A Bill of Materials (BOM) is a comprehensive, hierarchical list of all raw materials, sub-assemblies, components, and quantities required to manufacture a finished product. Think of it as a recipe for production, detailing not just what ingredients you need but also how they fit together in the assembly structure. The BOM serves as the foundation for virtually every manufacturing planning activity, from procurement and inventory management to production scheduling and costing.
A well-structured BOM organises items in levels or tiers. The top level represents the finished product, whilst subsequent levels break down into sub-assemblies and individual components. For example, a bicycle’s BOM might show the complete bike at level 0, major assemblies like the frame, wheels, and drivetrain at level 1, and individual parts like spokes, bearings, and bolts at lower levels. Each item in the BOM includes critical information such as part numbers, descriptions, quantities per assembly, units of measure, and often lead times or preferred suppliers.
Different types of BOMs serve different purposes. An engineering BOM (EBOM) reflects the product design as created by engineering, whilst a manufacturing BOM (MBOM) represents how the product is actually built on the shop floor, potentially grouping components differently for assembly efficiency. Some manufacturers also maintain service BOMs for aftermarket parts and support.
Accurate BOMs are essential for Material Requirements Planning (MRP), which uses the BOM structure to calculate exactly what materials need to be purchased or manufactured and when. They also drive job costing by establishing standard material costs for each product. In modern ERP systems, BOMs integrate with inventory, purchasing, and production modules, automatically triggering material requirements when orders are placed and ensuring that production has everything needed before work begins. Maintaining BOM accuracy through engineering change management processes is critical, as errors cascade through planning, purchasing, and production, causing shortages, excess inventory, and production delays.
