Baud Rate Calculator

Enter your microcontroller's clock frequency, desired baud rate, and oversampling factor to compute the UART baud rate register (UBRR) value, the actual baud rate the hardware will generate, and the resulting error.

Quick Facts

Formula
UBRR = round(F_OSC / (N x Baud)) - 1
Actual baud rate = F_OSC / (N x (UBRR + 1)); N is the oversampling factor, 8 or 16.
Tolerance
Keep error under about 2%
Most UART receivers need error below roughly 2% to decode framed bytes reliably.

Your Results

Calculated
UBRR register value
-
Load this into the UART baud rate register
Actual baud rate
-
Rate the hardware actually generates
Baud rate error
-
Deviation from your desired rate
Bit time
-
Duration of one bit at the actual rate

Ready

Enter your clock frequency and desired baud rate, then press Calculate.

Understanding the Baud Rate Calculator

This tool computes the values a microcontroller's UART (Universal Asynchronous Receiver/Transmitter) hardware needs to generate a target baud rate from a fixed clock, or oscillator, frequency. Because UART baud rate generators divide the clock by a whole number, the baud rate you actually get is almost always a close approximation of the baud rate you asked for — this calculator shows you exactly how close, and what register value produces it.

The formula

For the standard asynchronous UART baud rate generator used in AVR, PIC, and many other microcontroller families, the baud rate register value is:

UBRR = round(F_OSC / (N × Baud)) − 1

where F_OSC is the clock frequency in hertz, Baud is the desired baud rate in bits per second, and N is the oversampling factor — 16 for standard speed or 8 for double-speed (U2X) mode. Because UBRR must be a whole number, the register value is rounded, and the actual baud rate the hardware produces is found by reversing the formula:

Actual baud rate = F_OSC / (N × (UBRR + 1))

The percent error between the actual and desired baud rate is (Actual − Desired) / Desired × 100. A smaller error means the receiving device's sample point stays closer to the center of each bit.

Reading the results

The UBRR value is what you load into the hardware's baud rate register. The actual baud rate is the real bit rate the UART will transmit and receive at, which may differ slightly from your target. The error percentage tells you how far off that is, and the bit time is the duration of a single bit at the actual rate (1 ÷ actual baud rate), which is useful when measuring a signal on an oscilloscope or logic analyzer.

Common clock and baud rate combinations

Common oscillator frequencies for microcontrollers include 8 MHz, 11.0592 MHz, 16 MHz, and 20 MHz — some of these, like 11.0592 MHz, exist specifically because they divide evenly into standard baud rates, producing 0% error. Standard baud rates in serial communication are 1200, 2400, 4800, 9600, 19200, 38400, 57600, and 115200 bps; 9600 and 115200 are the most common defaults for debugging consoles and general-purpose serial links.

Getting accurate results

  • Use the exact clock frequency your microcontroller or crystal actually runs at, not a rounded value — small differences here directly change the error percentage.
  • Try both 16x and 8x oversampling if your error is too high; one setting sometimes divides more evenly into your clock and baud rate than the other.
  • If the error stays above about 2%, consider a different standard baud rate or a crystal frequency chosen to divide evenly, such as the common 11.0592 MHz choice.

Always confirm the final UBRR value and its acceptable error range against your specific microcontroller's datasheet, since some families define the formula, register width, or maximum tolerable error slightly differently.

Frequently Asked Questions

What is the UBRR register and why do I need it?
UBRR (USART Baud Rate Register) is the divisor value loaded into a microcontroller's UART hardware so it can derive the desired baud rate from its clock. It is calculated as UBRR = round(F_OSC / (N × Baud)) − 1, where F_OSC is the clock frequency and N is the oversampling factor (8 or 16). The hardware then generates the actual bit clock from that integer value.
Why is there an error between my desired and actual baud rate?
UBRR must be a whole number, but F_OSC / (N × Baud) − 1 is rarely a perfect integer. Rounding to the nearest whole number produces a small mismatch between the baud rate you asked for and the rate the hardware actually generates. This rounding error grows as a percentage when the clock frequency does not divide evenly by the chosen baud rate.
How much baud rate error is acceptable?
A commonly used guideline for asynchronous UART links is to keep the cumulative error under about 2%. Beyond that, the receiver's sample point can drift far enough from the center of each bit that framing errors and misread bytes become more likely, especially on longer data frames or noisy connections.
What is the difference between 8x and 16x oversampling?
Oversampling is how many times the receiver samples each incoming bit to determine its value. 16x oversampling, the default on most UARTs, samples more often per bit, which usually gives finer resolution and lower error at low-to-moderate baud rates. 8x oversampling, often called double-speed or U2X mode, samples half as often per bit, which can produce a closer baud rate match at high speeds relative to a slow clock, at the cost of a smaller timing margin per sample.

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