CRC-16/CCITT Checksum Calculator

How it is calculated

The input is read as hexadecimal bytes, two digits per byte, with spaces ignored. The register starts at 0xFFFF. For each byte, the byte is XORed into the top 8 bits of the register, then the register is shifted left one bit eight times. Whenever the bit shifted out is 1, the register is XORed with the polynomial 0x1021. After the last byte the register is the CRC, printed as 0x followed by four uppercase hex digits. There is no final XOR and no bit reversal, which is what the CCITT-FALSE variant means. Input with an odd number of digits, a 0x prefix or non-hex characters returns no result.

Common mistakes

• Typing the text itself instead of its hex bytes. The tool does not convert letters, so 123456789 must be entered as 313233343536373839, not as the digits.
• Adding a 0x prefix. Unlike some converters, this tool rejects 0x and returns nothing, so leave it off.
• Entering an odd number of hex digits. Every byte needs two digits, so 313 is rejected; write 0313 if the leading zero is meant.
• Comparing against a different CRC-16 variant. XMODEM, Modbus and others use other starting values, polynomials or bit orders, so the same data gives different results.
• Including the CRC bytes of the frame in the input and expecting the same answer. The CRC should be computed over the payload only, unless the protocol says otherwise.