What is a bill of materials?
A bill of materials is a structured list of the components, quantities and expected wastage required to produce one unit of a finished product. It is used to plan what to buy, to consume the right stock when production is posted, and to calculate what a unit actually cost to make.
What it contains
Each component, the quantity per unit of output, the unit of measure, and usually an allowance for normal wastage. Labour and overhead may be attached as well, which is what turns a parts list into a cost sheet.
Multi-level bills
A component can itself be a manufactured item with its own bill of materials. Costs roll up through the levels, so a finished assembly reflects what its sub-assemblies really cost rather than an assumed transfer price.
Planned versus actual
The BOM says what a run should consume; the production voucher records what it did. The variance between them is the most useful number a small factory produces, and it only exists if the BOM is maintained.
Not just for factories
Assembly, kitting and even a restaurant recipe are one-level bills of material. Anywhere inputs become a different output, a BOM is the thing that makes the output’s cost knowable.
Related questions
Do I need a bill of materials for every product?
No, and starting with all of them is how the exercise stalls. Write them for the highest-volume products first — they usually account for most of the cost and most of the variance.
Can a BOM include labour and overhead?
Yes, and it should if you want a cost per unit rather than a materials cost. Material alone answers only part of the question.
Is a recipe a bill of materials?
Functionally yes — a one-level BOM where inputs become a different output. That is how restaurant food costing works.
More definitions
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