc covers the inside and outside. These coatings are typically heavier than coil coatings. ASTM A53, for example, sets a minimum average of 1.8 oz/ft² for galvanized pipe.
Pre-galvanized tube is made from already coated strip, so it carries a sheet-type coating. The weld seam is then re-protected after forming. Compare both options on our hot-dip galvanized steel pipe & tube page.
How to Specify Galvanized Coating in Your Order
A clear specification avoids disputes at delivery. Include these points:
- Standard: EN 10346, ASTM A653, JIS G3302 or another standard your customer requires.
- Steel grade: For example, DX51D, S250GD or CS Type B.
- Coating designation: For example, Z275 or G90, written as a total for both sides.
- Spangle and surface treatment: Regular spangle or zero spangle; chromated, oiled or anti-fingerprint.
- Dimensions: Thickness, width, and coil weight or sheet length.
A common entry-level specification is DX51D Z100 for indoor and painted uses. For outdoor structures, most buyers move up to Z275.
FAQ
What Is the Thickness of Z275 Galvanized Coating in Microns?
Z275 carries about 20 µm of zinc on each side. The 275 g/m² figure is the total for both sides, so each side holds roughly 137 g/m². Dividing that by the density of zinc, 7.14 g/cm³, gives about 19 µm. EN 10346 lists 20 µm per side as the typical value for Z275.
Is G90 the Same as Z275?
They carry almost the same amount of zinc. G90 means 0.90 oz/ft² in total, which equals about 275 g/m². The difference is the standard behind them. G90 belongs to ASTM A653, while Z275 belongs to EN 10346. Single-spot limits and steel grade names differ, so order to the standard your customer specifies.
Is Coating Weight Given for One Side or Both Sides?
In EN 10346 and ASTM A653, Z and G numbers are totals for both sides. Each side carries roughly half of the stated value. Some orders need a different coating on each side, called a differential coating. These are written with two values, one for each side. Always state clearly which method your drawing uses.
Does a Thicker Zinc Coating Always Last Longer?
In the same environment, yes. Coating life increases roughly in line with zinc thickness. However, heavier coatings are harder to bend tightly and to spot-weld. Design details also matter a lot. Good drainage, no trapped water and a paint layer often add as many years as extra zinc. Choose the coating that fits both service and fabrication.
What Coating Weight Is Used for PPGI?
Base metal for PPGI commonly uses Z80 to Z150. The paint system adds a second barrier on top of the zinc, so a lighter coating often works well. For long-life roofing, coastal sites or industrial areas, a heavier zinc coating under the paint is worth considering. It protects cut edges and scratches for longer.
Can Galvanized Coating Thickness Be Checked on the Certificate?
Yes. The mill test certificate reports the coating mass found in testing, in the units of the ordered standard. For EN 10346 orders, it shows g/m² for both sides. For ASTM A653 orders, it shows oz/ft². Compare the triple-spot result with the designation on your order, such as Z275 or G90.
Conclusion
For indoor and painted parts, a lighter coating saves cost and forms more easily. For exposed outdoor steel, Z275 or G90 is the dependable baseline. We process galvanized coil and sheet in Wuxi and cut them to your size. Send us your specification for a quote.