Adler-32 Checksum Calculator
Compute the fast Adler-32 rolling checksum for UTF-8 text.
Combine two rolling sums modulo 65521
Adler-32 keeps two rolling sums over input bytes: A starts at one and accumulates byte values modulo 65521, while B accumulates each successive A. The final checksum places B in the high 16 bits and A in the low 16 bits. This page encodes text as UTF-8 first and returns exactly eight lowercase hexadecimal digits.
For the standard text 123456789, the checksum is 091e01de. Repeating the calculation gives the same value, while changing one byte usually changes both rolling sums. Adler-32 is computationally light and appears in formats such as zlib, but its distribution is weaker than CRC-32 for short messages and it should not be selected merely because the output looks hash-like.
Use Adler-32 for accidental errors, not attackers
This checksum detects many accidental changes but is linear and not collision resistant against deliberate manipulation. It is unsuitable for passwords, signatures, authenticated downloads, or hostile input. The byte calculation stays in your browser. Match the source encoding exactly when comparing systems, because UTF-8 punctuation, line endings, and invisible whitespace all affect the resulting bytes.
Frequently Asked Questions
Why does Adler-32 use 65521?
65521 is the largest prime below 65536 and serves as the modulus for both rolling sums.
Is Adler-32 a secure hash?
No. It is a fast accidental-error checksum with known collision weaknesses and no adversarial protection.
How is text converted before checksumming?
The page encodes the input as UTF-8 bytes, including whitespace and line endings.
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