Bill of materials for an item
List the items: rolled sections are picked from the EN catalogue, anything else is entered by dimensions. The bill gives the mass of each item and the total — without allowance, with the cutting allowance, and in money if you enter a steel price. The table downloads and goes with the order.
Short answer
The weight calculator works out one section, but the order covers the whole item. The bill gathers all the items together: catalogue sections come from the EN tables by kilograms per metre, non-standard items are worked out from the cross-section, a cutting allowance is added and the total is multiplied by the price. The welding side — filler, time and gas — comes separately from the weld schedule.
Rolled sections from the catalogue
| No. | Section | Length, mm | Quantity |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 | |||
| 4 |
By dimensions
| No. | Shape | a, mm | b, mm | Wall, mm | Length, mm | Quantity |
|---|---|---|---|---|---|---|
| 5 | ||||||
| 6 | ||||||
| 7 |
A hollow section takes a, b and the wall; a round tube takes the outside diameter in a and the wall; sheet and flat bar take the thickness in a and the width in b; a round bar takes the diameter in a.
Where the masses come from
Kilograms per metre for beams, channels and angles come from the same EN catalogue that prints the section weight table on this site: 136 entries, with no second copy of those numbers. Everything the catalogue does not hold — hollow section, round tube, sheet, flat bar, round bar — is worked out from the cross-sectional area by the same function as the metal weight calculator.
So the mass of an item here will agree with what the separate calculator on the neighbouring page gives: there is nothing for them to drift apart on.
The welding side: filler, arc time and gas are worked out in the weld schedule, and repeating them here would mean two answers to one question. The real offcut: the allowance is a flat percentage, and how much is actually lost is shown by laying the parts out on stock bars — a separate calculation. And labour: the bill covers material, not work.
How to use it
There are seven rows — four for catalogue sections and three for items by dimensions. That covers an assembly; a whole item is usually broken into assemblies anyway, because that is how it is built and how it is inspected.
The 5 % default suits straightforward cutting to length. Where the parts are short and come out of six-metre bars, the real offcut is often twice that, and it is better to lay the parts out on the bars than to guess a percentage.
Worked example: a platform frame
A frame of two beams, cross members, bracing, posts and base plates. Input:
| No. | Item | Length, mm | Qty | Mass from |
|---|---|---|---|---|
| 1 | IPE 200 | 3000 | 2 | catalogue: 22.4 kg/m |
| 2 | UPN 120 | 1500 | 3 | catalogue: 13.4 kg/m |
| 3 | L 50×50×5 | 800 | 4 | catalogue: 3.77 kg/m |
| 4 | box section 80×80×4 | 1200 | 4 | area 1216 mm² |
| 5 | plate 10×200 | 200 | 4 | area 2000 mm² |
The bill returns: IPE — 67.20 kg each and 134.40 kg in total; UPN — 20.10 and 60.30 kg; angles — 3.02 and 12.06 kg; box section — 11.45 and 45.82 kg; plates — 3.14 and 12.56 kg. Net total 265.14 kg, with a 5 % allowance 278.40 kg. Enter a price per kilogram and the bill multiplies it by the gross mass.
Where items 4 and 5 come from. Box section: 80·80 − 72·72 = 1216 mm² (corners taken as sharp — real corner radii reduce the area by a few per cent, so the mass errs slightly high). Plate: 10 · 200 = 2000 mm². Mass is area × length × 7.85 g/cm³. Catalogue beams take their mass from the table, because an I-section area cannot be worked out from its overall dimensions.
How to read net, allowance and gross
Net is the mass of the parts after cutting — what the frame weighs without welds. Allowance covers saw kerf, end trimming and offcuts that cannot be used. Gross is what you have to buy. The percentage is an approximation, and in this example it is far too low: two 3000 mm beams will not fit into one 6000 mm bar, because each part carries a 3 mm kerf (the cutting plan counts it for squaring the bar end too) — you need two bars, each leaving 2997 mm. Four 800 mm angles take 3212 mm of a bar and leave 2788 mm. If those offcuts do not go into another job, they are scrap, and you pay for them. The real waste for each section is shown by the bar cutting plan.
From the bill to the stockholder order
- Group items by section and steel grade — the stockholder sells bars, not parts.
- For each section count the bars in the cutting plan (stock length, usually 6 or 12 m, and saw kerf).
- Add the grade and the inspection document the drawing calls for (e.g. a 3.1 certificate to EN 10204) — mass alone is not enough to order.
- Compare the invoiced mass with the gross from the bill: a few per cent is rolling tolerance, more is a sign that a different section or length arrived.
Check the mass of a single section in the steel section weight table or the metal weight calculator; the welding side of the estimate — filler, time and gas — is in the weld schedule.
Common mistakes in a bill of materials
- Length in metres. The field takes millimetres: “3” instead of “3000” gives a 67 g beam.
- Plate dimensions swapped with length. For plate, a and b are thickness and width, length is separate. Swapping width and length changes nothing, swapping thickness and width does.
- Box section entered as a round tube. A Ø 80×4 tube and an 80×80×4 box differ in mass by about 20 %.
- One allowance for everything. Long beams cut at half a bar and short angles give completely different waste.
- No steel grade. S235 and S355 weigh the same, but they are different lines on the invoice and in the acceptance documents.
Frequently asked questions
Why are rolled sections picked from a list rather than entered by size?
Because the mass of a beam does not follow from its overall dimensions: it has root radii where the web meets the flange, and a “sum of rectangles” calculation is out by several per cent. The catalogue carries the nominal mass from the standard, and that is what is used.
Will the mass match the stockholder’s invoice?
Not necessarily. The bill carries the nominal mass to the standard, while the stockholder invoices against the certificate or the weighed batch; rolling tolerances usually put the two 1–4 % apart. Nominal is enough for an order and a quotation, not for checking an invoice.
What allowance should I use?
It depends on how you cut. Cutting to length from long bars, 3–5 % is enough. Where there are many short parts, the offcut is decided not by a percentage but by how they fall on a six-metre bar — and then a percentage only papers over not knowing.
Why does the cost cover material only?
Because the other half of an estimate is the welding, and it is already worked out in the weld schedule: filler, arc time and gas. They belong together in one estimate but should be calculated by two different tools; otherwise the same figure starts being produced two ways and one day the two disagree.
How do I count a plate with holes and cut-outs?
For ordering — as the full rectangle: that is how much plate you buy and how much goes under the torch or laser; the cut-outs are scrap. For the mass of the finished item (for transport or design checks) subtract the hole areas: a Ø 22 hole in 10 mm plate is about 0.03 kg, so a few holes make no difference, large cut-outs do.
Should welds be added to the structure mass?
Yes, if you want the finished item mass — but not in this bill. Weld metal is calculated by the weld schedule; add it to the net steel mass. Compared with the steel it is usually small, but in the estimate filler and gas are separate cost lines.
How do I turn kilograms into a number of bars to order?
Kilograms do not tell you how many bars to buy: two 3.5 m beams are two six-metre bars, not “7 m”. Put the lengths of one section’s parts into the cutting plan — it counts the bars and the real waste, and you can then correct the allowance percentage here.
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