What is a BOM? Bill of materials explained
A bill of materials (BOM) lists the components and materials needed to make a product, with their identities and quantities. For factory production, agree which product version it covers and which revision is approved before purchasing or assembly starts. Cambridge China Bridge helps buyers coordinate these details with factories through its staff in China.
What a BOM tells the factory
A BOM connects a finished product to the parts, materials and subassemblies used to make it. It tells purchasing what to obtain and production what to issue for assembly. A quotation describes the commercial offer; drawings describe shape and construction; the BOM identifies what goes into the product.
Start the document with the finished-product identity, variant, BOM revision, approval status and relevant drawing references. Include packaging and supplied accessories, or clearly reference a separate approved packing specification. Make exclusions explicit so the factory does not fill gaps by assumption.
Give each component an unambiguous identity
Use a stable buyer part reference alongside the factory reference. For bought-in components, record the manufacturer and exact manufacturer part number. For custom parts, reference the drawing and its revision, material grade, finish and relevant specification. A description such as 'plastic housing' cannot distinguish approved material from a cheaper substitute.
Keep photographs and approved samples as supporting references rather than the sole identity. Record whether components must be new and which alternatives, if any, are approved. The computer parts guide covers compatibility and component-condition checks.
State quantities and the purchasing basis
Record each component's quantity and unit of measure, and say whether the quantity applies per finished product, per subassembly or per batch. For materials consumed by weight, length or volume, use the corresponding unit. Keep production usage separate from purchasing allowances for scrap, samples and spares.
Calculate planned component demand from the agreed production quantity and BOM usage, then show allowances separately. Deduct usable stock only after checking its identity and revision. Identify whether the buyer or factory purchases each item using the component-purchasing guide. Agree how buyer-supplied parts will be received, counted and reconciled.
Approve revisions and define the changeover
Nominate who maintains the BOM and who can approve changes. Release an approved copy for the order and reference that revision in the purchase order. Keep earlier versions available as records, while removing them from purchasing and production instructions.
For each proposed change, record the affected component, previous and proposed specification, reason, approval and production changeover point. Decide what happens to stock already bought and goods already assembled. Check whether drawings, samples or testing need updating before release. Use the design change-control guide for the approval workflow and the substitution guide for replacement materials or components.
Check the released BOM against production
Before purchasing starts, ask the factory to confirm that its purchasing list matches the approved BOM and flag unavailable or ambiguous items. Before assembly, compare received component identities and revisions with that same document. Where a subassembly has its own BOM, check that the parent document references the approved subassembly version.
At inspection, compare accessible markings, material records and assembly details with the released BOM and approved sample. Record the BOM revision used for the production batch, together with any approved deviations. Keep unresolved differences open rather than silently editing the master document to match what was made.
Frequently asked questions
Is a BOM the same as a quotation?
No. A BOM identifies production inputs and quantities. A quotation states the commercial offer. Ask the factory to identify the BOM revision used for its quote.
Can I create a BOM in a spreadsheet?
Yes. Use consistent fields, a named document owner and clear approval status. Keep a released copy for each order so later edits do not obscure what was approved.
Should packaging be included in a BOM?
Include packaging, labels and supplied accessories, or reference a separate approved packing specification. State which document controls them.
What if the factory cannot buy a listed component?
Ask it to identify the affected item and propose an alternative with supporting specifications. Review the effect on fit, function, testing and stock before approving a revised BOM.