CRC-16 Calculator (Modbus, CCITT, XMODEM, Kermit …)

A CRC-16 is a 16-bit checksum that catches transmission errors in serial protocols, radio links, file formats and industrial buses. "CRC-16" is a family: Modbus, XMODEM, Kermit, X.25 and USB all use different polynomials, start values, bit orders and final XORs, so the same bytes give different results. This calculator has the common variants from the CRC catalogue, a custom mode, and shows the result in both byte orders.

How to use this tool

1. Enter the data as text or hex bytes (01 03 00 00 …).
2. Pick the variant your protocol uses, or enter custom parameters.
3. Read the CRC in hex and decimal, and in the byte order your protocol sends.
4. Compare with the table of all variants if you are unsure which one a device uses.

Formula

CRC = (remainder of M(x)·x¹⁶ ÷ P(x), starting from Init, with optional bit reflection) ⊕ XorOut

P(x) is the generator polynomial (0x8005 or 0x1021 for most variants). "Reflected" variants process each byte least-significant bit first and mirror the 16-bit result. Every variant is defined by its check value — the CRC of the nine ASCII characters 123456789.

Example

The text 123456789 gives 0x4B37 with CRC-16/MODBUS, 0x29B1 with CCITT-FALSE and 0x31C3 with XMODEM — the catalogue check values. The Modbus RTU request 01 03 00 00 00 0A has the CRC 0xCDC5, which goes on the wire low byte first: C5 CD.