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BLE ATT MTU: Size, Limits & Exchange Explained

By RF Wireless Expert Team

In the context of the Attribute Protocol (ATT) and Generic Attribute Profile (GATT), the ATT MTU defines the maximum size of an ATT packet (in bytes) that can be sent between a Client and a Server in a single transaction.

  • The Default Limit: By default, the starting ATT MTU size for a BLE connection is exactly 23 bytes. Because the ATT protocol header takes up 3 bytes (1 byte for the operation code, 2 bytes for the attribute handle), the default maximum payload of actual user data you can send in a single packet is 20 bytes.
  • Exchanging MTU Size: If an application needs to send larger chunks of data (to avoid the overhead of breaking data into 20 byte pieces), the devices can negotiate a larger packet size. The Core Specification defines a procedure called “Exchange MTU” (noted in the GATT feature requirements).
  • The Negotiation: During the connection setup, the Client can send an “Exchange MTU Request” to the Server, stating its maximum supported MTU (up to a theoretical maximum of 512 bytes). The Server responds with its own maximum capability. Both devices then agree to use the smaller of the two values for the remainder of the connection, allowing for much larger data payloads per packet.

References & Further Reading

  1. Bluetooth SIG : Bluetooth Core Specification Version 6.3, May 5, 2026.
  2. Bluetooth SIG : Bluetooth Technology Overview

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