Title: What Is a Bill of Materials (BOM) URL: https://zudoworks.com/answers/what-is-a-bill-of-materials/ Description: A bill of materials (BOM) lists every part, quantity and material needed to make one unit. Types, levels, scrap, revisions and common mistakes. Updated: 2026-11-03 # What is a bill of materials? A bill of materials (BOM) is the list of raw materials, components, sub-assemblies and consumables needed to make one unit of a product, with the quantity of each. A multi-level BOM nests sub-assemblies that have their own BOMs. In manufacturing software the BOM drives material requirements, stock issues to work orders, scrap allowances and the material part of the product cost. By Arunkumar V, CTO · 03 November 2026 · 3 min read [Try ZMOS free](https://app.zudoworks.com/signup) [Book a 20-minute demo](https://zudoworks.com/contact/) - BOM meaning - multi-level BOM - BOM vs routing - manufacturing BOM example ## What a BOM contains At minimum each line names a component, its quantity per parent and its unit of measure. Useful BOMs add more. A sheet-metal bracket might have one BOM line for sheet, expressed in kilograms or square metres with a scrap percentage, plus fasteners and powder coat. An assembled panel might have ten lines, two of which are sub-assemblies with their own BOMs. Keep units honest: if you buy sheet by weight and consume it by area, the BOM must hold the conversion. - A revision number and date, with what changed - Scrap or yield allowance per line - Whether the line is made in house, bought or sent for job work - Reference designators or positions, where it matters - Alternates or substitutes allowed - Notes such as cut length or finish ## Types of BOM Engineering BOMs (EBOM) describe the product as designed, usually straight from CAD. Manufacturing BOMs (MBOM) describe it as built: they add packaging, consumables, scrap and the order in which sub-assemblies are made. A sales or configurable BOM picks options at order time. Most small shops only need the MBOM, and the common mistake is to load the CAD export and treat it as one. A single-level BOM lists only direct components; a multi-level or indented BOM shows the whole tree. Phantom BOMs are groupings that never exist in stock, used to keep the tree tidy without creating sub-assemblies nobody counts. ## BOM and routing are different things The BOM says what goes in. The routing says what is done to it: the sequence of operations, the work centre for each and the expected time. Costing needs both: material from the BOM at issue cost, labour from the routing at work-centre rates. Some software merges them into one production template, which is convenient but can hide the distinction when you need to change one without the other. If the same part is made on two different machines depending on load, you need one BOM and two routings, not two BOMs. ## Revisions and the mistakes that cost money A BOM is live data. Drawings change, suppliers change, an operator finds a better way. Every change should produce a new revision with a date and a reason, and open work orders should keep the revision they were released with. Shops lose money in predictable ways: BOMs never updated after the first prototype, so planning buys the wrong quantities; no scrap allowance, so every job comes up short; consumables such as gas, abrasives and powder left out, so unit costs are understated. A quarterly check of the top twenty BOMs against what the floor actually issues catches most of this. ## Where ZMOS fits ZMOS holds bills of materials with revisions, scrap allowances and cost roll-up, and links them to work orders so material is issued against the BOM and costed at real issue cost. Custom fields can be added to the BOM form without code, for drawing numbers, finish or cut lengths. It is built for sheet-metal, machining and fabrication shops of 10 to 50 people. It is not a CAD or PLM tool: if your main need is managing engineering change on thousands of designed parts with CAD integration, you will need PLM software in front of it. [What ZMOS does](https://zudoworks.com/zmos/) - [BOMs with revisions in ZMOS](https://zudoworks.com/zmos/production-and-bom/) - [custom fields on every form](https://zudoworks.com/zmos/low-code/) - [job costing](https://zudoworks.com/zmos/job-work-and-costing/) - [BOMs in the ZMOS user guide](https://zudoworks.com/guide/#12-bills-of-materials) ## People also ask Short answers, each one complete on its own. ### Related answers - [What is a routing and a work centre?](https://zudoworks.com/answers/what-is-a-routing-and-work-centre/) - [How do I manage BOM revisions?](https://zudoworks.com/answers/how-to-manage-bom-revisions/) - [What is job costing in manufacturing?](https://zudoworks.com/answers/what-is-job-costing/) - [How do I handle scrap in production?](https://zudoworks.com/answers/how-to-handle-scrap-in-production/) - [What is a work order in manufacturing?](https://zudoworks.com/answers/what-is-a-work-order/) - [What is an immutable stock ledger?](https://zudoworks.com/answers/what-is-an-immutable-stock-ledger/) - [All 54 answers](https://zudoworks.com/answers/) ### Inside ZMOS [Zudo AI](https://zudoworks.com/zudo-ai/)[Stock ledger](https://zudoworks.com/zmos/stock-ledger/)[Low-code forms and rules](https://zudoworks.com/zmos/low-code/)[Barcode job cards](https://zudoworks.com/zmos/barcode-job-cards/)[Roles, audit log, API](https://zudoworks.com/zmos/roles-audit-api/)[Security](https://zudoworks.com/zmos/security/)[Free trial](https://zudoworks.com/zmos/free-trial/)[Why Zudo Works](https://zudoworks.com/why-zudo-works/) What is the difference between a BOM and a parts list? A parts list is often just the BOM printed from a drawing. In software, a BOM also carries quantities per parent, units, scrap, revision and make-or-buy flags, and it is linked to stock and work orders so it can drive purchasing and costing, which a parts list cannot. How many levels should a BOM have? As many as there are real sub-assemblies you stock or build separately. If a sub-assembly is always built and consumed inside the same job, make it a phantom or fold it into the parent. Extra levels mean extra work orders and extra stock postings for no benefit. Should scrap be in the BOM? Yes, as a percentage or fixed quantity per line, based on what the floor actually loses. Without it, planning underbuys and job costs look better than they are. Review it against actual issue and return quantities every few months rather than guessing once. Who should own the BOM in a small shop? One named person, usually production or engineering, with changes made only through revisions in the system. Purchasing and the floor read it; they do not edit it. If several people can change the BOM freely, the version the shop builds and the version that was costed will drift. Can a BOM include labour? Not normally. Labour belongs on the routing, as operations at work centres with expected times. Some simple systems put labour lines in the BOM to get a unit cost, which works for quotes but hides planned versus actual time.