What a bill of materials (BOM) is
A bill of materials (BOM) is the list of every component, raw material and subassembly needed to make one unit of a product, with the quantity of each. It shows what the product is made of and how the items nest in levels.
The BOM is the foundation of planning: MRP uses it to calculate purchasing and production, the production order copies it to pick materials, and product cost comes from it.
BOM visual map
The map shows the structure levels, the chair example, the explosion for 100 units and the material cost.
Types of BOM
| Type | Purpose | Owner |
|---|---|---|
| Engineering BOM (EBOM) | Describes the product as designed: every item on the drawing | Product engineering |
| Manufacturing BOM (MBOM) | Describes how the product is built: adds packaging, consumables and assembly sequence | Manufacturing engineering / planning |
| Single-level BOM | Lists only the direct children of one parent | Any function |
| Multi-level (indented) BOM | Shows the full tree, from finished product to raw materials | Planning / ERP |
| Phantom BOM | A subassembly that is never stocked: it flows straight into the next assembly | Planning |
| Modular or planning BOM | Groups options into modules to plan products with many variants | Planning / Sales |
The gap between EBOM and MBOM causes many errors: the drawing can be right while the list used on the shop floor is incomplete (no glue, label or carton). Whoever defines how to build the product maintains the MBOM.
Example: multi-level BOM of an office chair
Fictional product CE-100. Each quantity is per unit of the parent item. Level 0 is the finished product; levels 1 and 2 are subassemblies and components.
| Level | Code | Item | Quantity per parent |
|---|---|---|---|
| 0 | CE-100 | Assembled office chair | 1 pc |
| 1 | AS-10 | Seat assembly | 1 pc |
| 2 | EP-11 | Molded foam | 1 pc |
| 2 | CO-12 | Plywood base | 1 pc |
| 2 | TE-13 | Fabric | 0.8 m² |
| 2 | PA-90 | M6 screw | 4 pc |
| 1 | EN-20 | Backrest assembly | 1 pc |
| 2 | ES-21 | Plastic frame | 1 pc |
| 2 | TL-22 | Mesh | 0.5 m² |
| 2 | PA-90 | M6 screw | 2 pc |
| 1 | BA-30 | Star base | 1 pc |
| 1 | RO-31 | Caster | 5 pc |
| 1 | PI-32 | Gas cylinder | 1 pc |
| 1 | ME-33 | Adjustment mechanism | 1 pc |
The M6 screw (PA-90) appears in two places in the tree. A where-used query shows that it goes into both the seat assembly and the backrest assembly: if the supplier changes, both assemblies are affected.
Material explosion: what to buy for 100 chairs
The explosion multiplies the quantity ordered by the quantity at each level, top down. For a batch of 100 chairs:
| Item | Calculation | Gross requirement |
|---|---|---|
| Seat assembly | 100 × 1 | 100 pc |
| Molded foam | 100 × 1 | 100 pc |
| Plywood base | 100 × 1 | 100 pc |
| Fabric | 100 × 0.8 | 80 m² |
| M6 screw (AS-10) | 100 × 4 | 400 pc |
| Backrest assembly | 100 × 1 | 100 pc |
| Plastic frame | 100 × 1 | 100 pc |
| Mesh | 100 × 0.5 | 50 m² |
| M6 screw (EN-20) | 100 × 2 | 200 pc |
| Star base | 100 × 1 | 100 pc |
| Caster | 100 × 5 | 500 pc |
| Gas cylinder | 100 × 1 | 100 pc |
| Adjustment mechanism | 100 × 1 | 100 pc |
| M6 screw (total) | 400 + 200 | 600 pc |
Because the screw appears in two assemblies, MRP adds the requirements: 400 + 200 = 600 pcs. Scrap enters as yield: if fabric loses 5% in cutting, the requirement is 80 ÷ (1 − 0.05) = 84.2 m². If screws have 2% scrap, 600 ÷ (1 − 0.02) = 612.2, rounded up to 613 pcs.
Gross requirement = parent quantity × quantity per parent ÷ (1 − scrap)
Net requirement subtracts stock on hand. With 120 casters in stock, the net requirement is 500 − 120 = 380 pcs. That is the next step of MRP, fed by the master production schedule.
Material cost from the BOM (roll-up)
Adding up, from the bottom, each component's cost times its quantity gives the product's material cost (fictional values, no labor):
| Item | Calculation | Cost per chair |
|---|---|---|
| Molded foam | $18.00 | $18.00 |
| Plywood base | $12.00 | $12.00 |
| Fabric | 0.8 m² × $25.00 | $20.00 |
| M6 screw | 4 × $0.30 | $1.20 |
| Seat assembly | 18 + 12 + 20 + 1.2 | $51.20 |
| Plastic frame | $22.00 | $22.00 |
| Mesh | 0.5 m² × $30.00 | $15.00 |
| M6 screw | 2 × $0.30 | $0.60 |
| Backrest assembly | 22 + 15 + 0.6 | $37.60 |
| Star base | $45.00 | $45.00 |
| Caster | 5 × $3.50 | $17.50 |
| Gas cylinder | $28.00 | $28.00 |
| Adjustment mechanism | $40.00 | $40.00 |
| Total | 51.2 + 37.6 + 45 + 17.5 + 28 + 40 | $219.30 |
For 100 chairs the material costs 100 × $219.30 = $21,930.00, before scrap. Labor and overhead go into the production cost. The BOM also supports the make-or-buy decision for a subassembly.
How to build a BOM in 6 steps
- Start from the drawing and the process. List every item and check it against the physical part on the line, not just the drawing.
- Set levels by how you build. Create a level when a subassembly is built separately or stocked.
- Standardize codes and units. One code per item and one unit of measure (m², kg, pc) per code.
- Record quantity per parent unit. Scrap goes in the yield field, not added into the quantity.
- Control revisions. Each engineering change has a number, an effectivity date and an approval.
- Audit the BOM. Pick materials for one production order, build it and check for leftover or missing items. Record what you correct.
A correct BOM is also the basis for lot traceability and for the modules of an industrial ERP.
Common mistakes
- Outdated BOM. Engineering changes an item and the shop floor keeps using the old list.
- Mixing EBOM and MBOM. Consumables, packaging and process items go missing.
- Scrap built into quantities. Scrap varies by lot and belongs in its own field.
- Different units for the same item. Buying in meters and consuming in centimeters creates conversion errors.
- Too many levels. Subassemblies nobody builds or stocks only complicate MRP; use a phantom item.
- No owner. Define who creates, changes and approves; otherwise every function edits its own version.
Frequently asked questions
What does BOM stand for?
Bill of materials: the list of components and quantities needed to make one unit of the product.
What is the difference between a BOM and a routing?
The BOM says what goes into the product; the routing says how to make it: operations, resources and times. Both feed planning.
What is a BOM explosion?
It is the calculation of quantities at every level from the quantity of the finished product. In the example, 100 chairs need 500 casters and 600 M6 screws.
What is a multi-level BOM?
It is the list that shows the whole product tree, from finished goods to raw materials, with subassemblies indented at each level.
What is the difference between EBOM and MBOM?
The EBOM reflects the engineering design; the MBOM adds what is needed to build it, such as packaging and consumables, and the assembly sequence.
How does the BOM feed MRP?
MRP reads the BOM, inventory and the master schedule to calculate what to buy or make and when. Without a correct BOM the requirements come out wrong.
Sources
- ORLICKY, J. Material Requirements Planning. McGraw-Hill.
- VOLLMANN, T. E. et al. Manufacturing Planning and Control for Supply Chain Management. McGraw-Hill.
- SLACK, N. et al. Operations Management. Pearson.
Keep studying: trusted sources and links
- ASCM: Association for Supply Chain Management
- ABEPRO: Associação Brasileira de Engenharia de Produção (in Portuguese)
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About the author
Lean Manufacturing & Behavioral Management Specialist
Over 20 years in the automotive and metalworking industries (GM and Dana), Lean Manufacturing practitioner since 2006. SENAI instructor and mentor in Brazil’s Brasil Mais Produtivo program, delivering consulting, training and audits for 50+ companies, combining quality, productivity and people development.
