What a bill of materials needs to do in the trades, which types exist, and where Excel, ERP, and digital inventory processes hit their limits in practice.
TL;DR
Bill of materials (BOM) = all parts + quantities for a product or job. 4 types, one goal.
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.
Four basic types shape day-to-day practice:
| Type | Structure | When it fits |
|---|---|---|
| Quantity BOM | All parts unsorted, no hierarchy | Quick purchasing, manageable part count |
| Structural BOM | Hierarchical by assembly and production stage | Job with multiple sub-assemblies |
| Modular BOM | Each assembly as its own reusable list | Recurring assemblies across jobs |
| Variant BOM | One list, base configuration plus variant columns | Same product in multiple versions |
The quantity BOM is the simplest form. It lists all parts unsorted with their total quantity, no hierarchy, no assemblies.
When it fits: For standardized workflows with a manageable number of parts, where sequence doesn't matter and fast purchasing is the priority.
Example: Installing a washbasin. The quantity BOM contains: 1x washbasin, 1x faucet, 1x drain assembly, 2x angle valves, 2x flex hoses. Nothing more, nothing less.
The drawback: as soon as a job consists of multiple assemblies or parts lists need to be reused, the unsorted list quickly reaches its limits.
The structural BOM maps assemblies and production stages. Parts are ordered hierarchically by their installation sequence or assembly membership.
When it fits: When a job consists of multiple sub-assemblies and the sequence or responsibility during material preparation matters.
Example: A switchboard job contains the assemblies "DIN rail with breakers," "terminal block," and "cable duct." Each assembly has its own parts list. The structural BOM shows which parts belong to which unit.
A modular BOM keeps each assembly as a standalone, separately stored list. When a job consists of multiple assemblies, the individual lists are combined, not merged into one overall list.
Characteristics of the modular BOM:
The modular principle: Instead of maintaining one large overall list per job, you manage reusable building-block lists. A plumbing business might keep separate modular BOMs for "manifold," "pipe installation," and "controls." If an assembly changes, for example a component is replaced by the supplier, you update only that one modular list. All jobs using that assembly benefit automatically.
Advantage over quantity and structural BOMs: changes only need to be maintained once. No duplicate entries, no risk of outdated parts lists in individual jobs.
Typical use: for recurring assemblies that appear across different job types. Relevant in machine and plant construction, but also in businesses with standardized installation packages or serial production.
Distinction from the structural BOM: the structural BOM shows the assembly hierarchy of a single job. The modular BOM is modular: each assembly lives as a standalone list and is reused across jobs.
The variant BOM combines multiple product variants in a single list. A base column holds the standard configuration, additional columns describe the deviations per variant.
When it fits: When the same product is manufactured in different versions, e.g. a cabinet in three sizes or a system in two expansion stages. For one-off jobs or low variant counts, the effort rarely pays off.
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.
| 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 |
I see this again and again in conversations with businesses across different trades: 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.
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. I regularly see this as a sensible starting point for small businesses. You copy part numbers and quantities and have your basis.
But Excel hits a hard limit once things become operational:
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.
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.
Which approach makes sense depends less on the buzzword "ERP" than on item count, team size, repetition rate, and desired inventory accuracy.
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.
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 |
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.
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.
Depending on the trade, bills of materials look quite different:
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:
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.
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:
More on this topic in our guide on automating inventory management in 5 steps.
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.
repleno Founder
Christoph worked as an electronics technician in industry for five years and experienced firsthand how missing small parts can slow down processes. Later, as a project manager at P.S. Cooperation GmbH (Böllhoff Group), he introduced digital procurement processes for recurring parts at medium-sized companies and corporations. Today, he is building repleno to largely automate the procurement of consumables in small businesses.
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