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UART Baud Rate / Bit Timing Calculator
Compute UART bit time, frame time, throughput, and baud rate divisor error.
Bit time
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Frame time (start + data + parity + stop)
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Effective throughput
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Hover the frame to identify each bit. Data bits shown for an example byte, sent LSB first.
Baud rate divisor (UBRR-style)
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Actual baud rate achieved / error
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About this tool
Work out UART bit timing — the duration of a single bit, a full frame, and effective byte throughput — from a baud rate and frame format. Optionally enter an MCU clock frequency to compute the baud rate divisor and the resulting error percentage, the same check most datasheets publish in their baud rate tables.
Frequently asked questions
- Why does baud rate error matter?
- Because clock frequencies rarely divide evenly into a target baud rate, the actual bit rate is always slightly off — most UART receivers tolerate error up to roughly ±2–3% before bits start getting misread near the end of a frame, so checking this before committing to a clock/baud combination avoids flaky communication.
- What is the divisor calculation based on?
- It uses the common asynchronous UART baud generator formula divisor = round(Fclk ÷ (oversample × baud)) − 1, the same style of formula used by AVR (UBRR) and many other microcontroller UART peripherals with 8× or 16× oversampling.
- Does frame time include start and stop bits?
- Yes — the frame time is the bit time multiplied by the total bits actually put on the wire: 1 start bit, your chosen data bits, an optional parity bit, and your chosen stop bits.