Custom FIT fields save selected Data Field values into the activity file produced by the watch. Use them when the displayed value should be available after the activity, such as a computed pace, power target, lap metric, or session summary.

Message types

Each FIT field writes to one message type:

Record: samples during the activity according to Garmin's recording behavior.
Lap: writes the latest selected value when a lap is completed.
Session: writes the latest selected value when the activity session is reset or completed.

Record fields are not a sub-second event log. Smart Recording and the watch runtime decide when activity records are persisted.

Field schema

Each custom FIT field has:

A stable internal ID used by the project.
A display name.
A Connect IQ field ID.
A FIT data type: float, signed integer, unsigned integer, or string.
Units.
A record, lap, or session message type.
A primary value or formula binding.
Missing-value behavior.
An update policy.
Minimum API and device capability requirements.

Studio validates field ID collisions, duplicate names within a message type, units, value type, numeric range, message type, and device support before generation.

Null and type behavior

Choose Omit write when missing data should not create a FIT value. Choose Write zero only when zero is a meaningful fallback for the metric.

Numeric FIT fields require numeric formulas. String FIT fields require text formulas. Unsigned integer fields omit negative values instead of writing invalid data.

Compatibility

Custom FIT contribution depends on the target watch and Connect IQ API level. If the selected device does not support a message type or data type, Studio blocks the build and identifies the affected FIT field.

Synced activity platforms decide which custom FIT fields they display. A successful build and install does not guarantee that every third-party service will show every custom field.

Verification checklist

Before relying on custom FIT data:

Preview missing-data, lap, reset, and workout scenarios.
Build for the exact watch model.
Run a short test activity on the device.
Sync the activity and check where custom fields appear.
Keep field IDs stable after release so future builds do not reinterpret old data.