What is Adler-32 used for?

Adler-32 is the integrity check embedded in the zlib data format (RFC 1950), which wraps DEFLATE streams used by PNG, gzip's library, and many protocols. It is faster to compute than CRC-32 but slightly weaker for short inputs.

How is the value structured?

Adler-32 keeps two running sums modulo 65521. Sum A accumulates the bytes (starting at 1), and sum B accumulates A (starting at 0). The final 32-bit value is (B shifted left 16) OR A, which this tool also shows broken out.

Why does an empty string give 00000001?

Sum A is initialised to 1 and sum B to 0. With no bytes processed, A stays 1 and B stays 0, so the result is (0 << 16) | 1 = 1, shown as the hex 00000001.

No. Like CRC-32 it only detects accidental corruption, not deliberate tampering. It is also weak for very short messages. Use a cryptographic hash such as SHA-256 for any security purpose.

Why 65521 specifically?

65521 is the largest prime below 65536 (2^16). Using a prime modulus improves the error-detection properties of the modular sums while keeping each sum within 16 bits.

Adler-32 Checksum Calculator

Q: Is Adler-32 secure?

No. Like CRC-32 it only detects accidental corruption, not deliberate tampering. It is also weak for very short messages. Use a cryptographic hash such as SHA-256 for any security purpose.

Q: Why 65521 specifically?

65521 is the largest prime below 65536 (2^16). Using a prime modulus improves the error-detection properties of the modular sums while keeping each sum within 16 bits.

Adler-32 is a checksum algorithm invented by Mark Adler for the zlib compression library. It trades a little error-detection strength for speed, and appears as the trailing integrity check in every zlib-wrapped DEFLATE stream. This tool computes it over the UTF-8 bytes of your text.

How it works

Adler-32 maintains two 16-bit running sums, both taken modulo the prime 65521:

A = 1, B = 0
for each byte b:
    A = (A + b)  mod 65521
    B = (B + A)  mod 65521
result = (B << 16) | A

Sum A is a simple running total of the byte values; sum B is a running total of all the intermediate A values, which makes the result sensitive to byte order, not just the multiset of bytes. The 32-bit output packs B into the high 16 bits and A into the low 16 bits.

Tips and notes

The string "Wikipedia" produces the Adler-32 11E60398, a common reference value.
Because A and B are only mixed by addition, Adler-32 is comparatively weak on very short inputs — many tiny messages share checksums. CRC-32 is stronger there.
The modulus 65521 is prime, which is what keeps the sums from collapsing into predictable patterns.
As with any checksum, encoding matters: the result is over the UTF-8 bytes, so non-ASCII characters expand to multiple bytes before being summed.