UART Baud Rate Error Calculator

Clock & Baud Configuration

MCU Peripheral Clock (F_CLK)

Common: 8 MHz (STM32 internal), 16 MHz (Arduino), 72 MHz (STM32F103), 168 MHz (STM32F4)

Target Baud Rate

bps

Common: 9600 · 19200 · 38400 · 57600 · 115200 · 230400 · 1M

UART Oversampling Mode

STM32 supports both; AVR/Arduino always uses 16x (or 8x with U2X bit).

Prescaler / BRR Value

—

Actual Baud Rate Achieved

—

bps

Baud Rate Error

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All Standard Baud Rates — Error Scan

Shows all standard baud rates achievable within ±2% error for the current clock settings. ✅ = within spec, ⚠ = marginal, ❌ = unusable.

Baud Rate	BRR	Actual (bps)	Error	Status
Click Calculate to scan all baud rates

Understanding UART Baud Rate Error

UART (Universal Asynchronous Receiver-Transmitter) generates baud rates by dividing a peripheral clock frequency by an integer prescaler (BRR — Baud Rate Register). Because the division must be an integer, the actual baud rate is rarely exactly equal to the target, resulting in a small error.

The formula is: BRR = F_CLK / (Oversampling × Target_Baud). The BRR is rounded to the nearest integer, giving: Actual_Baud = F_CLK / (Oversampling × round(BRR)).

Acceptable Error Limits

< 1% — Excellent. Reliable communication even with noisy lines.
1–2% — Acceptable for most applications. UART spec typically allows ±2%.
2–3% — Marginal. May cause framing errors, especially at high baud rates or over long cables.
> 3% — Unreliable. Choose a different clock frequency or baud rate.

Tip: Use Crystal Clocks for UART

Internal RC oscillators (e.g., STM32 HSI at 8 MHz) have ±1–2% frequency tolerance themselves. For reliable high-baud UART, always use an external crystal oscillator so the clock error doesn't add to the baud rate error.

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Clock & Baud Configuration

All Standard Baud Rates — Error Scan

Understanding UART Baud Rate Error

Acceptable Error Limits

Tip: Use Crystal Clocks for UART