What is a Bill of Materials (BOM)?

What Is a Bill of Materials (BOM)?

A bill of materials is the complete recipe for making a product. It lists every raw material, part and sub-assembly the product needs, the quantity of each required for one unit, the unit of measure, and how the pieces relate: which part goes into which assembly, and which assembly goes into the finished product.

It looks like a simple engineering document, but almost everything in a factory depends on it. Planning uses it to calculate what to buy. The warehouse uses it to know what to issue to each production order. Costing uses it to build the standard cost. Sales relies on that cost when quoting. One wrong quantity in a BOM line is repeated in every production order, every purchase order and every price quote.

BOMs are not only for large plants. A joinery workshop building kitchens to order, a small date-packing facility and a print shop producing printed cartons all need precise knowledge of what each product consumes in order to price correctly and order materials on time.

This guide covers the elements of a BOM, the main types, a full worked example of a multi-level BOM, how requirements and costs are derived from it, how to manage engineering changes, the most common mistakes and the role of an ERP system.

The Elements of a BOM

Every line in a bill of materials carries a set of data. These are the fields no serious BOM should be missing:

The two fields most often neglected are scrap and unit of measure. A BOM saying a drawer needs 1.8 kg of steel with no scrap allowance will make planning under-order every time. A BOM listing screws in pieces while they are bought in boxes will confuse both the warehouse and purchasing unless the conversion factor is clearly defined.

Types of Bills of Materials

Single-level BOM

Lists every component directly under the finished product, with no sub-assemblies. Fine for simple products assembled from a few parts, but impractical once sub-assemblies are built and stocked separately.

Multi-level BOM

Mirrors the real product structure: the finished product at level 0, sub-assemblies at level 1, their components at level 2, and so on. This is the type most factories actually use, because it allows sub-assemblies to be planned at different times, stocked, and reused across several products.

Engineering BOM

Prepared by design and describes the product as designed, organized by drawings and functions. It does not necessarily reflect the manufacturing sequence or packaging.

Manufacturing BOM

Describes the product as it is actually built on the shop floor. It includes packaging, consumables such as welding wire and paint, and follows production stages. This is the BOM that planning, the warehouse and costing work from.

Sales or configurable BOM

Used when a product is sold with options such as color, size or accessories. Instead of a separate BOM for every combination, one BOM carries options and the customer's choices are selected on the sales order.

Phantom BOM

A sub-assembly defined for structure but never stocked or given its own production order, because it is built and used immediately on the line. Planning passes through it to its components without ordering it as a separate item.

Worked Example: A Four-Drawer Steel Filing Cabinet

Take a hypothetical office furniture plant in Dammam's industrial city making a four-drawer steel filing cabinet. Here is the multi-level BOM for one cabinet:

Exploding the BOM for a production order

A sales order arrives for 250 cabinets. Exploding the BOM means multiplying every quantity by the order size through all levels and adding scrap:

• Drawer assemblies: 250 × 4 = 1,000 drawers.
• Body steel: 250 × 14 = 3,500 kg, or 3,675 kg with 5% scrap.
• Drawer steel: 1,000 × 1.8 = 1,800 kg, or 1,890 kg with scrap.
• Total steel: 3,675 + 1,890 = 5,565 kg.
• Powder coating: 250 × 0.6 = 150 kg, or 154.5 kg with 3% scrap.
• Slides and handles: 1,000 of each.
• Screws: 1,000 × 8 = 8,000 pieces.
• Locks and packaging kits: 250 of each.

If steel is bought in 25 kg sheets, 5,565 kg means 223 sheets after rounding up, which is exactly why the conversion between consumption unit and purchase unit matters.

These are gross requirements. Planning then subtracts stock on hand and open purchase orders to arrive at net requirements. With 2,000 kg of steel in the warehouse, the net need is 3,565 kg. That is the logic of material requirements planning, covered in our guide to ERP in production planning.

Notice that the same steel appears at two levels. A planner reading only level 1 would forget that the drawers need another 1,890 kg. Multi-level BOMs managed in a system that aggregates demand across levels prevent exactly that mistake.

Rolling Up Standard Cost

Once you know each component's cost, you can roll material cost up from the bottom. With illustrative prices:

Material is not the full cost. Direct labor and manufacturing overhead, such as machine depreciation, power and supervision, are added, usually through the routing and labor hours. But the BOM is the starting point, and any error in it flows into everything after it. For full product costing, see our guide to cost of production.

Look at the effect of scrap alone. Without it, steel cost would be about SAR 4.80 lower per cabinet. Small, until you multiply by 10,000 cabinets a year: close to SAR 48,000 of cost missing from your pricing.

BOM and Routing: Two Halves of the Same Picture

The BOM answers "what is the product made of?" The routing answers "how is it made?": which operations, in what order, on which machine or work center, and how long each takes. For the cabinet, the routing might be cutting, bending, welding, powder coating, assembly and packing.

Linking the two matters because some materials are only needed at a given stage. Powder coating is not needed until the coating line. When each component is tied to the operation that consumes it, materials can be issued just in time instead of all at once at the start of the order. More on this in our article on production routing.

Managing Engineering Changes

Products change: a new slide supplier with different specifications, thinner steel to save weight, a better lock after a customer complaint. Every product change must reach the BOM, or the plant keeps buying and issuing obsolete parts. Sound change control includes:

• Numbered revisions: each change creates a new BOM version with an effective date, while earlier versions are kept for reference.
• A clear effective date: does the change apply immediately or once existing stock of the old part runs out? That decision determines whether old stock becomes dead stock.
• Cross-functional approval: engineering proposes; production, purchasing, quality and costing review the impact before approval.
• Notifying affected teams: purchasing amends open POs and the warehouse quarantines the old part if it is discontinued.

Building Your First BOMs: Five Steps

1. Start with best sellers

Do not try to build every BOM at once. Begin with the products that make up most of your volume, where accuracy will show up fastest in purchasing and cost.

2. Tear down a real unit

The best check is to take a unit off the line, or follow one through production, and record every component with its weight or count. Teams often discover parts that were never in any file, such as an adhesive or a rubber gasket.

3. Clean item codes first

Review inventory item cards, merge duplicates, and set a base unit of measure for each item with a conversion factor to the purchase unit. Tedious, but it prevents most later problems.

4. Compare with actual consumption

After entering the BOM, run several production orders and compare actual issues against the BOM. Early variances are normal and are your chance to fix quantities and scrap factors before planning depends on them.

5. Name one owner

In many plants nobody knows who updates the BOM. Give one function, usually production engineering, the authority to change it, with everyone else able to propose and review.

Common BOM Mistakes

• A paper BOM and a system BOM: when the floor works from an old printout while the system plans from a newer version, inventory variances are guaranteed.
• Leaving out consumables: welding wire, gas and adhesives are cheap per unit but run out suddenly if they are not in the BOM or the plan.
• Unrealistic scrap: a scrap factor set at launch and never revisited. Compare actual consumption with the BOM regularly and adjust.
• Duplicate codes: the same part under two codes shows a shortage on one and a surplus on the other.
• No revisions: editing the current BOM in place without saving the previous version makes it impossible to know what an older batch really cost.

The Role of ERP in Managing BOMs

Small plants often start with BOMs in spreadsheets. That works for a few products, but breaks down as levels, products and revisions multiply, because the file is not connected to inventory, purchasing or costing. With Saudi manufacturing expanding under the National Industrial Strategy launched in October 2022, which targets tripling industrial output and around 36,000 factories by 2035, many plants will move to more products and larger customers who expect traceability and cost-backed pricing. An ERP makes the BOM a single source every department relies on:

• Explodes the BOM automatically when a production order is created, calculating every component requirement across all levels.
• Compares requirements with available, reserved and incoming stock, and suggests purchase orders or production orders for sub-assemblies.
• Issues materials to the production order straight from the BOM, cutting ad hoc manual issues.
• Rolls up material cost from actual component prices and compares actual consumption with standard.

Tranquil's manufacturing solutions include multi-level BOMs, alongside material planning, production scheduling, quality checks, barcode integration and a production calendar, all connected to inventory, purchasing and accounting in one system.

Frequently Asked Questions

What is the difference between a BOM and a routing?

The BOM defines what the product is made of and in what quantities. The routing defines the operations, their sequence, the work centers and the time required. Planning and costing need both.

When do I need a multi-level BOM?

When the product contains sub-assemblies that are built separately, stocked, or used in more than one product. If it is assembled directly from purchased parts, a single-level BOM may be enough.

How do I set the right scrap factor?

Start with an engineering estimate, then compare it with actual consumption on production orders over several months. A persistent gap means the factor needs adjusting, or that the process has a problem worth investigating.

Is the engineering BOM different from the manufacturing BOM?

Usually, yes. The engineering BOM describes the product as designed; the manufacturing BOM adds packaging, consumables and production stage sequencing. The plant plans, issues and costs from the manufacturing BOM.

Does Tranquil support multi-level BOMs?

Yes, as part of Tranquil's manufacturing solutions together with material planning and production scheduling. Contact the Tranquil team to review your product structures and how to set them up.