# Build the bill of materials (/docs/product-development/bill-of-materials)

The bill of materials (BOM) lists every item a product consumes, per 1,000 pieces. Stock, suppliers,
incoming inspection and the material issue to a manufacturing order all follow from it. The engineer
(Daniel Tan) writes it. It can only be edited while the product, or the production method it belongs
to, is a **Draft**.

```mermaid
%% title: When the bill of materials can change
flowchart TD
  A["Product or method is Draft"] -->|"Add, edit, delete lines"| B["Lines saved"]
  B --> C["Submit for review"]
  C --> D["In review or Released"]
  D -->|"Any change"| E["Refused: start a new revision"]
  D -->|"New revision under an approved change note"| A
```

## Add components [#add-components]

<Steps>
<Step>

Open the product (for example `/r/PROD-A`). On the **Engineering** tab choose **Setup**, then choose
**Components** in the left rail. You can also open an operation and choose its **Components** tab.

</Step>
<Step>

Choose **+ Component**. If you added it from an operation, **Used at op** is filled in. Otherwise
choose the operation from the list of the production method's operations, for example
`10 · Moulding`; the list opens once the product or production method is chosen, and changing the
method clears it.

</Step>
<Step>

The dialog shows the product and production method it is for under its title, and gives the
**Line** the next free number. Choose the **Material or component**: each choice shows its item code,
unit and type. The **Unit** fills in from the item; another unit needs an approved unit conversion.
Enter the **Qty per 1,000 pcs** (above zero, not more than 100,000,000). The line under it works the
quantity out for you, for example "That is 0.025 g for each part. An order of 5,000 pcs needs 125 g."

</Step>
<Step>

When you add the line from the Bill of materials list instead, choose the **Product**, and the
**Production method** if the line belongs to one method. Leave the method empty for a line every
method of the product shares. Leave **Used at op** empty to issue the material at the start of the
order.

</Step>
<Step>

Save. Each line shows as `PROD-A · line 10`. A product can have only one line with the same number for
the same method.

</Step>
</Steps>

<GuideStep
  title="Components of an operation"
  description="The Components tab of the operation drawer lists what the operation consumes per 1,000 pieces."
  src="/guide/bill-of-materials/1.png"
  alt="Operation drawer with the Components tab selected showing the line, item and quantity per 1,000 pieces"
  width={1440}
  height={1000}
  callouts={[
    { x: 60, y: 14, caption: "The drawer tabs. Components is the last tab." },
    { x: 60, y: 35, caption: "Each component shows its line number and quantity per 1,000 pcs." },
    { x: 60, y: 80, caption: "+ Component adds a line used at this operation." },
  ]}
/>

## Units and unit conversions [#units-and-unit-conversions]

The **Unit** is the stock unit of the item. If a supplier sells the item in packs, purchasing and
receiving convert with an approved unit conversion. See
[Set up items and products](/docs/product-development/items-and-products#units-and-unit-conversions).

## What a released product does to its bill of materials [#what-a-released-product-does-to-its-bill-of-materials]

The BOM follows the document it belongs to.

- While the product is a **Draft**, you can add, edit and delete lines.
- Once the product is **In review**, **Released** or **Obsolete**, every line is read-only. The message
  names the product and its revision, for example `Product PROD-A rev A is in review. Start a new
  revision to change it.`
- A line that names a production method is locked with that method. A **Draft** method that revises a
  released one opens its own lines. A brand-new **Draft** method on a released product opens the lines
  that name it, but not the product's own.
- To change a released bill of materials, raise an engineering change note and start a new revision, see
  [Change a released product](/docs/product-development/engineering-changes).

A product cannot be submitted for review without at least one BOM line. See
[Release the product documents](/docs/product-development/document-release).

If the system refuses a step here, see [Product development: why the app refuses](/docs/product-development/refusals#build-the-bill-of-materials).

<NextSteps>

<NextStep href="/docs/product-development/methods-and-routings" title="Define the production method" description="Add the operations, machine options and parameters the components are used at." />

<NextStep href="/docs/product-development/items-and-products" title="Set up items and products" description="Create the items the lines point to." />

<NextStep href="/docs/product-development/refusals" title="Product development: why the app refuses" description="Every message on these pages and what to do." />

</NextSteps>