Arduino / ESP32 Baud Rate & UART Error Calculator
Pick your MCU's clock, type your desired baud rate, and see the actual baud rate the hardware will generate — plus the error percentage and whether it's safe.
Calculator
Results update as you type.
Examples
Pick a preset to populate the calculator above.
About this tool
Why UART baud rates aren't always exact
A UART doesn't generate your requested baud rate directly — it derives it from the MCU's clock by dividing down to the nearest whole number, since most hardware dividers can only count in integer steps. That rounding means the actual bit rate is usually slightly higher or lower than what you asked for. Classic AVR chips (Arduino Uno, Nano, Pro Mini) divide their clock by 16 first and then by an integer UBRR register, which is why 16 MHz famously can't hit 115200 baud exactly. Chips like the ESP32 use a more flexible clock divider (and on newer silicon, a fractional one), which is why ESP32 boards rarely show any meaningful UART error at all.
What this calculator does
You pick your MCU's clock source (or enter a custom clock) and type the baud rate you want. It works out the actual integer divisor the hardware would use, calculates the real baud rate that divisor produces, and reports the percentage error between what you asked for and what you'll actually get.
Why 2% matters
UART has no clock line — both ends just count bit-time based on their own local baud rate, so any mismatch stacks up bit by bit across each byte. The widely used rule of thumb is that error under roughly ±2% is safe, since standard 8N1 framing has just enough margin to still correctly sample the last bit of a byte. Go past that and you start seeing scrambled characters, especially at higher baud rates where each bit period is shorter and the same percentage error eats a bigger chunk of the sampling window.
Who should use it
Anyone wiring up serial communication between an MCU and a sensor, GPS module, PC, or another microcontroller — especially when picking a baud rate for an 8 MHz or 16 MHz AVR chip, where far more rates land outside safe tolerance than people expect.