UART Baud Rate & Transfer Speed Calculator
Calculate UART bit duration, frame time, and actual throughput (Bytes/s) for any baud rate, data bits, parity, and stop bits.
How it works
Universal Asynchronous Receiver-Transmitter (UART) communication transmits data serial bit-by-bit without a shared clock line. Timing relies on a predefined baud rate (symbols or bits per second).
Each UART frame consists of:
- 1 Start Bit: Always low (0) to synchronize the receiver.
- Data Bits: Usually 8 bits (1 byte), sent least-significant bit (LSB) first.
- Optional Parity Bit: 0 for None, 1 for Even or Odd parity checking.
- Stop Bits: 1, 1.5, or 2 high (1) bits marking the end of the frame.
Bit Duration = 1 / Baud Rate
Frame Size = 1 (start) + Data Bits + Parity + Stop Bits
Max Throughput (Bytes/s) = Baud Rate × (Data Bits / 8) / Frame Size
Related reading: Reverse engineering unknown serial firmware or locating baud rate generator divisors in Ghidra? See Reverse Engineering Firmware with Ghidra and AI in Antigravity.
Worked example
For standard 115200 8-N-1 configuration (115200 baud, 8 data bits, no parity, 1 stop bit):
- Bit Duration = 1 / 115,200 ≈ 8.68 μs
- Total Frame Size = 1 + 8 + 0 + 1 = 10 bits
- Frame Duration = 10 × 8.68 μs ≈ 86.81 μs
- Max Transfer Rate = 115,200 / 10 = 11,520 Bytes/s (≈ 11.25 KB/s)
Notice that because each 8-bit byte requires 10 bits on the wire, the protocol overhead is exactly 20%.
Frequently Asked Questions
Why is UART transfer rate slower than the baud rate divided by 8?
Each byte sent over UART requires framing overhead: 1 start bit, optional parity bit, and 1 or 2 stop bits. In standard 8-N-1 format, transmitting 8 data bits requires 10 bits on the wire (20% overhead), so effective speed is Baud / 10 Bytes/s.
What baud rate error percentage is acceptable for reliable communication?
Most UART hardware samplers sample each bit near its midpoint (e.g., 8× or 16× oversampling). Timing drift accumulates across the 10-bit frame. As a rule of thumb, total baud rate mismatch between transmitter and receiver must remain under ±2% to prevent framing errors.
Sources
- EIA/TIA-232 and asynchronous serial communication timing standards.
- Microchip & Texas Instruments UART baud rate generator application notes.
Results are for engineering assistance. Verify important calculations and check official datasheets. See the Engineering Disclaimer.