Technical and procurement guide · 10 August 2026
Sheet Metal Gauge Chart: Gauge, Inches and Millimeters
Why the same gauge number can mean different thicknesses for steel, galvanized steel, stainless steel and aluminum.
Answer first
The short answer
Sheet-metal gauge is not one universal linear unit. A gauge number must be tied to a particular material and gauge convention before it can be converted to inches or millimeters. For international procurement, specifying the nominal thickness, tolerance, material, product standard and whether coating is included is safer than ordering only by gauge number.
Technical explanation
What controls the result
Gauge tables developed historically and are material-specific. The numerical sequence is not a formula that applies equally to every metal.
One inch is exactly 25.4 millimeters, but converting a published gauge thickness does not remove the need for product-standard tolerances.
For coated sheet, distinguish base-metal thickness, total coated thickness and coating mass. Contract language should state which value controls acceptance.
Decision table
How to write a sheet-thickness requirement
| Ambiguous request | Missing decision | Procurement-ready wording |
|---|---|---|
| 16 gauge sheet | Material and convention | State material plus nominal mm/in thickness and tolerance |
| 1.0 mm galvanized | Base or coated thickness | Define base-metal thickness and coating designation |
| 3 mm plate | Product classification and tolerance | Name sheet/plate standard, alloy/grade and condition |
| 4 × 8 sheet | Unit, exact size and edge tolerance | State width × length, units, tolerance and quantity |
A conversion chart is a communication aid, not the acceptance specification.
Selection workflow
From service to an auditable specification
Define the service environment, design life and failure consequence before comparing grades or forms.
Name the governing product standard, ordered edition, dimensions, condition and acceptance criteria.
Confirm fabrication, joining, surface treatment, inspection documents and traceability with the supplier.
Approve a representative sample or document package before releasing the full order.
Limitations
What this guide cannot decide alone
- A family name, trade description or nominal grade does not prove suitability for a specific environment.
- Typical values and comparisons are screening tools; the ordered standard and certified lot data control acceptance.
- Cross-standard designations are not automatically interchangeable without composition, properties and product-form review.
Procurement checklist
Make supplier offers comparable
Grade or alloy designation and governing product standard
Product form, dimensions, tolerances and ordered condition
Surface finish, coating or corrosion-protection requirement
Quantity, unit weight basis, packing and destination
Inspection document, tests, marking and traceability
Delivery date, Incoterms and third-party inspection needs
Evidence
Sources and review
Project specifications and the current governing editions supersede this general guide.
National Institute of Standards and Technology · Exact inch-to-metre definition
ASTM International · Current ordered edition controls
ASTM International · Current ordered edition controls
Attributable sourcing request
Turn gauge into an unambiguous RFQ
Send material, gauge convention or target thickness, tolerance, sheet size, quantity and destination.