GI Sheet Gauge to mm Chart for Pakistan: Thickness, Weight and Zinc Coating

Karachi stockists’ published tables put 26 gauge GI sheet at 0.50 mm, 24 gauge at 0.60 mm, 22 gauge at 0.80 mm and 20 gauge at 1.00 mm. Under the American galvanized sheet gauge those same four numbers are 0.551, 0.701, 0.853 and 1.006 mm. Order by gauge against a counterparty working to a different chart and you can be short by up to 14 percent of the steel you thought you bought — 24 gauge is the worst case — while at 20 gauge the two charts agree to within 1 percent. The invoice looks correct either way.

Below is the conversion four ways, the weight per sheet, sheets and square feet per tonne, and the distinction that turns a thickness argument into a price argument: base metal thickness (BMT) versus total coated thickness (TCT). Throughout this page GP means galvanized plain — flat zinc-coated sheet — and GC means galvanized corrugated; IS 277:2003 uses the same GP and GC grade letters. On site both are chadar, and the gauge question is identical for both.

GI sheet gauge to mm chart: four conventions that disagree

There is no single correct gauge table. GalvInfo, the technical service of the International Zinc Association, is blunt: gauge numbers “were never regulated and did not refer to only one thickness, but instead to a range of thicknesses.”

GSG and MSG inch values are from Metal Supermarkets’ published chart; the millimetre values here are converted at 25.4 mm per inch and can differ by 0.001 mm from that chart’s own rounding. That chart lists no 30 gauge for bare mild steel. SWG is the old British Imperial Standard Wire Gauge, still quoted in some imported literature.

The Karachi column is Lokhandwala Group’s published GI sheet table, which runs from 32G at 0.25 mm to 10G at 3.00 mm. Tasleem Steel’s corrugated chart carries the same millimetre value on every gauge the two have in common — 30, 29, 28, 27, 26, 24, 23, 22, 20, 18, 16 and 14. Only 12G and 10G rest on Lokhandwala alone. Tasleem also lists 25G at 0.55 mm and 21G at 0.90 mm, which Lokhandwala does not carry.

Why the galvanized gauge runs thicker

The US Galvanized Sheet Gauge is the bare-steel Manufacturers’ Standard Gauge plus a fixed zinc allowance. GalvInfo records that the GSG decimal thicknesses “were equal to the steel sheet Manufacturers’ Standard Gauge (MSG), but with a 0.0037″ allowance for a relatively thick zinc coating of 1.25 oz/ft2 on each surface.” Comparing the two published charts confirms it gauge by gauge at +0.0037 or +0.0038 inch throughout.

That allowance no longer reflects what mills apply. At zinc’s density of 7,140 kg/m3, 0.0037 inch of zinc is roughly 670 g/m2 — about 2.2 oz/ft2 in total across both surfaces. A modern G90 / Z275 coating is 275 g/m2 total, about 0.0015 inch. So the galvanized gauge table carries roughly 2.4 times more coating than a standard G90 coil, on thickness and on mass alike. The heavy original coating survives in ASTM only as designation G210, which GalvInfo describes as being for “very specialized applications” and “generally not available on thinner gauge sheet.”

Where the Karachi numbers come from

Karachi gauge appears to be a trade label for a round metric thickness rather than a conversion from any published gauge standard. The numbers land on 0.30, 0.40, 0.50, 0.60, 0.80, 1.00, 1.20, 1.50 — values no gauge table produces. Against GSG the local figure runs 6 to 16 percent thin at 16G, 18G, 22G, 24G, 26G and 28G. Against SWG it runs the other way, 7 to 13 percent thick at 22G, 24G and 26G, but 8 percent thin at 16G. Only 20G and 14G land within 2 percent of GSG.

The common claim that Pakistan works to SWG is not supported by these two Karachi stockists’ own tables. Write the millimetre on the purchase order and treat the gauge number as description only.

Quick answers by gauge

What is 26 gauge GI sheet in mm?

0.50 mm on Karachi tables. 0.551 mm under US GSG, 0.457 mm under SWG.

What is 24 gauge GI sheet in mm?

0.60 mm on Karachi tables — the widest divergence on the chart, 14.4 percent below the GSG figure of 0.701 mm.

What is 22 gauge GI sheet in mm?

0.80 mm locally, against 0.853 mm GSG and 0.711 mm SWG.

What is 20 gauge GI sheet in mm?

1.00 mm locally, against 1.006 mm GSG — the one gauge where the local call and the American chart effectively agree.

What is 18 gauge GI sheet in mm?

1.20 mm locally, against 1.311 mm GSG and 1.219 mm SWG.

What is 16 gauge GI sheet in mm?

1.50 mm locally, against 1.613 mm GSG. Above 16G the local trade generally stops using gauge and quotes millimetres.

Per kilogram or per sheet: settle the price basis first

A gauge dispute only costs you steel if you are buying per piece. Buy per kilogram and a thin sheet does not short you on steel at all — it shorts you on covered area and on stiffness, because you get fewer square feet for the same tonne.

Published Pakistani price lists use more than one basis. Silver Steel Mills quotes roof sheet per running foot and per sheet; weight-per-sheet tables of the kind Lokhandwala publishes are what a per-kilogram rate has to be multiplied by. The conversion is the whole of it: rate per kg × calculated kg per sheet = rupees per sheet. Do that in both directions before comparing two quotes.

Two quotes can also differ on things that have nothing to do with steel: whether the rate is quoted inclusive or exclusive of sales tax, whether further tax applies to an unregistered buyer, whether it is ex-godown or delivered, and the payment terms. Ask for all four in writing, because they move the landed number more than a gauge argument does.

GI sheet weight calculation formula and sheets per tonne

Steel weight is volume times density, at 7.85 g/cm3. GalvInfo uses the same figure for sheet mass in SI units:

• Weight (kg) = Length (m) × Width (m) × Thickness (mm) × 7.85
• Equivalently, kg per square metre = thickness in mm × 7.85

The table is base steel only, at the thicknesses the Karachi trade calls each gauge. The 8 ft × 4 ft column is the size Lokhandwala publishes weights for; 10 ft × 3.5 ft is the roofing size Tasleem Steel and Silver Steel Mills list — but note that a corrugated sheet is billed on the flat blank, not the cover, so the 10 × 3.5 figure below is the flat area only. The formula above covers any other size.

Areas used: 8 × 3 ft = 2.2297 m2, 8 × 4 ft = 2.9729 m2, 10 × 3.5 ft = 3.2516 m2.

Against Lokhandwala’s own 8 × 4 ft weight column the formula agrees within 0.3 percent at 0.50, 1.00 and 2.00 mm. Six of the fifteen rows on that same chart — 0.25, 0.30, 0.60, 1.20, 1.50 and 3.00 mm — differ from the volume-times-density figure by 4 to 6 percent, in both directions. A published trade chart can disagree with its own stated thicknesses by more than the entire coating allowance, so do the arithmetic yourself.

Zinc coating: what it adds to weight and thickness

Coating is quoted as mass per unit area, not thickness, and the designation is always the total of both surfaces. IS 277 clause 7.1 puts it as “the total mass of zinc, both sides inclusive”. GalvInfo states the same for ASTM: “G90 requires a minimum average TST of 0.90 oz/ft2 total-both-sides”, and gives the conversion 1.00 oz/ft2 = 305 g/m2, so G90 = Z275. ArcelorMittal’s EN 10346 datasheet lists its coating weights as “double sided”.

Per-side figures for the EN 10346 classes are ArcelorMittal’s published values; for the ASTM-only classes they are calculated as half the total mass at a zinc density of 7,140 kg/m3, the same density GalvInfo uses for its conversion of 305 g/m2 to 0.0427 mm. The two methods agree within about a micron per side. Note that Z120 and Z180 are ASTM A653M metric designations and are not EN 10346 classes — order Z180 against EN 10346 and there is nothing to supply.

Worked example, one 8 ft × 4 ft sheet at 0.50 mm base metal with Z275: area 2.4384 m × 1.2192 m = 2.9729 m2. Steel = 2.9729 × 0.50 × 7.85 = 11.67 kg. Zinc = 2.9729 × 0.275 = 0.82 kg. Total 12.49 kg, the coating adding 7.0 percent.

One counter-intuitive point for anyone reconciling a coil against a weighbridge: zinc is less dense than steel, so a coated sheet weighs slightly less than solid steel of the same total thickness. GalvInfo’s formula is mass in kg/m2 = coated thickness in mm × 7.85 minus a coating factor, which for Z275 is 0.029 kg/m2. On thin gauges the zinc is a real fraction of what you are paying for: GalvInfo puts a 0.012 inch G90 coil at about 12.5 percent zinc by weight, against about 3 percent for 0.050 inch.

BMT or TCT: the difference that costs money

Read that same 0.50 mm the other way. If it is total coated thickness, the steel underneath is 0.50 − 0.039 = 0.461 mm and the steel content falls to 10.76 kg — about 8 percent less steel for the same nominal number.

The standards do not all answer this the same way. EN 10143:2006 clause 3.1 defines nominal thickness as the “whole order thickness of the metallically coated product,” noting it “includes both substrate and coating.” IS 277:2003 goes the other way: clause 13.1(c) lists “Thickness (uncoated sheets)” and clause 13.2.4 refers thickness tolerance back to the uncoated standards IS 1079 or IS 513. For the ASTM family, GalvInfo’s arithmetic works from the coated figure — its mass formula subtracts a coating factor from coated thickness times 7.85, and its charts are plotted against “Coated Sheet Thickness”.

The BMT, coating class and TCT columns are BlueScope Technical Bulletin TB-14, Table 3 (its ZINCALUME columns are omitted here); the difference column is calculated. BlueScope rounds up on the heavier classes — the zinc-density calculation gives 0.063 mm for Z450 and 0.084 mm for Z600 — so treat the last two rows as conservative.

BlueScope’s position is that coated steel should be specified “by nominating the base metal thickness (BMT),” because the base steel “provides structural load bearing capability, rigidity and other mechanical properties,” and it warns that “TCT quoted in isolation may lead to confusion and potential structural failure and must not be used in this manner.” It adds that measuring TCT is a valid check on whether the specified BMT was supplied, if you know the coating class.

On painted material the gap is wider, because the paint film is measured too: Tata Steel Colors gives 0.42 mm BMT = 0.47 mm TCT and 0.48 mm BMT = 0.53 mm TCT. Keep that in mind when comparing a colour-coated quote against plain GI, and see our comparison of PPGI, GI and Galvalume roofing sheets.

The operational rule on the shop floor is simple. A screw gauge on a galvanized sheet reads the coated thickness. Subtract the coating allowance for the class — 0.04 mm for Z275 — to get the base metal. None of the Pakistani stockist tables checked here states which basis its gauge numbers use, so put it on the order in words.

Which standard your material is actually written to

This is answerable, and it matters more than the gauge chart. In its sunset review of the anti-dumping duties on galvanized coil from China, Pakistan’s National Tariff Commission recorded that “customers in Pakistani market demand galvanized steel as per ASTM A653/653M and JIS G 3302 Standards whereas very few customers demand galvanized coils as per grades of EN 10346 standard specifications,” and that the two domestic producers, International Steels Limited and Aisha Steel Mills Limited, have produced and sold to the ASTM and JIS grades “because these two standards are mostly followed in the domestic market.”

That is consistent with the mills’ own published specifications. International Steels Limited gives its hot dipped galvanized range as 0.18 to 2.50 mm, maximum width 1,250 mm, coatings Z60 to Z275, “as per ASTM A653, JIS G3302 and equivalent standards”; its cold rolled range is 0.15 to 3.00 mm to ASTM A1008 and JIS G3141. See ISL GP coil for that range in context.

IS 277:2003 is an Indian standard with no force in Pakistan, and no local mill test certificate will be written to it. It appears on this page only because it is a public document that states the weight, coating and corrugation rules in plain language; where it is quoted below, treat it as a clearly written reference, not as the specification your supplier owes you.

Grade, spangle and surface treatment

Thickness and coating class are two of five things that decide whether a sheet behaves on your brake or under your powder coat. The other three are routinely left off local orders:

• Grade and formability. ISL publishes commercial grades CS Type A/B, SGCC and SGCH; drawing grades DDS, EDDS and SGCD1 to SGCD4; and structural grades including SS33 to SS80 and SGC340 to SGC570. The EN equivalents are the DX51D to DX57D family, which ArcelorMittal lists against EN 10346. SGCH is a hard grade and will not take the bend that SGCD will.
• Surface treatment. ISL applies a chromate rust inhibitor and offers the product skin passed or non-skin passed. Skin passing affects surface finish and stretcher strain; passivation affects how the sheet paints and how it survives storage.
• Spangle. Regular, minimised or zero spangle changes appearance and paint behaviour, and is an ordering choice, not something you can correct later.

Order 0.50 mm Z275 with none of that stated and you will get whatever is in the godown.

Coil width, sheet size and how corrugated is billed

Sheet sizes come out of coil widths. ISL lists standard widths of 914 mm, 1,000 mm and 1,219 mm with a 1,250 mm maximum — which is 3 ft, roughly 3.28 ft and 4 ft. That is why the market quotes 3 ft and 4 ft sheet and the mills quote millimetres, and why an odd width means trim scrap that someone pays for.

For corrugated there is a billing rule that catches people out. Area and mass are calculated on the width before corrugation — you pay for the flat blank, not the cover. IS 277:2003 Table 4, Note 3 states it directly: “For calculation of corrugated sheet mass, the width before corrugation considered while calculating the area.”

Tasleem Steel’s published weights show the rule in operation. Their 10 ft × 3.5 ft sheet at 0.50 mm is listed at 14.58 kg. Divide that by 3.925 kg/m2 and 3.048 m of length and the blank comes out at 1.219 m — exactly the 1,219 mm coil width, 14 percent more material than the 3.5 ft cover. Their 10 ft × 2.9 ft at 0.50 mm, 11.96 kg, implies a 1,000 mm blank, 13 percent more than the cover.

Tasleem’s 13 to 14 percent is a deeper loss than any Grade A row above, which means a different profile — reason enough to ask which profile you are being billed on before comparing two corrugated quotes. More on profiles on our corrugated steel page.

How to check what turned up

None of the arithmetic above helps unless the material is verified on arrival.

• Measure with a micrometer or screw gauge at several points across the width, away from the extreme edges, and average. Remember the reading is coated thickness; subtract the coating allowance to get base metal.
• Weigh and reconcile. Compare the weighed net against sheet count times calculated weight. If the numbers diverge by more than a percent or two, the thickness or the size is not what was ordered.
• Tie the mill test certificate to the material. An MTC belongs to a specific coil. Check the coil number on the certificate against the number on what was delivered, and check that the certificate states the coating class rather than accepting a verbal “275”.
• Look for storage stain. GalvInfo describes white rust as a corrosion stain that occurs “when sheets of galvanized steel that are in close contact (in a coil or stacked in lifts/bundles) get wet, either by direct water intrusion, or condensation between the surfaces.” It is a delivery and storage argument worth having at the gate, not after the sheet is on a roof.

Coating class and service life

Zinc protects steel two ways. It is a barrier, and where the steel is exposed “such as at a cut edge or scratch, the steel is cathodically protected by the sacrificial corrosion of the zinc coating” — which is why coating class, not steel thickness, sets how long a roof lasts.

GalvInfo’s service life chart, built from the American Galvanizers Association data, puts G90 (Z275) at about 20 years before 5 percent red rust in an average suburban environment. That is North American climate data and should not be read across to a Karachi rooftop unchanged, but GalvInfo’s own caution points the right way: corrosion rates in marine environments “are not so much changed” by falling air pollution, “since the rate of zinc loss is governed more by the amount of deposited sea salt”. Coastal exposure eats zinc, so specify the class upward near the sea rather than trimming it.

For reference, IS 277:2003 clause 7.3 recommends coating grade by thickness: 200 for 0.18 to 0.28 mm, 220 for 0.30 to 0.55 mm, 275 for 0.63 to 1.0 mm and 350 above 1.0 mm. ISL’s galvanized range runs Z60 to Z275.

Anti-dumping duty on galvanized coil from China

If you are importing rather than buying local, this is a line in the landed cost. The National Tariff Commission’s sunset review of the anti-dumping duties on galvanized steel coils/sheets imported from the People’s Republic of China, concluded 30 August 2022 (A.D.C. No. 37/2015/NTC/GCS/SSR-I/2021), defines the product under review as “galvanized steel coils/sheets coated with zinc of thickness ranging from 0.15 mm to 2.75 mm and of a width of 600 mm or more,” under PCT headings 7210.4110, 7210.4190, 7210.4990, 7212.3010, 7212.3090, 7225.9200 and 7226.9900.

The Commission decided to continue the duties for a further five years with effect from 8 February 2022, as ad valorem duties, at these rates:

The Commission also states that material below 0.15 mm and above 2.75 mm is outside the scope, that imports from sources other than China are not subject to these duties, and that the duties are in addition to other taxes and duties. Our note on anti-dumping duty on galvanized steel covers the trade remedy side, and local versus imported flat steel weighs the wider trade-off.

What to put on the purchase order

• Thickness in millimetres, not gauge — and state BMT or TCT.
• Coating class in g/m2 both sides (Z275) or the ASTM G-designation, and confirm it appears on the mill test certificate.
• Steel grade: SGCC, SGCH or SGCD under JIS G3302; CS, FS, DDS or EDDS under ASTM A653; DX51D to DX57D under EN 10346.
• Spangle and surface treatment: regular, minimised or zero spangle; chromate passivated, oiled, skin passed or not.
• The standard the mill test certificate will be written to — for locally produced coil, ASTM A653 or JIS G3302.
• Sheet or coil size in mm, and for corrugated, whether the width is before or after corrugation, and the profile.
• Basis of weight: whether weight per sheet includes zinc or is base steel only. Neither Karachi chart cited here states this, and both track the bare-steel calculation.
• Price basis: per kg or per sheet; sales tax inclusive or exclusive; further tax status; ex-godown or delivered; payment terms.

FAQ

Is 26 gauge GI sheet 0.50 mm or 0.55 mm?

On Karachi tables it is 0.50 mm — Lokhandwala Group and Tasleem Steel both publish 26G = 0.50 mm. Under the US Galvanized Sheet Gauge it is 0.551 mm; under British SWG, 0.457 mm. That is a 20 percent spread on one gauge number, so specify the millimetre.

Does a Pakistani gauge number mean base metal or total coated thickness?

None of the Pakistani stockist tables checked here states it either way, so do not assume. What a screw gauge reads is the coated thickness; subtract the coating allowance for the class — about 0.04 mm for Z275 — to get base metal. Have the supplier confirm the basis in writing on the order.

How do I calculate the weight of a GI sheet?

Weight in kg = length (m) × width (m) × thickness (mm) × 7.85. Add zinc separately: area (m2) × coating class (g/m2) ÷ 1000. An 8 × 4 ft sheet at 0.50 mm BMT with Z275 is 11.67 kg of steel plus 0.82 kg of zinc.

How many GI sheets are in one tonne?

At 8 ft × 4 ft, base steel only: about 86 sheets at 0.50 mm, 71 at 0.60 mm, 54 at 0.80 mm and 43 at 1.00 mm. In coverage that is roughly 2,742, 2,285, 1,714 and 1,371 square feet per tonne. Counting the coating reduces the sheet count by roughly the coating percentage — about 7 percent at Z275.

Why is my corrugated sheet billed on a wider size than it covers?

Because mass is calculated on the width before corrugation. On IS 277 Grade A a 1,000 mm blank gives 910 mm of cover, so you pay for about 10 percent more width than the roof shows; Tasleem Steel’s published weights imply 13 to 14 percent on their profiles. That is normal practice — but it must be stated, so that two quotes compare like with like.

Which standard should I ask for on the mill test certificate?

For coil produced in Pakistan, ASTM A653/A924 or JIS G3302 — the National Tariff Commission recorded these as the standards mostly followed in the domestic market, and International Steels Limited names both on its galvanized line. Ask for EN 10346 only if you have a specific reason, and check availability first.

Sources

• GalvInfoNote 1.1 Rev 4.0, December 2020, coating weight designations
• GalvInfoNote 1.6, selecting coating thickness and service life
• GalvInfoNote 1.10, useful galvanized sheet metrics and coating factors
• GalvInfoNote 3.1, how zinc protects steel
• GalvInfoNote 3.2, protecting galvanized sheet from storage stain
• ArcelorMittal datasheet E20, hot-dip galvanised steel, EN 10346 coating classes
• BlueScope Technical Bulletin TB-14, BMT and TCT
• EN 10143:2006, tolerances on hot-dip coated sheet and strip
• IS 277:2003, Galvanized Steel Sheets — Bureau of Indian Standards (Indian standard, cited as a public reference only)
• Tata Steel Colors, coated steel FAQs
• National Tariff Commission, conclusion of sunset review, galvanized steel coils/sheets from China, 30 August 2022
• International Steels Limited, product range
• Metal Supermarkets, sheet metal gauge chart
• Standard Wire Gauge reference values
• Lokhandwala Group, Karachi, GI sheet table
• Tasleem Steel, Karachi, corrugated weight chart
• Silver Steel Mills, Lahore, roof sheet sizes

Send your millimetre thickness, coating class, steel grade and sheet or coil size with any enquiry, so the specification is unambiguous from the start. Contact Raw Easy Corp for a current quotation.