Sheet metal thickness and gauge conversion

At a glance

A higher gauge number is thinner. mm = inches x 25.4, to 0.01 mm. SWG and BWG: published tables (secondary). US Standard: the US statute of 1893 (primary). AWG: its defining formula. The tables disagree, so never write a gauge without millimetres.

Specify millimetres, and why

A gauge number is not a thickness until you know which table it comes from. The figure below shows the same number, 16, in five tables: the thickest is 28 per cent thicker than the thinnest. The other gauge numbers disagree in the same way, by different amounts.

• No current UK standard defines gauge numbers. The BSI listing for BS 3737:1964, “Guide to series of basic sizes for metal sheet, strip and wire”, shows it as withdrawn, and BS 6722:1986, “Recommendations for dimensions of metallic materials”, as withdrawn on 31 October 2014. Some references say BS 3737 defines the SWG; we could not confirm that from the listing.
• The gauge numbers still turn up. A UK brass stockholder's listing gives sheet and tube sizes in swg as well as millimetres. A drawing that says “16 gauge” invites each supplier to read its own table.
• Write the millimetre value and its tolerance. Sheet is never exactly nominal. The permitted deviation comes from the product standard the sheet is bought to: for example EN 10051 for hot-rolled continuous strip and EN 10029 for hot-rolled plate, which we name from our research and have not read. Ask the partner which standard applies to the sheet they buy.
• Say whether the thickness is before or after finishing. Grinding and polishing remove metal: Euro Inox warns that too coarse a grit can leave the metal too thin.

Which table is which

The five systems you are likely to meet, and what we could read of each. The conversion table at the top of this sheet uses four of them; the Manufacturers' Standard figures appear only in the chart.

Where the tables nearly agree they still do not match. In the table, SWG and BWG give the same thickness only at 22 and 24, and differ at 10, 12, 14, 16, 18 and 20. A weight printed beside a gauge also follows the density that table assumes: the statute's 16 gauge row gives 12.21 kg per square metre at 1.5875 mm, which implies about 7.69 g/cm³ (our calculation), against the 7.85 usually used for steel. Take weights from the real metal and the real millimetre thickness, not from a gauge chart.

Going the other way: millimetres to gauge

Most common metric thicknesses sit within about 3 per cent of an SWG number, which is why people think the two systems line up. Two sizes that designers use all the time do not: 1.5 mm and 2.5 mm have no SWG equivalent. 1.5 mm lies between 17 swg and 16 swg, and calling it “16 gauge” is 8 per cent out in the UK table. The table is computed from the SWG figures above.

Weight per square metre

The mass of a sheet in kilograms per square metre is its thickness in millimetres multiplied by the metal's density in grams per cubic centimetre. That is worth knowing for fixings, lift loads and delivery.

These are nominal densities. Stainless values are from the Euro Inox tables of technical properties (2007), which follow EN 10088-1:2005: they differ by grade, so 304 and 316 do not weigh the same. Steel and aluminium are the usual round figures, and brass is the value one stockholder prints for CW508L. Allow for the thickness tolerance, and use the mill certificate or the partner's datasheet for a weight that matters.

What decides the thickness for each use

We give no recommended thickness for any use, because we found no authoritative source that does: the answer depends on the panel size, fixings, finish, how flat it must be and what carries it. What a source can tell you is what to ask. The partner workshop should supply the thickness and construction they have proven for your size, and a mock-up in the real light should prove it.

For a sense of range, Euro Inox's guide to stainless steel facades (2nd ed. 2002) records the thicknesses below in built examples. They are exterior cladding and 20 years old, so treat them as evidence that these thicknesses are used, not as a recommendation for an interior.

Flatness and oil canning

The usual way a thin panel disappoints is not strength but looks: a flat sheet shows waves in reflected light, called oil canning. The Metal Construction Association describes it as visible waviness in the flat areas of metal roof and wall panels, technically elastic buckling or stress wrinkling, and says many uncontrollable factors contribute, though the tendency can be reduced by attention to production, material selection, panel design and installation practice. It is written for exterior panels; the mechanism is the same indoors.

• Thickness helps, but so does shape. Euro Inox notes that pattern-rolled sheet is stiffer and can be thinner, and that folds in a panel add rigidity.
• A core can carry the flatness. A thin face bonded or wrapped over a stable core stays flat; our sheet metal fabrication journal describes brass worktops made this way.
• Gloss shows it. Euro Inox notes that reflective polished finishes are sometimes inappropriate where it would be difficult to ensure the necessary optical flatness, and that brushed or satin finishes are often preferred then.
• Mill finish matters. Euro Inox's finish table notes that skin passing of 2B sheet may be complemented by tension levelling. Ask the partner whether the sheet is levelled, and approve a mock-up in raking light, which shows ripples face-on lighting hides.
• Directional and patterned finishes are covered by the sheet on stainless finish codes. Where flatness is critical, say how it will be judged and in what light.

Sheet thickness [1.5] mm nominal, stated in millimetres on all drawings and schedules, supplied to the thickness tolerance of [product standard, edition to be confirmed]. Finished thickness after grinding and polishing not less than [1.4] mm. Gauge numbers are not to be used. Where a supplier quotes a gauge, the gauge table and the millimetre value are to be stated in writing.

Panels [__ x __ mm] are to be flat within [ ] mm over [ ] mm, judged [as installed] under [raking light] against approved mock-up [ref. __]. Stiffening by [folded return edges / bonded core] as drawn. The partner is to state the largest panel size proven at the stated thickness.

The weight per square metre of [panel / sheet] and the total weight of each assembly are to be stated on the shop drawing for the fixing design. Weights are to come from the actual metal and finished thickness, not a gauge table.

Standards and sources

• 15 U.S.C. section 206, standard gauge for sheet and plate iron and steel (US statute, 3 March 1893): the full table with decimal inches and mm was read.
• BS 3737:1964 (sheet, strip and wire sizes) and BS 6722:1986 (dimensions of metallic materials): BSI listings: both withdrawn (BS 6722 on 31 October 2014); texts not read.
• Euro Inox: Building Series Vol. 1, Guide to Stainless Steel Finishes (3rd ed. 2005); Vol. 2, Stainless Steel Facades (2nd ed. 2002); Vol. 5, Tables of Technical Properties (2nd ed. 2007, density by grade)
• Metal Construction Association, oil canning in metal roof and wall systems; and thickness tolerance standards such as EN 10051 and EN 10029 (named, not read)

Questions we are asked

How thick is 16 gauge in mm?

It depends on the table. In the UK gauges, 16 swg is 1.63 mm and 16 BWG is 1.65 mm. In the US Standard gauge set by statute it is 1.59 mm, in the US manufacturers' table for sheet steel about 1.52 mm, and 16 AWG, the US non-ferrous wire gauge, is 1.29 mm. Ask for the thickness in millimetres.

What is 1.5 mm in gauge?

There is no exact match in the British tables: 1.5 mm lies between 17 swg (1.42 mm) and 16 swg (1.63 mm). US 16 gauge sheet steel is about 1.52 mm in a secondary table, which is probably why 1.5 mm is often called 16 gauge (the studio's reading). Specify 1.5 mm and leave the gauge out.

What is the difference between SWG and BWG?

Both are old British systems where a higher number is thinner, but they give different thicknesses for most numbers: 10 is 3.25 mm in SWG and 3.40 mm in BWG, and 16 is 1.63 mm against 1.65 mm. In the table above they agree only at 22 and 24. SWG is met with wire and sheet; BWG is mostly met as tube wall thickness.

What thickness brass sheet do I need for a table top?

There is no single figure. It depends on whether the top is solid sheet or sheet wrapped over a core, on the span and on the edge detail. Our sheet metal journal notes that brass and bronze worktops are often made by wrapping the sheet over a moisture-resistant core, which supplies the rigidity. Ask the partner for the thickness and construction they have proven at your size, and state the finished thickness after polishing.

Why does my metal panel look wavy?

Probably oil canning: visible waviness in a flat panel, caused by elastic buckling. Thin sheet and wide panels make it more likely, and Euro Inox notes that reflective polished finishes can be inappropriate where optical flatness is hard to ensure. Stiffening by folds or pattern, a thicker sheet or a bonded core reduces it. Ask for a mock-up in raking light before production.

How do I work out the weight of a sheet?

Multiply the thickness in millimetres by the density in grams per cubic centimetre to get kilograms per square metre. For example, 2 mm of 304 stainless (7.9) is 15.8 kg per square metre. Use the real metal and finished thickness, not a gauge chart, and allow for the thickness tolerance.