Heart Rate Measurement Format

Use this page when you need the `0x2A37` payload structure, flags, and RR conversion details.

Heart rate data uses the standard Bluetooth SIG Heart Rate Measurement characteristic (0x2A37) on the Heart Rate Service (0x180D). This is not Polar-specific — any BLE heart rate monitor uses this format.

Payload Layout

[flags] [heart_rate ...] [rr_interval ...] [rr_interval ...]

Flags Byte (byte 0)

Bit	Meaning
0	HR format: 0 = UINT8, 1 = UINT16
4	RR intervals present: 0 = no, 1 = yes

Other flag bits (sensor contact, energy expended) are defined by the Bluetooth SIG spec but are not used in our implementation.

Heart Rate Value

If flag bit 0 = 0: a single uint8 at byte 1 → HR in BPM
If flag bit 0 = 1: a uint16 little-endian at bytes 1–2 → HR in BPM

RR Intervals

If flag bit 4 is set, the remaining bytes after the heart rate value contain one or more uint16 little-endian RR interval values.

Each value is in units of 1/1024 seconds. To convert to milliseconds:

rr_ms = raw_value × 1000.0 / 1024.0

Multiple RR intervals can appear in a single notification when multiple beats occurred between transmissions (the Polar H10 typically sends HR notifications once per second).

Typical Behavior

The Polar H10 sends heart rate notifications approximately once per second. The heart rate value reflects the current calculated BPM, while the RR intervals provide the precise inter-beat timing for the beats that occurred since the last notification.

Notes

The HR format flag (8-bit vs 16-bit) can change between notifications. Always check the flags byte for each notification.
An empty notification (no RR intervals, HR = 0) can occur during the initial connection phase before the sensor locks onto a reliable signal.