Most guys ordering ductwork know the size they need. Some know the gauge. A few say G90 because it was on the print or because somebody upstream called it out.
A lot fewer could tell you what G90 actually means.
That is not really their fault. A lot of shop and field language gets passed around that way. You hear it enough times, you learn when to say it, but nobody stops to explain what the numbers actually mean or why they matter.
So here it is in plain English.
Gauge: Start There
Gauge is the thickness of the base steel before the galvanized coating ever gets added.
And yes, the numbering is backwards. Lower number means thicker metal.
That trips people up all the time.
These are the gauges you will run into most in HVAC work:
- 26 gauge — 0.0179”. Common residential material. Light, easy to work with, fine for smaller low-pressure ductwork.
- 24 gauge — 0.0239”. The workhorse. Good for commercial work, heavier residential, larger fittings, and anything that needs a little more backbone.
- 22 gauge — 0.0299”. Heavier-duty work, bigger duct, higher pressure, longer runs.
- 20 gauge — 0.0359”. Industrial stuff, rougher environments, or jobs where lighter material would be asking for trouble.
On paper, the jump from 26 to 24 gauge does not look like much.
In your hands, it does.
Twenty-four holds shape better. It dents less. It stays truer. Seams feel better. Large pieces behave better. On a trunk line, plenum, or anything with some size to it, that difference matters fast.
SMACNA tables get more specific and lay out minimum gauges by duct size, pressure class, and application. That matters. But even without getting deep into tables, the basic idea is simple: as the duct gets bigger or the job gets tougher, the metal needs to get heavier.
What G90 Actually Means
G90 is not the grade of the steel.
It is the zinc coating.
Galvanized steel starts as plain carbon steel. Then it gets coated in zinc. That zinc is what helps keep the steel underneath from rusting.
That is the whole point.
The “G” number tells you how much zinc is on the sheet.
G90 means there is a total coating weight of 0.90 ounces of zinc per square foot, combined across both sides of the sheet.
That comes from ASTM A653, which is the standard behind the designation.
Here is the practical version:
- G60 — lighter zinc coating, cheaper material, less corrosion protection.
- G90 — heavier coating, better protection, standard for HVAC ductwork.
- G115 / G140 — heavier yet, usually for nastier environments where ordinary interior duct standards are not enough.
That is it. G90 is not magic. It just means more zinc.
And more zinc means more protection when the duct sees moisture, condensation, humid spaces, or the kind of real-world installation conditions that are never as clean as the drawing makes them look.
If everything stays dry forever, maybe you never notice the difference.
A lot of ductwork does not live in that kind of perfect world.
Why We Use G90 as the Default
At K & E Sheet Metal, we default to G90 because we would rather build it right than shave a few dollars off material most people are not thinking about closely anyway.
When somebody calls in a furnace transition, a return box, or a plenum, they are usually focused on size, fit, and getting the job moving. They are not always thinking about whether that piece is going into a damp basement, a humid mechanical room, an attic that sweats in summer, or some other space that is harder on metal than people expect.
If we default to the cheaper option, we are making that decision for them.
I would rather not do that.
The price difference between G60 and G90 on a normal order is usually not the part that breaks the job. The cost of premature rust, callbacks, or replacing metal that should have lasted a lot longer is a much worse place to save money.
There is also the shop side of it.
Cheaper material is not always just cheaper. A lot of the time it is more annoying to work with. Better-coated stock tends to be more consistent. It forms better. It behaves better. When you are making seams, bending metal, and trying to turn out clean work, consistency matters.
Guys in the field may never see that part.
The shop does.
Coil vs. Sheet
Most of what we build comes off coil stock.
That is exactly what it sounds like: a roll of galvanized steel feeding into machinery so we can make what we need efficiently.
Sheet stock is flat stock, usually something like 4x8 or 4x10. That gets used when the job calls for it, when the size makes more sense that way, or when it is just easier to pull a sheet than run a coil for a small quantity.
So when you see something like 24 4x8 G90, that means 24 gauge, 4-by-8 sheet, G90 coating.
If you see 26 coil G90, same idea. Twenty-six gauge coil stock with G90 coating.
Same kind of steel. Different form.
Aluminum is its own thing and shows up on specialty jobs. That is not galvanized. It protects itself differently. But for ordinary HVAC ductwork, galvanized steel is what most people are dealing with, and G90 is the number that usually matters.
What Actually Matters on Your Job
If you are ordering ductwork, the first number to pay attention to is gauge.
That tells you how heavy the metal is.
The next thing is whether the material is actually G90.
For normal interior HVAC work, that should not be some exotic upgrade. It should be the baseline. If a shop is quietly using lighter-coated material to save a few bucks, that is something you should know before the order gets made.
And if the job is going into a damp space, a humid space, a coastal environment, or anywhere moisture is likely to be part of the story, then material matters even more.
None of this is complicated once somebody finally explains what the numbers mean.
But it does matter.
Because once the metal is made and installed, those little spec-sheet numbers stop being paperwork and start being the thing your name is attached to.
K & E Sheet Metal works primarily in G90 galvanized steel across standard gauges for residential and commercial HVAC fabrication. Have a job with specific material requirements? Get in touch.