G90 vs Z275: Galvanized Coating Weight, Decoded

G90 vs Z275 is not a choice between two coatings. They are the same amount of zinc written in two unit systems, and the mill that quotes you one is quoting you the other. What actually goes wrong sits one layer down: the number is a mass, not a thickness, and it is a total across both faces, not a promise about the face that meets the weather.

Three specific misreadings account for almost every galvanized dispute I have seen land in a quality complaint. Here they are, with the clause each one hides behind.

G90 vs Z275: the same coating in two unit systems

ASTM A653/A653M covers continuously hot-dip galvanized steel sheet, and it publishes its coating designations twice — once in inch-pound units and once in SI. The conversion is a single constant:

1.00 oz/ft² = 305 g/m²

So G90, which is 0.90 oz/ft², is 275 g/m². That is Z275. Same coil, same coating, different order form. The letter tells you the coating chemistry — G for galvanize, zinc — and the number is the minimum average triple-spot test value, total both sides, measured per ASTM A924/A924M.

Inch-pound (order to A653)	oz/ft² total both sides	SI (order to A653M)	g/m² total both sides	Coating thickness, both sides	If split evenly, per side
G30	0.30	Z90	90	≈ 0.013 mm	≈ 6 µm
G40	0.40	Z120	120	≈ 0.017 mm	≈ 9 µm
G60	0.60	Z180	180	≈ 0.026 mm	≈ 13 µm
G90	0.90	Z275	275	≈ 0.038 mm	≈ 19 µm
G115	1.15	Z350	350	≈ 0.049 mm	≈ 25 µm

The thickness columns are derived, not specified. The GalvInfo Center publishes the relationship as 1.00 oz/ft² = 0.00168 in = 305 g/m² = 0.0427 mm, based on a zinc density of 446 lb/ft³ (7,140 kg/m³). Its own worked example rounds G90 to "about 1.6 mils (0.0016 inches, or about 42 microns)" total with "about 0.0008 inches (21 microns) of zinc on each surface"; the constant above gives about 38 µm total. You are arguing over a micron or two, and that gap is itself the point — coating weight is measured by weighing, stripping and re-weighing a coupon, because thickness cannot be measured accurately enough to specify directly.

Heavier designations exist — G165, G210 — but they are used for specialised applications and are generally not available on thinner gauge sheet, so writing one into a spec for 0.5 mm sheet is a quiet way to make your enquiry unquotable.

European mills quote the same Z-plus-number form under EN 10346 for continuously hot-dip coated flat products. The number means the same thing — grams of zinc per square metre, both faces. The acceptance testing is written to the European standard rather than to A924, so a Z275 on an EN certificate and a Z275 on an ASTM certificate are the same target, not automatically the same paperwork. Say which standard you are buying to.

Trap 1: total both sides is not half on each side

This is the expensive one. G90 vs Z275 is a units question; total-both-sides versus per-side is a money question. Both designations are total-both-sides figures, and the standard does not require the coating to split evenly.

Look at what A653/A653M actually requires for a single coil:

Designation	Triple-spot test average	Single-spot test, total both sides	Single-spot test, any one side
G90	0.90 oz/ft² minimum	0.80 oz/ft² minimum	normally not less than 0.32 oz/ft²
Z275	275 g/m² minimum	235 g/m² minimum	normally not less than 94 g/m²

Read the last column again. A Z275 coil that splits its zinc evenly carries 137.5 g/m² per face. The any-one-side floor is 94 g/m² — roughly a third less. A coil stamped Z275 can carry 94 g/m² of zinc on the face that meets the weather and still be a legitimate Z275.

The rule behind that number is that one side will normally be at least 40 % of the specified minimum single-spot total-both-sides value. For G90 that is 40 % of 0.80, which is the 0.32 oz/ft² in the table.

For a roof panel, a cladding sheet, an enclosure, or a duct — anything where one face is exposed and the other is not — this matters more than the headline designation does. Corrosion performance for galvanized coatings is roughly linear in coating thickness: a G60 coating is twice the thickness of a G30 and lasts roughly twice as long to 5 % red rust, and a G90 is about 50 % thicker than a G60 and performs about 50 % better. Lose a third of the zinc on the exposed face and you have moved a rung down that ladder without changing a single line on the purchase order.

What to do about it. If one face carries the corrosion duty, stop specifying total-both-sides. Order to the per-side designations instead.

Trap 2: 60G60G and 13Z13Z read backwards

In 2007 ASTM added single-side, single-spot coating designations to A653/A653M. They take the automotive form: the numerals come first, and that signals a per-side requirement.

60G60G — minimum 60 g/m² and maximum 110 g/m² of zinc on each surface, for any single spot
13Z13Z — under A879/A879M for electrogalvanized sheet, minimum 0.13 oz/ft² and maximum 0.23 oz/ft² on each surface

Now the rule that catches everyone, stated as plainly as the source states it: for hot-dip galvanize, ASTM uses G preceding the numerals for inch-pound units and Z for SI units, total both sides in each case. For electrogalvanize, G following the numerals means SI and Z means inch-pound, single spot and single side in each case. The same two letters flip meaning depending on which side of the number they sit.

So "Z275" and "13Z13Z" both contain a Z, and one is 275 g/m² across both faces while the other is 0.13 oz/ft² minimum on each face. A purchasing clerk pattern-matching on letters will get this wrong, and the mill will ship exactly what the order said.

There is one more consequence worth internalising. Per-side designations set a maximum as well as a minimum; total-both-sides designations set only a minimum. And you cannot convert precisely between the two systems, because a total-both-sides figure is a triple-spot average that tolerates an uneven split, while a per-side figure is a mandatory floor at every single spot.

The published worked example makes it concrete: a G60 coating has a minimum total-both-sides thickness of about 1.0 mil, which is very close to twice the 12.5 µm per-side minimum of a 90G90G. They look equivalent. They are not — the G60 can put its zinc wherever it likes within the tolerance, and the 90G90G cannot.

Trap 3: a coating designation is a floor, not a target

A653 Table 1 designations specify a minimum. There is no maximum. Actual production coatings run a few percent above the minimum precisely so the mill can guarantee it.

That has two practical consequences that show up in different departments.

For engineering, "we specified G90" tells you nothing about the upper bound, which matters when the sheet has to be formed, welded or painted — heavier zinc is harder on tooling and changes paint adhesion behaviour. If an upper limit matters, you need a per-side designation, which carries one.

For sales, two suppliers both quoting Z275 are not necessarily selling the same thing, and neither is lying. If your coil genuinely runs heavier, the designation will never show it — only the measured value on the mill test certificate will. Put that number where the buyer reads it, next to the designation, not buried in an attachment — the same discipline that makes a Ra vs Rz surface roughness figure useful rather than decorative.

Galvanneal uses a different alphabet: A40 and ZF120

Zinc-iron alloy coated sheet — galvanneal — is produced under the same A653/A653M standard but designated differently:

Inch-pound	Requirement	SI	Requirement
A25, A40, A60	oz/ft² triple-spot average, total both sides	ZF75, ZF120, ZF180	g/m² triple-spot average, total both sides

A40 is 0.40 oz/ft² total both sides, with a single-spot floor of 0.30 oz/ft² total both sides and normally not less than 0.12 oz/ft² on any one side. ZF120 is 120 g/m² total both sides, single-spot 90 g/m² total both sides, normally not less than 36 g/m² any one side. Per-side galvanneal designations follow the same numerals-first form: 45A45A is 45 g/m² minimum and 75 g/m² maximum on each surface.

The coating contains roughly 8 to 10 % iron, so it is slightly denser and slightly thinner than a pure zinc coating of the same mass — a difference small enough to ignore for specification purposes, and large enough to explain why galvanneal and galvanize of nominally equal weight do not behave the same under paint.

If your enquiry says "A40 or equivalent" and the mill reads it as G40, you have swapped a zinc-iron alloy for pure zinc. Same number, different coating, different weldability.

ASTM A653 vs A123: sheet is not fabrication

The single most common category error in export enquiries: specifying G90 for a fabricated part.

Per the American Galvanizers Association, "G90 is not a galvanizing specification; it is a coating thickness designation" — it belongs to continuous sheet galvanizing, where coil runs through a zinc bath at speed. ASTM A123/A123M is the standard for products that are fabricated first and then hot-dip galvanized in a batch kettle, and it specifies coating thickness by steel category and thickness, not in oz/ft² of sheet.

The two processes do not produce the same coating. Continuous sheet coating is thin and formable — it survives bending after coating. Batch coating is thicker and alloyed, protects a finished assembly better, and cannot be formed afterwards.

So: bend it then dip it, and you are in A123. Coat the coil then form it, and you are in A653. A drawing that calls out "hot-dip galvanized G90" on a welded frame is asking for a process that does not exist for that part, and every quote you receive back will silently reinterpret it.

How to write the coating line so it cannot be misread

Getting G90 vs Z275 right is the easy half. Six fields turn a coating call-out into something a mill, an inspector and a buyer all read the same way. This is the line I would put on an industrial spec diagram or a quotation sheet.

Standard and edition — ASTM A653/A653M or EN 10346, stated explicitly, because the same designation is tested differently
Designation exactly as printed — Z275, not "275 zinc"; 60G60G, not "60G"
Basis — write "total both sides" or "each side" in words next to the designation, even though the format already implies it
Which face carries the duty — if one side is exposed, say so, and specify a per-side minimum for that face
Maximum, if one matters — needed for forming, welding or paint adhesion; only per-side designations carry one
Test method and certificate — ASTM A924/A924M, with the mill test certificate values reported, not just the designation
Substrate thickness alongside coating — heavier designations are not available on thin gauge, so pair the coating with the sheet metal gauge to mm call-out rather than specifying them in separate documents

FAQ

Is G90 the same as Z275?

Yes. G90 vs Z275 is a units difference, not a coating difference: G90 is 0.90 oz/ft² of zinc, total both sides, and 1.00 oz/ft² converts to 305 g/m², so 0.90 oz/ft² is 275 g/m². They are the inch-pound and SI designations for the same coating weight in ASTM A653/A653M.

How thick is a Z275 coating in microns?

About 38 to 42 µm across both faces, or roughly 19 to 21 µm per face if the zinc splits evenly, derived from the relationship 1.00 oz/ft² = 0.0427 mm. Thickness is not what the standard specifies — coating weight is — so treat any micron figure as a conversion rather than a requirement.

Does Z275 mean 275 g/m² on each side?

No, and this is the costliest misreading in the whole system. Z275 is 275 g/m² total across both surfaces. The single-spot floor on any one side is normally not less than 94 g/m². If you need a guaranteed amount on one face, order a per-side designation such as 60G60G, which sets both a minimum and a maximum on each surface.

Can I specify G90 for a welded steel frame?

No. G90 belongs to ASTM A653/A653M continuously galvanized sheet. A fabricated assembly that is galvanized after welding falls under ASTM A123/A123M, which specifies coating thickness by steel category rather than in oz/ft² of coil.

How do I get a coating spec onto a product image without it becoming a wall of text?

Put the number where the buyer's eye already is: on the part it describes. A coating call-out sitting in a table three screens down gets skipped; the same value on a leader line pointing at the exposed face gets read. Doing that well means the annotation has to be pinned to the real geometry — the measured edge, the actual face — so that "Z275, each side, exposed face" lands on the surface it refers to and the overall dimensions on the drawing are the measured ones. That is the difference between measurement-driven annotation software and an AI image generator: one places a value against something it measured, the other produces a plausible-looking number next to a restyled photo. Keep the annotation layer live and editable, and a change from Z275 to Z350 is one field, not a redraw — the same argument that applies to a powder coating specifications call-out.

G90 vs Z275: Same Zinc, Three Ways to Read It Wrong