Baud Rate Calculator

How the Baud Rate Calculator works

The baud rate calculator computes UART timing parameters including bit period, frame duration, prescaler register values, and baud rate error percentage for any system clock frequency. Embedded developers use it to configure UART peripherals on STM32, ESP32, AVR, and other microcontrollers, and to verify that the actual baud rate achievable with a given crystal matches the target within ±2% tolerance.

Standard UART baud rates

The most common UART baud rates are 9600, 19200, 38400, 57600, 115200, 230400, 460800, and 921600 baud. At 9600 baud, each bit is 104.17 µs wide. At 115200 baud, each bit is 8.68 µs. The UART protocol samples each bit at its midpoint, so the receiver's oscillator must be accurate enough that accumulated timing error across 10 bit-times does not exceed ±50% of one bit period.

Frame format and timing overhead

A standard 8N1 UART frame consists of 1 start bit + 8 data bits + 1 stop bit = 10 bits total. At 115200 baud, a single byte takes 86.8 µs to transmit. Including 2 stop bits (8N2) adds 8.68 µs per frame. At 9600 baud with 8N1, maximum throughput is only 960 bytes/second — because 2 out of every 10 bits are framing overhead, not payload data.

Prescaler calculation for STM32 and AVR

STM32 UART BRR = f_clock / baud_rate (with integer and fractional parts for USART2 on APB1). For a 16 MHz AVR with UBRR = (f_osc / (16 × baud)) − 1, a 115200 baud rate gives UBRR = 7.68 — rounded to 8, producing an actual rate of 111,111 baud and a −3.5% error, which just exceeds the safe threshold. The calculator shows the exact error so you can choose between double-speed mode (U2X=1) or a more suitable clock frequency.

Baud rate error and crystal selection

Common crystals of 8 MHz, 16 MHz, and 12 MHz do not divide evenly to produce standard baud rates. Using 11.0592 MHz — a non-standard crystal chosen specifically for UART accuracy — gives exactly 0% error at 9600, 19200, 38400, 57600, and 115200 baud. The calculator's error column quickly identifies which baud rate and clock frequency combinations are error-free versus which incur unacceptable drift.