G40 is a coating designation defined under the American standard ASTM A653 for continuously hot-dip galvanized steel coil. The letter G indicates a pure zinc coating (galvanized), as distinct from A which denotes a zinc-iron alloy coating (galvannealed). The number 40 represents the coating weight: a G40 coating has a total double-sided zinc weight of 0.40 ounces per square foot (oz/ft²). This corresponds to approximately 0.20 oz/ft² on each side. In metric terms, G40 is equivalent to approximately 120 grams per square metre (g/m²) total, or about 8 to 10 micrometres of zinc per side.

The coating is applied by immersing cold-rolled steel coil into a molten zinc bath at approximately 450°C (840°F), forming a metallurgically bonded zinc layer. This zinc provides corrosion protection through two mechanisms: it acts as a physical barrier preventing moisture and oxygen from reaching the steel surface, and it serves as a sacrificial anode, meaning the zinc corrodes preferentially to protect the underlying steel even if the coating is scratched.

G40 is classified as a moderate-level coating weight within the ASTM A653 G-series, which ranges from G30 (lightest) to G235 (heaviest). Compared to G30 (0.30 oz/ft²), G40 offers enhanced protection suitable for general outdoor exposure. Compared to G60 (0.60 oz/ft²) or G90 (0.90 oz/ft²), G40 provides a more cost-effective balance for less demanding outdoor environments. G90, for example, has roughly three times the corrosion resistance of G30, meaning G40 falls in the mid-range of the G-series in terms of service life.

G40 is typically specified for applications requiring moderate corrosion resistance at an economical cost. Common uses include:

● Roofing coils and gutters for residential construction

● Agricultural equipment and light structural components

● HVAC ductwork and ventilation systems

● Automotive body panels and underbody parts

● White goods (refrigerators, washing machines, air conditioners)

● Metal furniture, shelving, and office equipment

● Signage, fencing, and door frames