Which CRC-16 variant does this use?

It uses CRC-16/CCITT-FALSE: polynomial 0x1021, initial value 0xFFFF, no input or output reflection, and no final XOR. This is the variant most people mean by 'CRC-16 CCITT', used by XMODEM-style and many serial protocols.

How can I check the result is correct?

The standard CRC check string is the ASCII text "123456789". For CRC-16/CCITT-FALSE it must produce 0x29B1. If your value matches, the implementation is working correctly.

Why are there so many CRC-16 variants?

Different protocols chose different polynomials, initial values, reflection settings, and final XOR masks (CCITT, MODBUS, XMODEM, IBM, and more). They all use 16-bit arithmetic but produce different results, so you must match the exact variant your protocol specifies.

Is CRC-16 suitable for security?

No. CRC-16 is an error-detection checksum that catches accidental corruption, not a cryptographic hash. It is easy to forge, so never use it to protect against deliberate tampering.

CRC-16 Checksum Calculator

This tool computes a 16-bit CRC checksum of any text or raw hex bytes and shows it in hexadecimal and decimal, updating live. CRC-16 is the workhorse of serial and fieldbus protocols, offering much stronger accidental-error detection than an 8-bit CRC while staying compact.

How it works

CRC-16 computes the remainder of the input, viewed as a polynomial over GF(2), divided by the generator polynomial 0x1021 (x^16 + x^12 + x^5 + 1). This implementation uses the CRC-16/CCITT-FALSE configuration: a 256-entry table is built from the polynomial in the most-significant-bit-first direction, the running value is initialised to 0xFFFF, and for each byte the high byte of the CRC is XORed with the input to index the table — crc = (crc << 8) XOR table[(crc >> 8) XOR byte]. There is no reflection and no final XOR.

Example and notes

The canonical CRC check string 123456789 produces 0x29B1 with CRC-16/CCITT-FALSE, which you can use to verify the calculator. If you need MODBUS (0xA001 reflected, init 0xFFFF) or XMODEM (init 0x0000) instead, the result will differ — confirm which variant your protocol requires before comparing values.

CRC-16 catches accidental corruption only — it is not cryptographic and is easy to forge. Everything is computed in pure JavaScript in your browser; nothing is uploaded.