I cannot find the answer from official document and also surprised no one asked the same question.

What is the default baud rates if I didn't call Serial.begin?

1 Answer 1


There is no default baud rate. When the sketch starts, the serial port is disabled (thus non-functional). Calling Serial.begin() enables the port.

  • Really appreciate! I just found even though I use Serial.println in loop function. The TX LED light won't blink unless I added Serial.begin in setup function. Commented May 8, 2022 at 8:52
  • 1
    Just for completeness: On some alternative boards that are also compatible with Arduino IDE (like e.g. ESP8266 and ESP32 boards), some information is transmitted on serial before your sketch even starts. The default baud rate then is dependent on the chip and/or board.
    – orithena
    Commented May 9, 2022 at 8:25

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