The recipe for a product
A bill of materials, or BOM, is the list of everything needed to make one unit of a product: each raw material, component and packing item, with the quantity used per unit. It is the manufacturer’s recipe, and in a restaurant the same idea is simply called a recipe.
Everything downstream reads from it. Purchase uses it to know what to buy, the store to know what to issue, costing to know what a unit should cost, and planning to know whether an order can be made with the stock on hand.
What a BOM contains
Beyond the list, a useful BOM also records the unit of measure for each line, the wastage or scrap allowance, and which operation consumes each component, so material can be issued at the right stage.
| Component | Quantity per unit | Notes |
|---|---|---|
| MS sheet, 2 mm | 0.45 kg | Includes a 5% cutting wastage allowance |
| M8 bolt | 2 nos | |
| Powder coating | 0.02 kg | Applied at the coating operation |
| Carton | 1 no | Packing |
Single-level and multi-level BOMs
A single-level BOM lists the direct inputs to one product. A multi-level BOM goes further: when an input is itself made in-house, such as a sub-assembly, it has its own BOM, and the levels nest like a tree.
Multi-level structures matter for planning, because a sales order for the finished product creates requirements at every level below it. They also matter for costing, because the cost of a sub-assembly rolls up into the product above it.
BOM and routing
The BOM says what goes into a product; the routing says how it is made: the sequence of operations, the work station for each, and the time each takes. Together they give both the material cost and the conversion cost of a unit, and they tell the floor what to do in which order.
Keeping BOMs honest
- Version the BOM. When a design or a supplier changes, create a new version rather than editing the old one, so past orders keep the recipe they were made with.
- Compare actual consumption with the BOM on every order. A persistent gap means either the BOM or the floor practice is wrong.
- Record wastage as an allowance in the BOM, not as an unexplained difference at stock count.
- Give one person ownership of BOM changes, so the store, costing and planning are never working from different recipes.
In short
- A BOM lists every input and its quantity per unit of a product; it is the manufacturer’s recipe.
- Purchase, stores, costing and planning all read from it, so a wrong BOM spreads everywhere.
- Multi-level BOMs nest sub-assemblies; routing adds the operations and work stations.
- Version BOMs and compare consumption against them on every order.