A bill of materials is easiest to understand from a concrete example. This piece walks through a worked BOM, explains what each part of it is for, and then makes an honest point about templates: a spreadsheet template is fine for learning, but a real, working BOM belongs in a system.
A simple worked example
Take a simple product, a wooden stool. Its single-level bill of materials, the list of its immediate components, might read like this:
- Seat panel, quantity 1
- Leg, quantity 4
- Cross-brace, quantity 4
- Wood screw, quantity 12
- Wood glue, quantity 1 (a small measured amount)
- Protective foot pad, quantity 4
That list, with quantities, is the bill of materials for the stool. It says exactly what the stool is made of. From it, a manufacturer can plan: to build 50 stools, it needs 50 seat panels, 200 legs, 200 cross-braces, 600 screws, and so on. That multiplication is the BOM doing its job.
What the columns are for
A working BOM is more than a list of names and counts. The columns that matter, and why:
Component or part number. A unique identifier for each item, so there is no ambiguity about which part is meant. Names alone are not enough; two similar parts need distinct numbers.
Description. A human-readable name for the part, so the BOM is understandable to the people using it.
Quantity. How many of the part go into one unit of the product. This is the number planning multiplies.
Unit of measure. Whether the quantity is in pieces, metres, litres, kilograms. The glue in the example is measured, not counted, so its unit of measure matters.
Reference or position. Where the part goes, useful on the floor for assembly.
A BOM with these columns, accurately filled, is enough to plan, purchase, and build.
Single-level versus multi-level in the example
The stool BOM above is single-level: it lists the stool's immediate components directly. If the legs were not bought ready-made but were themselves manufactured, cut, shaped, and sanded from raw timber, then a leg would have its own BOM, and the stool would have a multi-level BOM: the stool is made of legs and other parts, and a leg is made of timber. Most real products are multi-level. The example is single-level only because it is deliberately simple.
The honest point about templates
It is easy to find a BOM template, a spreadsheet with the right columns, and for learning what a BOM is, that is genuinely useful. But for running a real manufacturing operation, a spreadsheet BOM has serious limits, and it is worth being honest about them.
A spreadsheet BOM is hard to keep accurate. When a product changes, someone has to remember to update the spreadsheet, and the version on the floor may not be the version that was updated. A spreadsheet BOM does not connect to anything: it does not feed MRP, it does not drive purchasing, it does not roll up cost automatically, all of that has to be redone by hand. A spreadsheet BOM has no real version control, so nobody is sure which revision is current. And a multi-level structure, sub-assemblies within sub-assemblies, is awkward to represent in a flat spreadsheet.
For a real operation, the BOM belongs in a manufacturing system, where it is structured, version-controlled, and connected: a change is made once, the current revision is unambiguous, and the BOM feeds planning, purchasing, production, and costing automatically. A template teaches the concept. A system runs the operation.
The takeaway
A bill of materials is, concretely, a list of components with quantities and units, and a worked example makes that plain. A template is a fine way to learn the structure. But a working BOM, the one a manufacturer plans and builds from, belongs in a system, because that is what keeps it accurate, current, and connected to everything that depends on it. For how we approach manufacturing systems, see our manufacturing work.