Bill of Materials in Trades: Structure, Use, and Practical Limits
TL;DR: A bill of materials (BOM) documents all parts and quantities for a job or product. In the trades, quantity and structural BOMs cover most needs. Excel works as a planning tool but hits its limits once multiple people work operationally with inventory. What matters is not the system, but whether the list is maintained, unambiguous, and actually used day-to-day.
What is a bill of materials?
A bill of materials (BOM) is a complete list of all materials, parts, and quantities needed to manufacture a product or carry out a job.
In practice, you'll encounter four basic types that differ in complexity and purpose:
Types of bills of materials
- Quantity BOM: The simplest form. It lists all parts unsorted with their total quantities (e.g. for a standard installation). Ideal for quick purchasing.
- Structural BOM: This maps assemblies and production stages. If you're building a switchboard, for example, the list contains sub-components for each module, ordered by the sequence they're installed.
- Modular BOM: Each assembly gets its own separate BOM. A plumbing company might maintain separate lists for "manifold," "pipe installation," and "controls." The lists are combined as needed. This makes changes to individual assemblies easier without touching the overall list.
- Variant BOM: Combines multiple product variants in one list. One column shows the base configuration, additional columns show deviations per variant. Relevant when you manufacture the same product in different versions.
In the trades, quantity and structural BOMs cover most businesses. Modular and variant BOMs become relevant mainly in serial production or complex custom work.
Normative basis: DIN 6771 and ISO 7200
DIN 6771 defined the formal structure of bills of materials in Germany until 2007. Required fields included position, quantity, description, part number, and unit. It was replaced by EN ISO 7200, which applies internationally and additionally requires fields like creator, issue date, and document type.
In practice, these standards are rarely formally applied in the trades. However, the required fields (item description, quantity, unit, reference number) provide useful guidance for any business looking to structure their BOMs properly.
In industry, there's also a distinction between Engineering BOM (design) and Manufacturing BOM (production). In the trades, a bill of materials is usually more pragmatic: a list of parts you need for a specific job: item description, quantity, unit, and ideally a clear reference to storage or staging location.
BOM vs. materials list vs. order list
| Term | What it is | Typical use |
|---|---|---|
| Bill of Materials (BOM) | All parts + quantities for a product or job | Recurring jobs, custom work |
| Materials list | Simple shopping list without structure | One-off projects, ad-hoc purchases |
| Order list | What needs to be reordered (based on stock levels) | Ongoing inventory replenishment |
When does a trade business need a BOM? Whenever jobs require recurring similar materials, multiple people assemble materials, or mistakes in preparation have direct impact on deadlines and costs.
Why tradespeople need bills of materials
The knowledge of which parts are needed for a job exists in most businesses. But it's tied to individual people. As long as the master tradesperson assembles the materials themselves, it works. Problems arise as soon as others need to take over that task.
What goes wrong without a BOM
- Wrong parts packed. The apprentice prepares the job and grabs 1.5 mm² cable instead of 2.5 mm². It's only noticed on site during installation.
- Knowledge isn't transferable. A new employee can't prepare jobs independently because the assembly is documented nowhere. The master tradesperson becomes the bottleneck.
- Incomplete costing. During quoting, items are missing because the parts list was created from memory. Margins shrink or the customer gets a change order.
- Quality varies. Without a defined parts list, the result depends on who prepared the job. Reproducibility is missing.
BOMs with Excel: Why it often falls short
Excel is an excellent tool for planning phases. If you work alone and use BOMs as templates for costing, a simple spreadsheet is often the fastest path. You copy part numbers and quantities and have your basis.
But Excel hits a hard limit once things become operational:
- No shared real-time data: Who enters the withdrawal when the journeyman picks up materials at 4:30 PM? As soon as multiple people work with the same list, planned and actual stock quickly diverge.
- Limited traceability in the team: Who adjusted which quantity when, which version is current, and on what basis was the reorder placed? These questions quickly become unclear with spreadsheets in daily use.
- Error-prone in practice: Spreadsheets are error-prone in practice, especially when multiple people maintain them in parallel. According to research by Prof. Raymond Panko (University of Hawaii), roughly 88% of all business spreadsheets contain errors. In the stockroom, a single transposed number can mean materials don't get reordered.
Excel often "knows" what you planned, but not automatically what was actually withdrawn, consumed, or reordered. This is exactly where planning document and operational material process diverge.
How businesses make BOMs operationally useful
A bill of materials alone doesn't manage inventory. In practice, it's usually combined with a second process: material withdrawal, reordering, picking, or job preparation. Businesses use different approaches depending on size and effort.
Typical implementations in practice
- Excel or simple spreadsheets: suitable for small teams, low repetition rates, and manageable item counts.
- Paper lists or job folders: work with clear processes but are prone to media breaks and outdated information.
- ERP or inventory management systems: useful when purchasing, costing, inventory, and job management need to work together.
- Mobile inventory and scanning processes: helpful when material movements need to be captured quickly, decentrally, and as directly as possible in daily work.
Which approach makes sense depends less on the buzzword "ERP" than on item count, team size, repetition rate, and desired inventory accuracy.
Digital BOMs: What software can do today
Anyone looking to use bills of materials not just as a planning document but operationally will sooner or later encounter the term BOM explosion. The principle: you book a job, the system automatically deducts all parts from the stored BOM from inventory.
Using the washbasin example: the technician starts the job in the software. The system books out 1x washbasin, 1x faucet, 2x angle valves, 2x flex hoses, and the remaining items automatically from the stockroom. If an item drops below the reorder point, the system alerts or reorders directly.
In practice, this full explosion is an ERP feature that requires clean master data. For many small businesses, the more pragmatic entry point is: view BOMs digitally, book withdrawals via scanning, and use reorder points with automatic alerts. BOM explosion only pays off with high repetition rates or serial production.
What information every BOM should contain
Regardless of the system: a bill of materials only becomes useful in daily work when entries are unambiguous, understandable, and maintainable.
| Information | Why it matters |
|---|---|
| Item description | Prevents confusion with similarly named parts |
| Quantity | Defines how much material is needed per job or assembly |
| Unit | Avoids errors between piece, meter, set, or packaging unit |
| Part number or internal reference | Simplifies purchasing, reordering, and system assignment |
| Optional: storage or staging location | Reduces search time and simplifies material preparation |
BOM template download
As a starting point, we've created a print-ready BOM template. It includes the required fields from the table above, space for 20 line items, and header/footer fields for company, job, and approval. The structure follows the fields of ISO 7200.
All three templates are free to use. For most businesses, they're a good starting point before committing to a digital system.
Practical example: BOM in a plumbing business
For a single outlet or light switch, practically nobody builds a bill of materials. BOMs pay off for recurring sets or assemblies.
A simple example from plumbing. Job: Install washbasin.
| Pos. | Material | Qty | Unit |
|---|---|---|---|
| 1 | Washbasin | 1 | Pcs |
| 2 | Basin faucet | 1 | Pcs |
| 3 | Drain assembly | 1 | Pcs |
| 4 | Siphon/trap | 1 | Pcs |
| 5 | Angle valve | 2 | Pcs |
| 6 | Flex hose | 2 | Pcs |
| 7 | Washbasin mounting kit | 1 | Set |
| 8 | Sanitary silicone | 1 | Cartridge |
Simple, realistic, reusable. Digital BOMs make sense for exactly these standard packages: create once, pull up for every washbasin job, have materials prepared.
BOMs by trade
Depending on the trade, bills of materials look quite different:
- Electrical installation: Cables (NYM-J 3x1.5 / 5x2.5), distribution boards, circuit breakers, junction boxes, switches, outlets. A typical BOM runs 15-30 line items per sub-distribution. Quantities often mix meters (cable) and pieces (components).
- Metalwork: Steel profiles (IPE, HEA), sheet metal, bolts, welding consumables. Quantities in kg, pieces, and meters. BOMs frequently include cut dimensions and surface treatment specifications.
- Carpentry: Panels (plywood, MDF), hardware, glued timber, edge banding. BOMs are typically organized per piece of furniture or installation, with cut dimensions.
When an ERP makes sense and when it doesn't
Not every business needs an ERP just because they use bills of materials. In many cases, a clean, standardized BOM with clear responsibilities is sufficient to start.
An ERP typically becomes useful when:
- Purchasing, inventory, costing, and job management need to be tightly integrated.
- Multiple people work simultaneously with the same material data.
- Item master data, prices, suppliers, and stock levels need to be centrally maintained.
- The BOM is part of a larger overall process, such as serial production or project-based material planning.
For smaller businesses, the biggest hurdle is often not the software but data quality. An unclear or inconsistent BOM remains error-prone even in the best system.
Next steps
In practice, what matters less is which system is used, but whether the BOM is structured transparently and actually used day-to-day. A sensible start often looks like this:
- Identify your top 5 jobs. The ones that recur most frequently.
- List the parts. Item name, quantity per job, storage location.
- Check terms and units. Everyone involved must understand the same items the same way.
- Define a maintenance process. Who updates the list when materials or quantities change?
More on this topic in our guide on automating inventory management in 5 steps.
Conclusion
A bill of materials in the trades is not an industrial tool but a practical work aid. It creates clarity about material requirements, reduces dependence on individual knowledge, and simplifies recurring workflows. What matters is not whether a business immediately adopts an ERP, but whether the BOM is maintained, understood, and connected to the actual material flow in daily operations.
