What is a Bill of Materials?


If you work in manufacturing, you’ve likely heard the expression bill of materials. But what exactly is a bill of materials and what is it for?


A bill of materials (BOM) is a meticulous list of every material and/or pieces needed to complete a certain end product or action. You can think of it as a list of ingredients.


When Should You Use a Bill of Materials?

There are two main types of BOMs. There are manufacturing bills of materials (mBOM), which list all the necessary materials to manufacture a certain product. The mBOM represents the product “as shipped”.


For example, a mask manufacturer can use a BOM to list the amount of fabric, thread, elastic and filters that it needs for each product. Gathering all the information in a single list improves stock management, schedule production, forecast output, and helps set realistic delivery deadlines. 


Engineering bill of materials (eBOM), the second most common type of BOMs, are used to plan possible substitutes and alternates. Manufacturers treat eBOM as the product “as designed”.


In this case, we look at our list of ingredients and replace with others that achieve the same result, but adapted to different needs – such as replacing dairy with non-dairy options, for example. It’s because of eBOMs that you usually find interchangeable parts for various models of cars, printers, computers, and so on. Stock managers appreciate the effort!


However, technically speaking, almost every product and service may have a BOM. Imagine managing check-ins at a hotel, for example. Every time a guest checks in, housekeepers need a list of materials to get the room ready for them – a pair of slippers, a bathrobe, a few bottles of shower gel, shampoo, a shower cap, and a set of towels, among other things. You probably already have something similar in place. If so, congratulations – you just made your first BOM.


Why are Bills Of Materials Important In Maintenance?

Now let’s get to the heart of the matter. How can bills of materials be useful in maintenance?


BOMs are extremely useful to streamline maintenance work. Imagine that an asset fails, and your staff doesn’t have a list of materials to fix it, or (worse!) they still have to check the manufacturer’s guidebook to understand what to do. They will have to rely on confusing diagrams to find the correct part number, and they may not have the right tools with them. Suddenly, corrective maintenance begins to feel like you’re assembling IKEA furniture. 


Because you don’t want your technicians to spend hours trying to decode diagrams, looking for the right tools, and working in vain, you can organise bills of materials in your CMMS. You can then link them to each work order, and technicians can easily check what they need for each asset. It’s the same with routine preventive maintenance orders. If you have an updated BOM and a spare part is out of stock, the task is suspended – instead of noticing it when maintenance work is already underway. 


Right at the beginning, we compared a BOM to a list of ingredients. Now, try to remember the last time you forgot a key ingredient for that recipe you wanted to cook. You had everything planned for chicken curry but… forgot the curry? You were about to make some spicy Mexican food and… forgot the chili? Regardless of the missing piece, you probably ended up ordering dinner, which cost you a lot more than organising your pantry and making a shopping list, which are free.


In maintenance, it’s pretty much the same. You can organise your materials, manage  stocks, spot discontinued products and interchangeable parts with a BOM. Besides, you’re able to calculate meticulously the cost of raw materials for each product or service that you’re offering.


How to Prepare a Bill of Materials

So far, we have explained what BOMs are, why you should use them, and how they fit into your maintenance work. But – there’s always a but – you must be very thorough when preparing these bills. If a bill of materials is wrong or incomplete (missing components, errors in the stock count, the price of each component is not accurate, specifications are lacking, etc.), you risk shutting down production completely. 


Whenever technicians use an incomplete bill of materials, their work will take much longer to finish. They will waste time trying to understand how the asset operates, identifying the missing parts or tools and, obviously, searching for them. If there is a stockout, tasks might stay pending for a long time, which increases the MTTR and the size of your backlog. 


Because all of this is the exact opposite of what we want, be sure to include all of this data in your bill of materials (use all that apply):


  • part name 
  • identification number in the CMMS
  • description (specify what type of intervention is needed)
  • number of parts needed
  • unit of measurement and ratio (i.e., x ml of lubricating oil)
  • weight
  • dimensions
  • manufacturer
  • manufacturer’s product number
  • preferred supplier
  • substitutes
  • price estimate
  • other specifications (i.e., fitting type, colour, etc.)


This information is usually organized in a chart or tree diagram. In this case, it starts from the global asset and moves on to each component. Tree diagrams are a very common structure in engineering BOMs.


Bill of materials | Estrutura de produtos

Source: Techtarget


As an example, here’s a BOM for a guitar tuner on a chart:


Bill of Materials example


The style (chart or diagram) you should select depends on the number of components you need to list, and your team’s work habits. In any case, as long as you don’t forget any of the information we listed above, it’s guaranteed that your work orders will become much more efficient.