UART Baud Rate Error Calculator

Work out uart baud rate error instantly with clear inputs, formula shown and shareable results.

About UART Baud Rate Error Calculator

The UART divides its clock by an integer, so most clock and baud combinations cannot be hit exactly. Rounding the divisor produces an error, and since the receiver samples the stop bit after ten bit times, accumulated error must stay under roughly 2% to avoid framing errors.

Formula

Baud divisor

divisor = round(f / (baud x oversampling)); error = (actual - target) / target

Worked example

With the default values below, UART Baud Rate Error Calculator returns Baud rate error of -3.5494%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Peripheral clock: 16 MHz
Target baud rate: 115,200 baud
Oversampling: 16 x

Results

Baud rate error: -3.5494% %
Integer divisor: 9
Actual baud rate: 111,111.1 baud
Assessment: Too much error - framing errors likely

Step by step

1Values used
2Baud divisor
3Baud rate error= -3.5494 %
4Integer divisor= 9
5Actual baud rate= 111,111.1 baud
6Assessment= Too much error - framing errors likely

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is 2% the limit?

Error accumulates over the frame. After 9-10 bit times a 2% error has shifted sampling by 20% of a bit, which starts clipping the stop bit.

Why do 14.7456 MHz crystals exist?

Because they divide exactly into every standard baud rate, giving zero error. 16 MHz does not, which is why 115200 baud from 16 MHz carries about 2.1% error at 16x oversampling.