A Bill of Materials (BOM) is a central document in any manufacturing or assembly project. It records the parts, quantities, suppliers and dates the team is working with; gaps or incorrect data can create follow-up work during the project.
This guide covers everything you need to know about creating a professional BOM — from the essential columns to include, to the common mistakes that cause projects to go wrong.
What Is a Bill of Materials?
A Bill of Materials is a comprehensive list of all the parts, components, assemblies, and materials needed to build a product. It's the single source of truth for your project — used by engineers, purchasing teams, workshop supervisors, and project managers alike.
A good BOM doesn't just list part numbers and quantities. It connects every component to a supplier, a delivery date, a stock level, and a cost — giving you a complete picture of your project at any moment.
Types of Bill of Materials
Engineering BOM (eBOM)
Created during the design phase, the eBOM lists components as designed — typically derived from CAD software. It reflects the product as it was designed, not necessarily as it will be built.
Manufacturing BOM (mBOM)
The mBOM is used on the shop floor. It includes all the components, sub-assemblies, and raw materials needed to actually manufacture the product, often including items not in the eBOM (such as fasteners, adhesives, or packaging).
Assembly BOM
For companies building custom machinery or equipment, the assembly BOM is the most practical format — listing parts by assembly group, with quantities per machine and quantities required for a specific order.
Essential Columns in a Professional BOM
A well-structured manufacturing BOM should include the following columns as a minimum:
- Assembly Number — groups parts by sub-assembly (e.g. hydraulic unit, control panel, frame)
- Part Number — your internal reference for each component
- Description — clear, unambiguous description of the part
- Drawing Reference — link to the relevant engineering drawing
- Quantity Per Machine — how many of this part go into one unit
- Quantity Required — total quantity for the order (qty per machine × number of machines)
- Stock — current stock level of this part
- To Order — quantity needed from suppliers (required minus stock)
- Minimum Order Quantity (MOQ) — supplier's minimum order
- Order Quantity — adjusted quantity based on MOQ
- Price Each — unit cost from supplier
- Total Price — extended cost for this line
- Supplier — preferred supplier for this component
- Supplier Reference — supplier's part number or catalog reference
- Date Required — when this part is needed on site
- Delivery Time — supplier's lead time
- Status — where is this part in the procurement workflow?
- Notes — any additional information
Structuring Your BOM: Flat vs Hierarchical
A flat BOM lists all components in a single level — useful for simple products with no sub-assemblies. A hierarchical (or multi-level) BOM groups components into assemblies and sub-assemblies, reflecting the actual build structure.
For many assembly and manufacturing projects, a hierarchical structure is useful. It allows you to:
- Assign purchasing responsibility by assembly
- Track delivery status at assembly level
- Identify which assembly is causing a delay
- Reuse common sub-assemblies across multiple projects
The BOM Workflow: From Parts List to Delivery
A BOM isn't just a static list — it should drive your entire procurement workflow. Here's how a well-managed BOM flows through a project:
- Create the BOM — export it from your CAD system as a spreadsheet, or build it manually from engineering drawings
- Check stock — identify which parts are already in stock and can be used
- Generate quote requests — send RFQs to suppliers for items without a current price
- Raise purchase orders — order everything needed, with correct quantities and references
- Track deliveries — monitor incoming parts against required dates
- Update status — mark parts as received and flag any shortages
Common BOM Mistakes to Avoid
No Version Control
Without version control, you can't be certain everyone is working from the same BOM. Always date-stamp your BOM and record who made changes and when.
Missing MOQ Data
Failing to account for minimum order quantities leads to either ordering too little (and having to reorder at higher unit cost) or ordering too much (and tying up cash in excess stock).
No Stock Integration
Ordering parts you already have in stock creates an avoidable duplicate order. A good BOM system deducts available stock from the quantity to order.
Ignoring Lead Times
Knowing when a part is needed and knowing the supplier's lead time are both essential. A BOM that doesn't include both makes it impossible to know whether you're at risk of a late delivery.
BOM Software vs Spreadsheets
For small and growing manufacturers, purpose-built BOM software brings quantity calculations, stock deduction, purchase order generation and delivery tracking into a workflow linked to the live BOM. These functions can be difficult to coordinate across separate spreadsheet files.
AssembleFlow includes all of these features in a clean, intuitive interface designed for engineers and project managers — not IT specialists. Import your existing Excel BOM and start your free 14-day trial today.
Frequently Asked Questions
What columns should a bill of materials include?
At a minimum: assembly, part number, description, drawing reference and issue, quantity per machine, supplier, price, need-by date and lead time. A status column shows where each part is in the buying process.
What is the difference between an engineering BOM and a manufacturing BOM?
The engineering BOM lists parts as designed, usually exported from CAD. The manufacturing BOM adds what is needed to actually build the product, such as fasteners, adhesives and packaging.
Should a BOM be flat or multi-level?
A flat list suits simple products. For machines with sub-assemblies, a multi-level or assembly-grouped BOM lets you assign purchasing and track delivery by assembly.
Free tools: the Excel templates (BOM, PO tracker, cost and quote worksheet), the margin calculator and a product tour need no sign-up.