What a bill of materials is and what it holds

A bill of materials, or BOM, is the parts list for something you make. It names every part, sub-assembly and raw material that goes in, and how much of each.¹

Think of it as the recipe card for a product. A cook's card says two eggs. A BOM says two brackets, one fan, eight screws.

A sub-assembly is a smaller unit you build first, then fit into the bigger product. A raw material is stuff you cut, mix or shape, like sheet steel or resin.

What one BOM record holds

Software stores a BOM as a record. Different tools label the fields differently, but the maker's manual for one ERP system shows the usual shape.²

• The item being made. The product or sub-assembly this list builds.²
• The quantity it makes. You enter how many of the item this list produces.²
• The component lines. Each line is a part, raw material or sub-assembly, with the amount needed.²
• A warehouse per line. You pick a source warehouse so stock can be tracked.²
• Operations, if you want them. A BOM may also contain the manufacturing operations needed to make the item.²

That's the core. A product name, a count, and a list of parts with amounts.

Parts go inside parts

BOMs are hierarchical, meaning they work in levels.¹ The top is the finished product. Below it sit the sub-assemblies and parts. A sub-assembly can have its own parts underneath, and so on.

That's why the same part can show up on several lists. A bolt might sit in a frame sub-assembly and also in the final fit-out. Each list only needs to say what goes into that one build.

The next page, Single-level and multi-level BOMs, shows how the levels are laid out on screen.

One product, more than one list

Not every BOM answers the same question. A BOM can define a product as it's designed, as it's ordered, as it's built, or as it's maintained.¹

So a designer's list and a shop-floor list can differ for the same product. Know which one you're looking at before you order parts from it.

Why the amounts matter

A BOM is not just paperwork. In a system that links lists to stock, the quantities you type feed other things. The ERPNext manual says you must always maintain correct BOMs to make accurate purchase requests.²

A wrong count on a line means a wrong order later. Count carefully when you set the list up. Check a fresh record against the real product on your bench.

Fields you may meet that aren't in the core

Some systems add extras to a record. Examples from the same manual include an "Is Default" option, so a list gets picked by default in work orders, and an "Is Active" option for switching a list off.²

You don't need these on day one. A small shop can start with item, quantity and component lines, then add fields when a real problem calls for them.

If you're comparing tools, check the maker's own help pages for the exact field names. They vary, and so do the options.