Nominal thickness
This is the target the mill rolls the sheet to, and CSA S136 calls it the design thickness. Delivered sheet lands a little above or below it. A spec that gives one figure with no other word usually means nominal.
British Columbia
British Columbia
WHAT A GAUGE NUMBER MEANS IN EACH METAL
Name the metal and write the millimetres beside the gauge number whenever a supplier could read it two ways.
Confusion between sheet metal gauge and thickness trips up many orders. A gauge number names a thickness, and that thickness changes with the metal. Sixteen gauge mild steel is about 1.52 mm, galvanized about 1.61 mm, and aluminum about 1.29 mm. A lower number means thicker sheet. This guide gives the millimetres for each gauge and metal and explains why they differ. It also shows what to write so every supplier prices the same sheet.
Find the number on your order, then read across to the metal.
| Gauge | Mild steel (mm) | Galvanized (mm) | Stainless (mm) | Aluminum (mm) |
|---|---|---|---|---|
| 10 | 3.42 | 3.51 | 3.57 | 2.59 |
| 12 | 2.66 | 2.75 | 2.78 | 2.05 |
| 14 | 1.90 | 1.99 | 1.98 | 1.63 |
| 16 | 1.52 | 1.61 | 1.59 | 1.29 |
| 18 | 1.21 | 1.31 | 1.27 | 1.02 |
| 20 | 0.91 | 1.01 | 0.95 | 0.81 |
| 22 | 0.76 | 0.85 | 0.79 | 0.64 |
| 24 | 0.61 | 0.70 | 0.64 | 0.51 |
| 26 | 0.46 | 0.55 | 0.48 | 0.40 |
| 28 | 0.38 | 0.47 | 0.40 | 0.32 |
| 30 | 0.30 | 0.40 | 0.32 | 0.25 |
Stainless tables differ between sources, and this column uses the one based on inch fractions, where 16 gauge is 1/16 in. Other charts list 16 gauge stainless at 1.52 mm, and that gap is one more reason to write the millimetres on a stainless order.
Sixteen gauge, four metals, drawn to scale
Each bar is the sheet's edge, magnified 100 times
Each metal got its numbers from a different system, long before anyone ordered in millimetres. That history explains every gap in the table.
The steel numbers began as a set weight per square foot. Sixteen gauge steel weighs about 12.2 kg per square metre, and its thickness is worked back from that weight. This steel table, the Manufacturers' Standard Gauge or MSG, is the one most Canadian steel charts use.
Each galvanized figure is the steel one plus about 0.09 mm, an allowance for a heavy zinc coat on both faces. If you measure galvanized sheet, you're measuring the steel and its coating together.
Aluminum follows the Brown and Sharpe system made for wire. That's why 18 gauge aluminum is about 1.02 mm, and the same number in steel is about 1.21 mm. If you order aluminum, name the thickness.
Different suppliers list stainless steel by different gauge tables, even for the same grade. If a stainless order gives only the gauge, two shops can price two different sheets, and the millimetres prevent that.
The steps between gauges aren't even, either. In mild steel, 10 to 12 gauge drops about 0.76 mm, but 22 to 24 drops only about 0.15 mm.
The standards that govern sheet metal fabrication agree on one point. They want the thickness written as a measurement, and they treat the old numbers as a convenience at best.
| Standard or body | What it covers | What it says about gauge and thickness |
|---|---|---|
| ASTM A480 | Stainless steel sheet and plate | Thickness is ordered as a decimal or a fraction. It calls gauge an outdated term, and says the number can't be grounds for rejecting sheet |
| ASTM A924 | Galvanized and other coated steel sheet | Thickness is given to 0.01 mm, and it's the steel and the coating together |
| Canadian Sheet Steel Building Institute | Sheet steel used in Canadian construction | The numbers aren't regulated, and design and ordering need a decimal thickness |
| CSA S136 | Design of cold-formed steel in Canada | Design thickness is the base steel only, and the delivered sheet can be as thin as 95% of it |
| National Master Specification, metal ducts | Ductwork on Canadian public projects | Duct thickness comes from SMACNA's tables for each duct's size and pressure, not from a gauge chart |
For ductwork, the thickness comes out of SMACNA's tables for the duct's size and pressure. That's covered in duct pressure class and gauge.
A thickness on a spec answers two questions. Is it the target or the floor, and does it count the zinc?
This is the target the mill rolls the sheet to, and CSA S136 calls it the design thickness. Delivered sheet lands a little above or below it. A spec that gives one figure with no other word usually means nominal.
This is the floor. CSA S136 lets 22 gauge steel with a 0.76 mm design thickness arrive at 0.72 mm, so 0.72 mm is its floor. If a spec says minimum, every sheet must reach that figure.
Base metal thickness leaves out the zinc, and coated thickness includes it. If your spec says "minimum base metal thickness", the coating doesn't count toward it.
Mills can also deliver galvanized sheet a little over or under. Under ASTM A924, sheet 0.58 to 1.09 mm thick and up to 813 mm wide can vary 0.10 mm either way. If a thinner sheet would cause a problem, write a minimum thickness.
Owner specs are the written requirements a building's owner issues with the drawings. Two published Canadian ones use different tables for galvanized, and the comparison below is an illustration, not a job.
| Spec or order | Table it reads gauge by | What 24 gauge comes to |
|---|---|---|
| A UBC spec that writes "0.71 mm thick (24 ga), Z275" | The galvanized table, which counts the zinc | About 0.70 mm, steel and zinc together |
| A City of Winnipeg spec that writes "0.76 mm (22 MSG)" | The steel table, MSG, which leaves the zinc out | About 0.61 mm of bare steel, before the zinc |
| An order that only says "24 gauge galvanized" | Whichever table the supplier happens to use | Either 0.61 or 0.70 mm, with no way to tell |
The two readings sit about 0.09 mm apart, roughly 15%. A mill's tolerance lets each sheet land a little over or under its figure. This gap is different, because every sheet on the order is built to the other figure. Both specs avoid the problem by writing the millimetres beside the gauge.
Each of these orders can be read as more than one thickness. Adding the metal and the millimetres leaves only one.
| What the order says | What goes wrong | What to write instead |
|---|---|---|
| "Heavy duty" | Every supplier pictures a different thickness | "1.52 mm (16 ga) mild steel" |
| "16 gauge" with no metal named | It could mean anything from 1.29 to 1.61 mm | "16 ga galvanized, 1.61 mm coated" |
| "16 gauge stainless" | Tables list it at 1.52 or 1.59 mm | "304 stainless, 1.59 mm (0.0625 in.)" |
| "16 gauge aluminum" | Aluminum suppliers list sheet in decimal inches, such as 0.050 in. | "Aluminum, 0.050 in. (1.27 mm)", a listed size close to 16 gauge's 1.29 mm |
| A gauge and a millimetre figure that disagree | Nobody knows which one to build to | Ask the engineer which governs, then write that one |
| "Same as the old one" | Nobody knows how thick the old part is | "Measured 1.2 mm on a clean edge," naming the metal |
If you measure an old galvanized part with calipers, the reading includes its zinc. Measure on a clean, flat edge, and say it was coated.
Most of the time the thickness isn't yours to pick. It comes from a standard or the designer's drawings, and knowing where to look saves a round of questions.
| Kind of work | What sets the thickness | Where to find it |
|---|---|---|
| HVAC ductwork | The engineer, through SMACNA's tables | The duct construction table or mechanical spec |
| Grease duct from a cooking hood | NFPA 96, the grease duct standard, at no thinner than No. 16 gauge mild steel or No. 18 stainless. Its own millimetre figures vary by edition, so ask the inspector which applies | The mechanical drawings and the local inspector |
| Flashing and roofing sheet | The architect's spec | The roofing or sheet metal section of the spec |
| Stainless counters and food-service tables | The kitchen designer or the owner's spec | The equipment schedule or the drawing |
| Brackets, covers, and equipment parts | The designer, based on load and stiffness | The part drawing, or ask us to suggest one |
Grease duct thickness is covered in full in grease duct specifications explained. Counter thickness and its effect on price are in what affects the cost of a custom stainless steel countertop.
Every order is read for its metal and thickness before any sheet is cut. When a gauge and a millimetre figure disagree, we ask instead of guessing.
Reading the metal with the gauge. We match each gauge to the right table for its metal, and check any millimetres written beside it.
Checking our ranges. We work mild steel about 0.30 to 1.90 mm, galvanized 0.40 to 1.99 mm, and aluminum 0.25 to 1.63 mm. That's 30 to 14 gauge, and stainless runs 26 to 14 gauge, about 0.48 to 1.98 mm.
Following SMACNA on ductwork. Every duct job follows SMACNA's construction standards, and where the engineer's spec asks for more, we build to the spec.
Asking about anything unclear. If a thickness is missing or unclear, we ask before pricing, or suggest one when you'd like us to.
Checking before release. Each piece is checked against the drawing before it leaves. If a piece is wrong and the mistake was ours, we fix or remake it at no charge.
Outside those ranges, we'll tell you before pricing whether we can take the work on. Our materials and finishes page lists the metals we work, and cutting and bending shows how the sheet becomes a part.
A gauge on its own works when everyone already reads it the same way. Anywhere that's in doubt, write the millimetres too.
If a thinner sheet will do the job and the spec allows it, we'll say so before you pay for heavier metal. We'd rather the order get smaller than have you pay for steel the job doesn't need.
Thicker sheet also costs more to buy and bend, and why stainless fabrication costs more shows where that money goes on stainless.
Tick these off, and the thickness on every part reads one way. The full list for a quote is in What Duct Duct Goose Needs to Quote Custom Metalwork. More metal choices are in Choosing Materials and Finishes.
A finished drawing with every thickness marked and a sketch that says "about as thick as the old one" are both fine places to start. We'll read the metal and the thickness, ask about anything unclear, and price the sheet you meant.
Standard ductwork and fittings price instantly on WebDuct, the online pricing and ordering portal, with a company account. Stainless, kitchen exhaust, production parts, installs, and other custom work start with a request or a call.
At Duct Duct Goose, we specialize in sheet metal fabrication and stainless steel services for commercial and industrial applications, including HVAC systems, ducts, and custom metalwork. Please note that we do not offer services for automotive-related projects, such as vehicle parts, repairs, or modifications.
