Bill of Materials (BOM)
📜 Product structure definition
Purpose	Define what goes into a product
Contains	Components, quantities, levels
Used by	MRP, CRP, Costing
Accuracy target	≥98% (Class A MRP II)
Part of	MRP II

Bill of Materials (BOM)

A Bill of Materials (BOM) is a structured list of all components, sub-assemblies, and raw materials required to manufacture a finished product, together with the quantity of each needed per unit. Within MRP II, the BOM is the structural backbone that enables MRP to "explode" end-item demand into component-level requirements.

Contents

Product structure tree
Types of BOMs
Low-level coding
Accuracy and maintenance
Relationship to other modules
See also

Product structure tree

A BOM is typically visualised as a tree. Consider a simple bicycle:

graph TD
    BIKE["Bicycle
(Level 0)"] --> FRAME["Frame Assembly
(Level 1) ×1"]
    BIKE --> WHEEL["Wheel Assembly
(Level 1) ×2"]
    BIKE --> CHAIN["Chain
(Level 1) ×1"]
    BIKE --> SEAT["Seat
(Level 1) ×1"]
    FRAME --> TUBE["Steel Tubing
(Level 2) ×3m"]
    FRAME --> PAINT["Paint
(Level 2) ×0.5L"]
    WHEEL --> RIM["Rim
(Level 2) ×1"]
    WHEEL --> TIRE["Tyre
(Level 2) ×1"]
    WHEEL --> SPOKE["Spokes
(Level 2) ×36"]
    WHEEL --> HUB["Hub
(Level 2) ×1"]

    style BIKE fill:#cedff2,stroke:#333
    style FRAME fill:#d5e8f5,stroke:#333
    style WHEEL fill:#d5e8f5,stroke:#333
    style CHAIN fill:#d5f5d5,stroke:#333
    style SEAT fill:#d5f5d5,stroke:#333

Each level represents a stage of assembly. MRP processes the BOM level by level (top-down) to derive dependent demand at each level.

Types of BOMs

Engineering BOM (EBOM) — Designed by engineering; defines the product as designed. May include design alternatives and revision history.
Manufacturing BOM (MBOM) — Reflects how the product is actually built. May differ from the EBOM (e.g., different assembly sequence, phantom sub-assemblies).
Planning BOM — An artificial BOM used to group product families for MPS and S&OP planning. Uses percentage breakdowns to forecast option mixes.
Modular BOM — Organises components by option/feature module for configure-to-order products. Simplifies the MPS for products with many variants.
Phantom BOM — Represents a sub-assembly that is built and consumed immediately (never stocked). MRP "blows through" phantoms, treating their components as belonging to the parent.

Low-level coding

Some components appear at multiple levels (e.g., the same bolt used in both the frame and the seat). Low-level coding assigns each item the lowest level at which it appears anywhere in any BOM. This ensures that MRP only nets the item once, at the lowest level, after all gross requirements from higher levels have been accumulated — preventing premature ordering.

Accuracy and maintenance

BOM accuracy is critical to MRP II success. Oliver Wight's Class A standard requires ≥98% BOM accuracy. Inaccurate BOMs cause MRP to order wrong quantities, leading to shortages on the shop floor or excess inventory. Maintenance practices include:

Engineering Change Orders (ECOs) with effectivity dates
Regular BOM audits (comparing physical products to the recorded BOM)
Single-source-of-truth policy: one master BOM record, not multiple copies

Relationship to other modules

graph TB
    BOM["BOM"] -->|"product structure"| MRP["MRP"]
    BOM -->|"routing & operations"| CRP["CRP"]
    BOM -->|"component list"| SFC["SFC"]
    BOM -->|"buy items"| PUR["Purchasing"]
    MPS["MPS"] -->|"end items"| MRP

    style BOM fill:#cedff2,stroke:#333