Bill of materials (BOM): meaning, format and example

What a bill of materials is

A bill of materials, or BOM, is the list of everything needed to make one unit of a product, with the quantity of each item. It is the recipe the factory works from. Stores use it to issue material, purchase uses it to know what to buy, and accounts use it to work out what the product costs.

What goes in a BOM

A usable BOM has, for each line:

• Component. The raw material, bought-out part or sub-assembly.
• Quantity per unit. How much one finished piece consumes.
• Unit of measure. Kilograms, metres, numbers. The unit must match how you stock the item.

And for the BOM as a whole:

• The product it makes, and the quantity the BOM is written for (usually 1).
• Operations, if you track them: the steps and the work centre for each.

A worked example

A machining shop makes a brake caliper bracket from EN8 steel bar.

To make 25 brackets, the order needs 25 × 2 = 50 kg of bar. If 100 kg is in stock, 50 kg is left afterwards, and a second order for 15 brackets needs another 30 kg.

A product with more parts looks the same, with more lines:

The last line means one carton holds 20 units.

Single-level and multi-level BOMs

A single-level BOM lists only what goes directly into the product. A multi-level BOM has components that are themselves made in-house and have their own BOMs. In the example above, the machined bracket is a sub-assembly with its own BOM of steel bar. Keep each level as its own BOM; do not flatten everything into one list, or you cannot stock or cost the sub-assembly.

Common mistakes

1. Wrong unit. The BOM says 2 and stores read it as 2 metres when it meant 2 kg. Write the unit on every line.
2. Ignoring scrap. If 2.0 kg of bar yields a 1.7 kg part, the BOM quantity is 2.0, not 1.7. Put in what is consumed.
3. Leaving out small items. Fasteners, labels and packing run out precisely because nobody counted them.
4. One BOM, several versions. When the drawing changes, make a new BOM and retire the old one, so old orders still show what they actually used.

From spreadsheet to ERP

A BOM in a spreadsheet is a document. A BOM in an ERP does work: raising a manufacturing order for 25 units reserves the right quantity of each component, shows what is short, and reduces stock when production is recorded. See how production works in ARUKZ ERP.