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UART calculator

UART Transmission Time Calculator

Estimate how long an asynchronous serial transfer occupies the wire, including start, parity, and stop-bit overhead.

Frame settings

Estimated result

Bits per frame
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Total transmitted bits
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Data field bits
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Transmission time
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payload bit rate
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Formula

bits/frame = 1 + data + parity + stop
time = total bits ÷ baud rate

Formula and assumptions

One start bit is added to the selected data bits, an optional parity bit, and the selected stop-bit duration. Bits per frame = 1 + data bits + parity bit + stop bits. Total bits = frame bits × Number of frames. Transmission time = total bits ÷ baud rate. Payload bits count only the selected data field bits per frame; in 8-bit mode, one frame corresponds to one 8-bit data byte. This is not a Unicode character or string count, and does not assume packing of arbitrary 8-bit bytes in non-8-bit modes.

Worked example

At 115,200 baud, 100 frames (8-bit data mode) in 8-N-1 framing use 10 bits per frame and 1,000 line bits. The transfer takes about 8.681 ms, while useful payload bit rate is 92,160 bit/s.

How to use this calculator

  1. Enter values using the units shown beside each field.
  2. Choose the framing or hardware options that match your design.
  3. Select Calculate, then compare the result with your timing or bandwidth budget.
  4. Check the assumptions and your component reference manual before committing a design.

Common mistakes

  • Counting only eight data bits and ignoring framing overhead.
  • Confusing baud rate with bytes per second.
  • Assuming software latency, buffering, or flow control is included in wire time.
  • Selecting a frame format that the remote endpoint does not support.

Frequently asked questions

Does baud rate equal payload bit rate?

No. Start, parity, and stop bits consume line time, so payload bit rate is lower than the baud rate.

Why is one start bit assumed?

Conventional asynchronous UART frames use one start bit. This calculator models that common format.

Can a UART use 1.5 stop bits?

Some UART modes support it. Confirm support in both endpoints before using that selection.

Related tools and guides

Engineering disclaimer. The UART calculator provides estimates for education and early design work. Confirm critical values with measurements, device documentation, and a qualified review. Read the full disclaimer.