Dev & Computing · 波特率（Baud Rate） · Updated 2026-10-25
What Is Baud Rate?
Definition
Baud rate counts symbols (signal changes) per second; bit rate counts bits per second. They coincide only when one symbol carries one bit — if a symbol encodes 4 levels (2 bits), a 9600-baud line actually carries 19200 bps. In embedded work, UART serial speeds (9600/115200) are baud rates; a mismatch between the two ends garbles every byte.
Formula
Bit rate (bps) = baud × log₂M (M = number of symbol states)
Worked Example
Two devices talk over UART, one at 115200 and the other at 9600: the receiver slices levels at the wrong rhythm and the log is all "?" marks. Setting both ends to the same baud is step one of serial debugging.
Common Misconception
Saying "9600 baud = 9600 bps" is coincidentally right for plain UART, but in QAM modulation (Wi-Fi, modems) one symbol carries 8-10 bits, so baud and bps differ by an order of magnitude and must not be interchanged.
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