What is a Bill of Materials?
A Bill of Materials (BOM) is a complete list of everything needed to make a product — raw materials, components, sub-assemblies, hardware, and consumables, with quantities for each. Think of it as a recipe. For a job shop, every part you make has a BOM, whether it's written down or in someone's head.
Single-level vs multi-level BOMs
A single-level BOM lists the direct materials for one part: "This bracket needs 1.5 lbs of 6061 aluminum and 4 socket head cap screws." Simple, flat, easy to manage.
A multi-level BOM adds depth: "This assembly needs 3 brackets (each with their own BOM), a housing (with its own BOM), and 12 fasteners." Each sub-assembly has its own BOM that rolls up into the parent. Multi-level BOMs are essential when you build assemblies from components you also manufacture.
Why BOMs matter for costing
Your BOM is the foundation of your material cost. If your BOM says a part needs 2 lbs of steel and you're quoting based on that, but the actual part consumes 2.4 lbs (because of kerf waste and drops), you're underquoting by 20% on material alone.
Accurate BOMs also drive MRP. If your BOMs are wrong, your material planning is wrong, which means you order too much (cash tied up) or too little (idle machines). Getting BOMs right isn't glamorous, but it's the foundation of accurate costing and planning.
Managing BOMs effectively
Key principles: (1) Create BOMs during quoting, not after — if you're estimating material costs anyway, capture the BOM at the same time. (2) Link BOMs to part revisions — when the drawing changes, the BOM should update. (3) Track actual consumption vs BOM quantities — the gap tells you where your estimates are wrong. (4) Store BOMs in your ERP, not in spreadsheets — so they connect to inventory, purchasing, and costing automatically.
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