# What is a bill of materials?

> A bill of materials lists every part and quantity needed to build one unit of a product. Definition with a costed example and how Atta uses it.

Updated 2026-11-06. Source: https://atta.sh/glossary/bill-of-materials

A bill of materials, or BOM, is the list of components and quantities needed to make one unit of a product. Costing a BOM adds up the cost of each component, and it tells you what to buy before a production run.

## Example

A small company builds a sensor kit. One unit needs 1 circuit board ($6.00), 2 connectors ($0.85 each), 1 enclosure ($2.40) and 4 screws ($0.05 each).

Rolled-up material cost: 6.00 + 1.70 + 2.40 + 0.20 = $10.30 per unit. A run of 50 units needs 50 boards, 100 connectors, 50 enclosures and 200 screws.

## In Atta

An item can have a BOM. A production run (`buildItem`) is one transaction: it writes `consumed` rows to the stock ledger for each component and one `produced` row for the finished item at the rolled-up cost, keyed to the run so it can't happen twice. If a component is short, the run is refused with both figures.

A work order holds the same plan over time and consumes nothing until it is marked done. A kit is different: it's sold and never stocked, and selling one withdraws its parts.

## Levels

A BOM can nest. The sensor kit's circuit board might itself have a BOM of chips and resistors. Costing rolls up from the bottom, so a $0.10 change in a resistor price moves the kit's cost by the quantity used. Keep BOM quantities per one finished unit and scale them by the run size when you order.

## Questions

### What is the difference between a BOM and a kit?

A BOM is built into a stocked item. A kit is a bundle sold as one line and made of parts at the time of sale.