> ## Documentation Index
> Fetch the complete documentation index at: https://docs.brudcast.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Custom fields
> Define your own contact data, pick a field type, fill it in on a contact, and use the field as a merge tag.
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Custom fields hold data Brudcast has no built-in place for: a customer's plan, their city, their
renewal date. Define a field once, then fill it in on each contact and use it as a merge tag.
**Before you start:** a field's key and type are fixed the moment you create it, so decide what
the field holds before you add it. Values can't be brought in by a CSV import; you fill them in on
each contact, collect them through a form, or write them through the API.
## Create a field
Go to **Contacts > Custom Fields** and select **Add Field**. Enter a **Field Name**, such as
"Company Size". This is the label people see on the contact page.
The **Field Key** is generated from the name, for example `company_size`. It's how you refer to
the field in merge tags and the API. A key starts with a lowercase letter and holds lowercase
letters, digits and underscores, up to 100 characters. Each key must be unique in your
organization.
Pick a **Field Type**, then select **Create Field**.
## Field types
| Type | What it stores | Editor on the contact page |
| --------------------- | -------------------------------- | -------------------------- |
| **Text** | Any text | A text box |
| **Number** | A number | A number box |
| **Date** | A calendar date | A date picker |
| **Boolean** | Yes or no | A switch |
| **Select (Dropdown)** | One value from a list you define | A dropdown |
A **Select (Dropdown)** field needs at least one option and can have up to 50. You can also give a
field a **Description** and mark it **Required**.
You can't change a field's key or type after you create it. To change either, create a new field
and copy the values across. Everything else about a field, such as its name and description, can
be edited later.
The key and type are fixed because they're what everything else points at: merge tags in templates
and campaigns, form mappings, and your own code calling the API. Renaming a key would quietly break
every message using it, and a tag that no longer matches a field sends as empty text rather than
failing loudly.
## Fill in a value
Open a contact and select the pencil icon on the **Custom Fields** card on the **Overview** tab.
Values can also arrive on their own:
* **From a form.** Map a question to **Custom field** so answers land on the contact. A field only
accepts questions that produce the right kind of value. See
[Map answers to contact fields](/forms/create-a-form#map-answers-to-contact-fields).
* **From a migration.** Custom fields come across from your old provider before your contacts do, so
contacts land with their values. See [Run a migration](/migrations/run-a-migration).
* **From your app.** Write values through the [Platform API](/api-reference/introduction).
A CSV import is the exception: it maps the built-in columns only, so custom field values can't be
imported from a spreadsheet.
## Use a field as a merge tag
Each field's key works as a merge tag, such as `{{company_size}}`. Insert one with **Insert
variable** wherever you write copy, and it's filled in per recipient when the campaign sends.
Five tag names are reserved and always come from the contact record: `first_name`, `last_name`,
`full_name`, `email` and `unsubscribe_url`. A custom field can't take one of them over. See
[Placeholders and merge tags](/templates/placeholders-and-merge-tags).
## Related
Personalize a message with the fields you just created.
Fill in a contact's fields, tags and notes.
Collect custom field values from the people who sign up.