Bill of materials for an item
List the positions: rolled sections are picked from the EN catalogue, anything else is entered by dimensions. The bill gives the mass of each position 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 the positions together: catalogue sections come from the EN tables by kilograms per metre, non-standard positions 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 a position 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 positions — four from the catalogue and three 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.
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.
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