# Field types and safe field type conversion

*Platform App Builder Study Guide · Data Modeling and Management · Last updated October 25, 2026 · https://www.certifyforce.com/study-guide/platform-app-builder/data-modeling-and-management/field-types-and-safe-field-type-conversion*

How to pick the right custom field type for text, identifiers and locations, which type conversions lose data or are blocked, how to work around a formula reference, the 85 million conversion limit for deployments, and the picklist and global value set limits that decide when a lookup is the better design.

- Certification: Salesforce Platform App Builder
- Exam section: Data Modeling and Management
- Study guide: https://www.certifyforce.com/study-guide/platform-app-builder

**Official exam objective:**

> Given a scenario, determine the considerations when selecting or changing a field data type.

## What the exam expects

- Choose a custom field type from the data: length, formatting, leading zeros, location, generated numbers.
- Predict which conversions can lose data, and what else breaks when data is lost.
- Recognise conversions Salesforce blocks, and the workaround for a field referenced in a formula.
- Explain the 85 million conversion limit on deployments and package upgrades.
- Apply picklist, multi-select and global value set limits, and know when a lookup beats a picklist.

Key terms:
- **Field type conversion:** changing an existing custom field's data type; it runs in the background.
- **Restricted picklist:** accepts only admin-defined values, even through the API.
- **Global value set:** a restricted value list shared by picklist fields on many objects.
- **Compound field:** one field made of several stored parts, such as Geolocation.

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Text and Text Area | Up to 255 characters | [Custom Field Types](https://help.salesforce.com/s/articleView?language=en_US&id=platform.custom_field_types.htm&type=5) |
| Text Area (Long) | 256 to 131,072 characters; default 32,768 | [Custom Field Types](https://help.salesforce.com/s/articleView?language=en_US&id=platform.custom_field_types.htm&type=5) |
| Number | Leading zeros are removed | [Custom Field Types](https://help.salesforce.com/s/articleView?language=en_US&id=platform.custom_field_types.htm&type=5) |
| Geolocation | Counts as three custom fields | [Custom Field Types](https://help.salesforce.com/s/articleView?language=en_US&id=platform.custom_field_types.htm&type=5) |
| Auto Number | Max 30 characters; Text converts to it without data loss | [Converting Field Types](https://help.salesforce.com/s/articleView?id=platform.notes_on_changing_custom_field_types.htm&language=en_US&type=5) |
| Long text to Text | Truncated to the first 255 characters | [Converting Field Types](https://help.salesforce.com/s/articleView?id=platform.notes_on_changing_custom_field_types.htm&language=en_US&type=5) |
| After data loss | List views on the field are deleted | [Converting Field Types](https://help.salesforce.com/s/articleView?id=platform.notes_on_changing_custom_field_types.htm&language=en_US&type=5) |
| Lead-mapped field | Its type can't be changed | [Converting Field Types](https://help.salesforce.com/s/articleView?id=platform.notes_on_changing_custom_field_types.htm&language=en_US&type=5) |
| Deployment limit | 85 million custom field type conversions | [Converting Field Types](https://help.salesforce.com/s/articleView?id=platform.notes_on_changing_custom_field_types.htm&language=en_US&type=5) |
| Restricted picklist | 1,000 values, active and inactive combined | [Picklist Limitations](https://help.salesforce.com/s/articleView?id=platform.picklist_limitations.htm&type=5) |
| Global value set on over 13 objects | Deactivate only; no replace or delete | [Picklist Limitations](https://help.salesforce.com/s/articleView?id=platform.picklist_limitations.htm&type=5) |
| Multi-select import | Imported value replaces current selections | [Picklist Limitations](https://help.salesforce.com/s/articleView?id=platform.picklist_limitations.htm&type=5) |

## Check yourself

- Why does a Geolocation field use more of an object's field limit than a Text field?
- When can a Text field become Auto Number without losing data?
- What happens to images after a rich text field becomes a long text area?
- Why can a package upgrade that changes field types fail on a large object?

## Sources

- [Considerations for Converting the Field Type of a Custom Field](https://help.salesforce.com/s/articleView?id=platform.notes_on_changing_custom_field_types.htm&language=en_US&type=5)
- [Custom Field Types](https://help.salesforce.com/s/articleView?language=en_US&id=platform.custom_field_types.htm&type=5)
- [Picklist Limitations](https://help.salesforce.com/s/articleView?id=platform.picklist_limitations.htm&type=5)
- [Error 'Cannot change type due to existing data' when you update custom field data types](https://help.salesforce.com/s/articleView?id=000385523&language=en_US&type=1)
- [Object Relationships Overview](https://help.salesforce.com/s/articleView?id=platform.overview_of_custom_object_relationships.htm&type=5)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 15 linked practice questions, and unlimited timed mock exams are included with the Platform App Builder pack (USD $19): https://www.certifyforce.com/practice-exams