Developer · Checksums
Checksum calculator.
CRC-32, Adler-32, Fletcher-16 and Fletcher-32 of any text or file. Everything runs in your browser — nothing is uploaded.
Input
Checksums
Enter text or choose a file, then press Calculate.
What these checksums are
CRC-32 here is the CRC-32/ISO-HDLC variant (used by zip, gzip and PNG): polynomial 0x04C11DB7, initial value 0xFFFFFFFF, reflected input and output, final XOR 0xFFFFFFFF. Its published check value for the ASCII string 123456789 is CBF43926.
Adler-32 (zlib) keeps two sums modulo 65521, starting at 1 and 0, and outputs s2 × 65536 + s1. For 123456789 it is 091E01DE.
Fletcher-16 sums bytes modulo 255; Fletcher-32 sums 16-bit little-endian words modulo 65535 (an odd final byte is padded with zero). Both start at 0 and output sum2 in the high half. Published vectors: Fletcher-16 of abcde is C8F0, Fletcher-32 of abcde is F04FC729.
Sources: Catalogue of parametrised CRC algorithms (CRC-32/ISO-HDLC); RFC 1950 section 8.2 (Adler-32); Fletcher's checksum (test vectors).
Frequently asked questions
Are my files uploaded?
No. Files are read locally in your browser and never leave your device.
Why does my CRC-32 differ from another tool?
CRC-32 has many variants. This tool uses CRC-32/ISO-HDLC (the common zip/gzip/PNG one); other variants such as CRC-32C or CRC-32/BZIP2 give different values for the same input.
Are checksums secure?
No. They detect accidental corruption only and are trivial to forge. Use SHA-256 or stronger for integrity against tampering.
Error detection, not security
CRC and Fletcher/Adler sums catch accidental changes to data. They provide no protection against deliberate modification.