A drawing that says “18 ga” instead of “1.2 mm” looks precise. It is not. Gauge is an index number from a particular standard, and the same number means different thicknesses depending on the material and the standard being used. Two factories can read that line differently and quote two different parts.
A sheet metal gauge chart is useful as a translation aid, but it is not a substitute for a dimensioned drawing. This guide shows the approximate conversion most buyers need, explains why aluminium and stainless steel do not follow the same scale, and sets out what to write in an RFQ so every supplier prices the same part.
Why a gauge number is not a thickness
Gauge systems were built around wire and sheet manufacturing, not around modern CAD. A lower gauge number means thicker material, but the step between numbers is not uniform. On top of that, the US Standard scale used for carbon steel is not the scale used for aluminium, and stainless steel has its own convention again. A note that says only “16 ga” leaves the supplier to choose the scale.
That choice affects more than weight. Bend radius, weld settings, laser cutting parameters, forming tonnage and the flat pattern all follow from actual thickness. If two shops assume different thicknesses, the parts may not fit the same assembly even though both are “correct” against their own reading of the drawing.
Common steel gauge to mm conversion (approximate)
The table below uses the US Standard scale for carbon steel. Treat it as a translation aid for early conversations, not as a specification. Always confirm the actual thickness against the mill certificate for the batch you are buying.
| US Standard gauge (steel) | Approx. mm | Approx. inch | Typical use |
|---|---|---|---|
| 10 ga | 3.42 | 0.1345 | Heavy brackets, structural plates |
| 12 ga | 2.66 | 0.1046 | Load-bearing supports |
| 14 ga | 1.90 | 0.0747 | Frames, machine guards |
| 16 ga | 1.52 | 0.0598 | Enclosures, panels |
| 18 ga | 1.21 | 0.0478 | Cabinets, general sheet work |
| 20 ga | 0.91 | 0.0359 | Light covers, electronics housings |
| 22 ga | 0.76 | 0.0299 | Thin panels, shields |
| 24 ga | 0.61 | 0.0239 | Light cosmetic parts |
Aluminium and stainless steel follow different scales. If your part is aluminium, write the thickness in millimetres and name the alloy, rather than quoting a gauge number at all.
What to put on the drawing instead
State the thickness in millimetres, name the material grade and condition, and add the applicable general tolerance note. Keep the gauge number only as a reference in a note if your engineering team uses it internally. A drawing written this way survives translation between a US engineer, a Chinese fabricator and a quote comparison spreadsheet without anyone having to guess.
The same logic applies to tolerances: a general note covers most features, and only the functional dimensions need tight values. See ISO 2768 and drawing notes for how those lines change a quote. If the part could be made by more than one process, settle that first with CNC vs stamping vs sheet metal.
How gauge changes the quote
Thicker sheet needs more forming force, slower cutting and more robust packaging. It also changes the minimum bend radius: a radius that forms cleanly in 20 ga may crack in 14 ga. If a supplier quotes a thicker interpretation of “18 ga” than you intended, the price difference is not a margin problem — it is a different part. Ask for the assumed thickness on the quotation itself, next to the material grade.
Ask for an itemised quote with material, cutting, forming, finishing and inspection on separate lines. The CNC quote cost breakdown shows the lines a comparable quote should contain, and the same discipline applies to sheet metal. Our custom metal parts sourcing guide explains how we compare them on landed cost rather than unit price.
Tolerances, material certificates and inspection
A thickness callout is only useful if the delivered material matches it. Ask for the mill certificate, and write the inspection requirement into the purchase order: what is measured, against which dimension, and who signs off on the first article. A supplier that can show calibration records for its measuring equipment is giving you evidence a thickness note cannot. That verification sequence is in how to verify a Chinese machining supplier and the buyer RFQ checklist.
Five mistakes we see on sheet metal RFQs
- Writing only a gauge number, with no material named and no millimetre equivalent.
- Assuming an aluminium part follows the steel scale.
- Leaving the bend radius off the drawing and discovering cracks during forming.
- Specifying a finish on the whole part when only one face is visible.
- Comparing quotes that assumed different thicknesses, then choosing the cheapest.
Frequently asked questions
Is 18 gauge the same thickness for steel and aluminium?
No. Gauge scales differ by material. A gauge number that maps to roughly 1.2 mm in carbon steel will not map to the same thickness in aluminium. Write the thickness in millimetres and name the grade to remove the ambiguity.
Should I use gauge or millimetres on a drawing for a Chinese factory?
Use millimetres as the controlling dimension. If your team works in gauge, keep it as a secondary note. Metric dimensions travel cleanly through CAD, CAM and inspection reports without a conversion step.
Why did two suppliers quote different prices for the same gauge?
Usually because they assumed different actual thicknesses, different materials or different finishing scope. Ask each to state the assumed millimetre thickness and material grade on the quotation, then compare the itemised lines rather than the totals.
Do I need a material certificate for sheet metal parts?
For structural, regulated or corrosion-exposed parts, yes — and ask for it before production, not at the port. For simple cosmetic panels it may be unnecessary. Decide at the RFQ stage, because it affects which mill the supplier buys from.
What to do next
Use the table above as a rough check, then rewrite the thickness callout on your drawing in millimetres before you send anything. Our CNC machining RFQ checklist lists the other files and specifications a shop needs. If you want a second pair of eyes on a drawing, send it over and we will review it before you commit to a supplier.