What is a Bill of Materials?
Definition
A Bill of Materials (BOM) is a structured, hierarchical list of every raw material, component, sub-assembly, and part required to manufacture one unit of a finished product. It defines what goes in, how much of each item is needed, and how each part relates to the finished output. It's the complete recipe for building your product.
Think of building a bicycle. A BOM would list every single part needed — the frame, handlebars, wheels, pedals, chain, bolts, brake cables, tyres, saddle, and every individual screw. Each component's exact quantity, specification and how it fits into the overall assembly would be defined. Before a single piece of steel is cut, the BOM tells everyone involved exactly what's needed.
In manufacturing, the BOM serves as the central source of truth that connects engineering, procurement, production and inventory. When a customer places an order, the BOM tells your MRP system what materials to check, what to order, when production should start and what the expected cost will be. Everything downstream depends on the BOM being accurate.
BOM hierarchy — how a product breaks down into its components
What does a BOM contain?
An effective BOM isn't just a parts list — it's a structured document that gives everyone from engineering to procurement to the shop floor exactly the information they need. Here are the essential fields every BOM should include:
Part name & part number
A unique identifier and human-readable name for every item. Part numbers enable unambiguous identification — especially when multiple parts look similar.
Quantity required
The exact quantity of each part needed to make one unit of the parent assembly. This is what MRP uses to calculate purchase requirements.
Unit of measure
Whether the quantity is measured in each, kg, metres, litres or another unit. Mixing up units of measure is one of the most common causes of BOM errors.
Manufacturer & supplier
Who makes or supplies each component. Critical for procurement, alternative sourcing, and managing supply chain disruptions.
Description & specification
Enough detail to distinguish this part from similar ones — dimensions, material grade, finish, tolerance. Prevents costly substitution errors.
Part phase / lifecycle status
Whether a part is active, in development, obsolete or being phased out. Using an obsolete part in production causes costly rework and delays.
Procurement type
Whether the part is bought (purchased externally), made (manufactured in-house), or kitted. This determines how MRP generates requirements for it.
Notes & revision history
Any additional context — why a specification changed, what to watch for during assembly, which revision this represents. Version control keeps everyone on the same page.
Single-level vs multi-level BOMs
BOMs can be structured in two main ways — and the right choice depends on your product's complexity:
Single-level BOM
A flat list of all components needed to make the finished product. Shows only the immediate components — not their sub-components. Simple, fast to create, suitable for straightforward assemblies.
Multi-level BOM
A hierarchical tree that shows every component at every level — from the finished product down through sub-assemblies to individual raw materials. Reveals parent-child relationships and enables MRP to plan materials at every level.
Industry best practice: Use a multi-level BOM for any product with sub-assemblies. It gives your MRP system the full picture it needs to plan materials correctly at every level. Single-level BOMs can work for very simple products but quickly become inadequate as complexity grows.
The four main types of BOM
Different departments need to use product information in different ways. That's why BOMs come in several types — each tailored to a specific use case:
Manufacturing BOM
mBOM — How the product is actually built
The BOM that drives production. It defines every component, sub-assembly, and material needed to build a shippable product — including packaging, consumables, and the sequence of operations. This is what Brytebuild uses to plan jobs and deduct stock.
Used by: Production, planning, MRP, inventory
Engineering BOM
eBOM — How the product is designed
Created by engineers during the design phase, often based on CAD drawings. Reflects design intent — what the product should be. The eBOM is the starting point; the mBOM adapts it for how the product is actually manufactured on the shop floor.
Used by: Engineering, design, R&D
Sales BOM
sBOM — What the customer receives
Used when a product is sold as a configurable bundle. The sales BOM defines what the customer receives and what options they can choose. Essential for made-to-order and configure-to-order manufacturing environments.
Used by: Sales, quoting, customer service
Service BOM
Service / Maintenance BOM
Lists the parts, consumables and instructions needed to maintain or repair a product after it's been delivered. Helps service teams identify required spare parts quickly and ensures products can be serviced throughout their operational life.
Used by: Service, maintenance, after-sales
Why a well-structured BOM matters
A BOM isn't just administrative overhead — it's the foundation that every other manufacturing process is built on. Get it right and everything downstream works smoothly. Get it wrong and the errors cascade through every system that depends on it.
Accurate procurement
With exact material requirements defined, purchasing can order the right quantities at the right time — eliminating both shortages that halt production and overstock that ties up capital.
Reliable cost estimation
With quantities and components listed, calculating the total material cost of a product becomes straightforward — enabling accurate quoting, margin analysis and pricing decisions.
Controlled inventory
MRP uses BOMs to automatically calculate what materials are needed and when. Stock deductions happen automatically as jobs are completed — keeping inventory counts accurate without manual reconciliation.
Reduced waste and errors
When production knows exactly what goes into each item, the risk of wrong components, incorrect quantities or missing parts drops dramatically — along with the rework and scrap those errors cause.
Faster quality investigations
When a product has a defect, the BOM immediately shows which component is likely at fault. If a specific component is consistently failing, the BOM trace reveals which batch to investigate.
Scalable production planning
With accurate BOMs, your production planning scales cleanly. Adding volume doesn't mean more manual calculation — the BOM tells the system everything it needs to expand seamlessly.
98%
Industry best practice requires 98% or higher BOM accuracy for effective MRP planning. Even a 2% error rate cascades through every calculation — wrong materials ordered, wrong quantities, wrong timing. A BOM that's almost right is almost useless to your MRP system.
What happens with a poorly managed BOM
Many manufacturers have BOMs — but manage them in spreadsheets, email chains or shared drives. The BOM exists on paper; the problem is that it's never quite current, never quite complete, and never quite accessible to the people who need it. The consequences compound over time:
Version chaos — which BOM is current?
When BOMs are managed in spreadsheets, it's almost impossible to know which version is the correct one. Production might be working from a three-month-old version while engineering has updated the design twice since then. Every batch built from the wrong BOM is a quality risk.
Wrong materials ordered — stockouts and overstock
Inaccurate quantities in the BOM mean MRP calculates the wrong purchase requirements. The result is chronic shortages of some components and excess stock of others — both of which cost money and disrupt production.
Production stoppages — a component missing that "should be there"
When a BOM doesn't list a component accurately, procurement doesn't order it. The first time this is discovered is when a production job starts and that component isn't in stock. The job stops. The order is delayed. The customer is unhappy.
Cost estimates that bear no relation to actual costs
An inaccurate BOM produces inaccurate cost estimates. You price jobs based on what you think goes in them — not what actually goes in them. The margin you thought you were making disappears on the shop floor.
BOM errors don't just affect the job they're discovered on. They affect every job that uses the same BOM — which means a single inaccurate entry can silently be causing problems across dozens of production runs before anyone notices.
How BOMs power your MRP system
A Bill of Materials on its own is useful. A Bill of Materials connected to an MRP system is transformative. Here's what happens when the two work together:
A customer order arrives
Your sales team books an order for 50 units of Product X. The order is logged in Brytebuild, triggering the planning process.
MRP explodes the BOM
Brytebuild multiplies the BOM quantities by 50. It knows that Product X requires 3 units of Component A, 12 units of Component B, and 0.5 kg of Material C per unit — so it calculates 150, 600 and 25 kg respectively.
Available inventory is checked
Brytebuild checks what's already in stock. If 80 units of Component A are available and 150 are needed, only 70 need to be purchased. The net requirement is calculated automatically.
Purchase orders and job cards are generated
Purchase orders are raised for shortfall materials. Production jobs are scheduled with the correct BOM items, work instructions, and quality checks attached. The team knows exactly what to make, what to order, and when.
Inventory updates automatically as jobs complete
When a production job is marked complete in Brytebuild, all components in the BOM are automatically deducted from stock. No manual adjustment, no spreadsheet update — the inventory is always accurate.
This is why BOM accuracy is so critical. If the BOM says a product needs 3 units of Component A but it actually needs 4, every single calculation in steps 2–5 is wrong — and the error is invisible until production starts.
How Brytebuild makes BOM management simple
In Brytebuild, creating and managing BOMs is built directly into the item management system. Set up an assembly-type item, add your components, and your BOM is live — connected automatically to scheduling, inventory, purchasing and your work instructions.
Easy BOM creation
Create a BOM in Brytebuild by setting up an assembly item and adding components. No complex configuration — built for manufacturing teams, not IT departments.
Multi-level sub-assembly support
Build complex multi-level BOMs with unlimited sub-assemblies. Each sub-assembly has its own BOM, and Brytebuild handles the explosion and netting automatically for every production job.
Version control built in
Update a BOM once and production immediately works from the new version. Full version history is retained — you can always see which BOM version was used to build any item.
Automatic inventory deduction
When a job is completed, all BOM components are deducted from stock automatically. Your inventory count is always current — without anyone having to update a spreadsheet.
Linked to work instructions
Attach work instructions directly to your BOM items. Operators see the relevant instruction for each component at the right step — BOM and work instructions working as one system.
Connected to scheduling & purchasing
Schedule a production job and Brytebuild automatically checks BOM requirements against current stock. Shortfalls trigger purchase order recommendations — no manual material planning required.
Best practices for BOM management
Start with a physical audit
Before digitising your BOMs, walk through a real production build and count every component. Compare it against what's listed. Most manufacturers discover discrepancies they didn't know existed.
Include scrap factors for consumable components
If production scraps 5% of a component during assembly, the BOM should list 1.05 per assembly — not 1.0. Ignoring scrap factors causes MRP to chronically under-order.
Assign a BOM owner for each product family
Every BOM needs one person responsible for its accuracy. Without clear ownership, changes slip through, the BOM drifts from reality, and no one is accountable for the errors.
Run a "where-used" check before changing a component
Before modifying a component specification, identify every BOM that uses it. Changing a raw material spec can affect dozens of finished products. Unexpected ripple effects cause production problems.
Implement a formal change process
Every BOM modification should be logged, reviewed and approved before it goes live in production. Informal BOM changes made on the fly are one of the leading causes of manufacturing quality issues.
Set lifecycle status on every component
Mark components as active, in development, or obsolete. Using an obsolete component because the BOM wasn't updated after it was discontinued is an embarrassingly common and entirely preventable mistake.
Use MRP software — don't manage BOMs in spreadsheets
A BOM in a spreadsheet is disconnected from procurement, inventory and production. MRP software connects your BOMs to every other system, making accuracy automatic rather than a discipline exercise.
Audit BOMs regularly against real production
Set a schedule to physically verify your top BOMs every quarter. Pick your highest-revenue products and compare the system BOM against what actually goes into a build. Fix discrepancies immediately.
Common questions about BOMs
They're functionally the same thing — and many food manufacturers literally call their BOM a recipe. Both define what goes into a product and in what quantities. The term "Bill of Materials" is standard in discrete manufacturing (components, assemblies) while "recipe" is common in process manufacturing (food, pharmaceuticals, cosmetics). In Brytebuild, both work identically.
In Brytebuild, you create a BOM by setting up an "assembly" type item in your item library and then adding the component items to it with their quantities and units. Sub-assemblies are themselves assembly items — so multi-level BOMs build naturally. The BOM is then automatically available to your scheduling, inventory and purchasing modules.
In Brytebuild, updating a BOM component immediately reflects across all future jobs that use it. The old version is retained in history, so you can always see which version was used for any given production run. Before changing a component, use the where-used report to identify every product BOM affected by the change.
Yes — BOM management is included in all Brytebuild plans, including Guide Only (from £11/user/month). You can create and manage unlimited BOMs with multi-level sub-assemblies on any plan. The 14-day free trial gives you full access to build and test your BOMs before committing.
Brytebuild supports multi-level BOMs with unlimited depth. You can nest sub-assemblies within sub-assemblies as deeply as your product structure requires. Each sub-assembly is itself an assembly item with its own BOM, and Brytebuild handles the full explosion through all levels automatically when planning production.
Ready to build your first BOM in Brytebuild?
Full BOM management included on all plans. 14-day free trial — no setup fees, no card required.