Steel is bought by the tonne and used by the square metre or the metre, so every estimating job starts with converting one into the other. Get it wrong and it costs: a roof five percent short means a second shipment and a splice nobody wanted, while a heavy order ties up money in offcuts. The arithmetic is simple — one density constant and a handful of section formulas — but it has to be applied with real product dimensions and tolerances, not nominal ones. This article works through the conversion product family by family, using the specifications on our product pages.

The Base Arithmetic: Area per Tonne

Carbon steel has a density of about 7.85 t/m³, so one square metre of one-millimetre sheet weighs 7.85 kg, and one tonne buys roughly 127 square metres of 1.0 mm sheet. The general rule is: square metres per tonne = 127 divided by the thickness in millimetres. Half-millimetre sheet covers about 255 m² per tonne; 2 mm plate about 64 m²; 6 mm plate about 21 m². The plate thickness classification on our hot rolled versus cold rolled plate page — thin plate under 4 mm down to 0.2 mm, medium plate 4-60 mm, extra-thick plate 60-115 mm — spans a coverage range from over 600 m² per tonne at the thinnest to barely 12 m² per tonne at the heaviest. Coatings barely move the number: even a heavy zinc layer is tens of microns, a fraction of a percent of the steel mass. Structural grades do not change it either — a tonne of Q235 plate covers the same area as a tonne of Q345, worth remembering when a designer upgrades a grade without repricing.

Gauge Numbers and Tolerance Bands

Estimates fail most often at the thickness input. Gauge numbers are not millimetres — a 26 gauge galvanized coil and a 0.4 mm galvanized coil are close in gauge-speak but only one states a metric thickness — so every take-off should be done in millimetres. Then comes tolerance. The Z275 galvanized coil specification carries a thickness tolerance of 0.01 mm and a width tolerance of 2 mm; stainless and H-section material is commonly held to one percent. A worst-case combination can move an order by a couple of percent — real money at 25-tonne scale. Invoicing absorbs some of this: sections are commonly invoiced by theoretical weight, as stated on the ASTM A36 universal beam page, so estimate and invoice agree by construction. But the hot dip galvanized H-beam page invoices by actual weight — the zinc layer adds mass the theoretical figure does not know about.

Coils: Length, Coverage and the Galvalume Bonus

For coil-fed work the useful question is not area but length: divide the coil weight by the weight per square metre, then by the width. A 4-tonne coil of 1.0 mm material at 1250 mm width holds about 410 running metres — enough to plan decoiler changes around. Coil weights sit in published bands: 3-5 t on the Z275 coil, 3-8 t on lighter hot rolled Q195 coil stock, 3-14 t on DC01 cold rolled sheet, and 3-18 t across the wider hot rolled coil family. One correction improves galvalume estimates: the coating on 55% aluminium-zinc is about 3.75 g/cm³ against 7.15 for pure zinc, so at equal coating mass an aluzinc roofing sheet yields about three percent more area per tonne — the fact the galvalume explainer uses when comparing substrates.

Sections: Theoretical Weight from Dimensions

Sections are estimated from their cross-section. For an H-beam, weight per metre in kg = 0.00785 × (2 × flange width × flange thickness + web depth × web thickness), all in millimetres. Take a light 150-deep section with 9 mm flanges and a 6 mm web: 0.00785 × (2 × 150 × 9 + 138 × 6) gives roughly 27.5 kg/m, so a 12-metre length weighs about 330 kg and a tonne buys just three lengths. Beam pages run from the compact 50x150 mild steel H-beam to sections with 400 mm flanges and 900 mm webs on the steel beam category, in 6-12 m lengths with a 1-tonne MOQ. Because invoicing is theoretical, estimator and mill use the same table — but tolerance is still one percent, so leave margin in fabrication drawings.

Tube, Pipe and Rope: Length Is the Product

Hollow sections convert the other way: the buyer needs metres and the tonnage follows. For round tube, weight per metre in kg = 0.02466 × wall thickness × (outer diameter − wall thickness). A 60.3 mm tube with a 3 mm wall weighs about 4.2 kg/m, so a tonne is roughly 235 running metres. The range is enormous: our square stainless pipe runs from 10x10 to 100x100 mm, while the large-diameter thick-wall stainless pipe page covers diameters to 2500 mm with walls to 150 mm, where a metre can top a hundred kilos. Carbon pipe follows schedules rather than decimals — the ASTM A53 pipe page lists 1/2" to 36" nominal sizes in SCH 40, 80 and 160 walls, delivered in plain, beveled or threaded ends. Wire rope inverts the logic: it is selected by diameter and breaking force, not converted from tonnage. The custom stainless wire rope page lists constructions from 1x7 to 6x37 in diameters of 0.8-21.4 mm, and the rope net table pairs mesh sizes with breaking forces from 2.17 to 7.38 kN.

When the Metal Is Not Carbon Steel

Density corrections matter the moment the alloy changes. Austenitic and duplex stainless weighs about 7.93 g/cm³ against 7.85 for carbon steel — a one percent increase, visible across a full duplex stainless coil order. Titanium is the dramatic case: at 4.51 g/cm³ it is only 57 percent of the weight of steel, so a tonne of titanium round bar is nearly twice the length of a tonne of steel bar — one reason titanium is priced in kilograms, with a 100 kg MOQ on the rod page, rather than by the tonne. The titanium rod category carries the density and grade data. None of this changes the formulas, only the constant — and the right constant avoids ordering light on a dense alloy.

From Estimate to Order

The last step turns the number into a real order. Add processing losses: nesting rarely beats a few percent on simple rectangles, and the shear and cutting services quote from the gross plate, offcuts included — the custom processing menu lists the full range. Check the commercial minimums: stainless ships from 1 tonne, most galvanized coil from 5 tonnes, DC01 from 25 tonnes, and 25 tonnes is exactly one 20-foot container, making freight and MOQ the same conversation. Finally, sanity-check the estimate against product reality — a galvanized steel plate order and a 304/304L stainless coil order at the same coverage need different thicknesses once finish and forming are counted, and the 430 strip width list will not match the carbon widths. Ten minutes with the real specifications on each product page — thickness bands, coil weights, tolerances, invoicing basis — separates an estimate that survives the mill from one that dies at the first invoice.