CRC32 Checksum Calculator
Compute CRC32 checksums of text or a file in your browser, in hex and decimal, across five CRC-32 variants.
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zlib.crc32 — Other CRC-32 variants (CRC-32C, BZIP2, MPEG-2, POSIX)
- CRC-32CISCSI · Castagnoli
— - CRC-32/BZIP2unreflected
— - CRC-32/MPEG-2no final XOR
— - CRC-32/POSIXcksum polynomial
—
How it works
Type or paste text and all five CRC-32 variants are computed as you type; switch to
File to checksum a download, an archive or a firmware image straight from
disk. Each variant is a full parameterisation of the Rocksoft model used by the CRC RevEng
catalogue: polynomial, initial register value, input and output reflection, and final XOR.
The first row is CRC-32/ISO-HDLC — polynomial 0x04C11DB7, init
0xFFFFFFFF, reflected in and out, XOR 0xFFFFFFFF — the variant
specified for gzip (RFC 1952), PNG (RFC 2083), ZIP and Ethernet, and the one that
crc32, Python's zlib.crc32 and Java's
java.util.zip.CRC32 return. CRC-32C uses the Castagnoli
polynomial 0x1EDC6F41 from iSCSI (RFC 3385), the one behind the SSE4.2
instruction and the ext4 and Btrfs metadata checksums. Every implementation here is
table-driven and verified against the catalogue's check value: the checksum of the ASCII
string 123456789 is 0xCBF43926 for ISO-HDLC,
0xE3069283 for CRC-32C, 0xFC891918 for BZIP2,
0x0376E6E7 for MPEG-2 and 0x765E7680 for POSIX.
Text is checksummed as UTF-8 bytes, matching what command-line tools do, so a value here
lines up byte for byte with crc32 file or a zlib.crc32 one-liner.
One caveat on the last row: CRC-32/POSIX is the raw catalogue algorithm, while
the POSIX cksum utility additionally feeds the message length through the
register, so cksum prints a different number for the same file. Paste any
value into Compare with a known CRC and the tool tells you which variant it
matches, in hex or decimal, case and 0x prefix ignored. Everything runs in
your browser: no request leaves the page, nothing is uploaded, and it works offline. Do
remember that a CRC is an error-detection code, not a hash — it reliably catches accidental
corruption but is trivial to forge, so never use it as a security check. Use SHA-256 for
that.
Frequently asked questions
Which CRC32 does this calculator use?
All five at once. The top row is CRC-32/ISO-HDLC (polynomial 0x04C11DB7, init 0xFFFFFFFF, reflected in and out, final XOR 0xFFFFFFFF), which is the CRC used by ZIP, gzip (RFC 1952), PNG (RFC 2083) and Ethernet, and the value returned by the crc32 command, Python's zlib.crc32 and Java's java.util.zip.CRC32. Below it you also get CRC-32C (ISCSI/Castagnoli, RFC 3385), CRC-32/BZIP2, CRC-32/MPEG-2 and CRC-32/POSIX. Each one is verified against the CRC RevEng catalogue check value: for the string 123456789 they are CBF43926, E3069283, FC891918, 0376E6E7 and 765E7680.
What is the difference between CRC32 and CRC32C?
They use different polynomials. Plain CRC-32 uses 0x04C11DB7, the classic IEEE 802.3 polynomial. CRC-32C uses 0x1EDC6F41, chosen by Castagnoli for better error detection on short messages; it is the polynomial in the SSE4.2 crc32 instruction and is used by iSCSI, ext4, Btrfs and many storage formats. The two produce completely different values for the same input, so always check which one a specification asks for before comparing.
Is a CRC32 checksum secure?
No. A CRC is an error-detection code, not a cryptographic hash. It reliably catches accidental corruption such as a flipped bit in transit or on disk, but anyone can craft a different file with the same CRC in milliseconds. Use it for integrity checks against random damage, and use SHA-256 when you need to prove a file was not tampered with deliberately.
Are my files uploaded anywhere?
No. The checksum is computed entirely in your browser with JavaScript: your text and files are read on your device, no request leaves the page, and the tool keeps working offline once loaded. That makes it safe for private archives, firmware images and confidential documents.