How is Adler-32 calculated?

Adler-32 keeps two running sums modulo 65521 (the largest prime below 2^16). Sum A starts at 1 and adds each byte; sum B adds the current A after every byte. The checksum is (B << 16) | A, giving a 32-bit value.

How can I check the result is correct?

The standard check string is the ASCII text "123456789", which produces 0x091E01DE. A single empty input produces 0x00000001, since A starts at 1 and B stays 0. Matching values confirm the implementation.

Is Adler-32 better than CRC-32?

Adler-32 is faster to compute than CRC-32 because it only does additions, but it is weaker for very short messages and detects fewer error patterns. It is used in zlib and PNG where speed matters and data is rarely tiny.

Is Adler-32 a cryptographic checksum?

No. Adler-32 detects accidental corruption only and is trivial to forge. Never use it for security, passwords, or integrity against deliberate tampering — use SHA-256 or BLAKE3 instead.

Adler-32 Checksum Calculator

Q: How can I check the result is correct?

The standard check string is the ASCII text "123456789", which produces 0x091E01DE. A single empty input produces 0x00000001, since A starts at 1 and B stays 0. Matching values confirm the implementation.

Q: Is Adler-32 better than CRC-32?

Adler-32 is faster to compute than CRC-32 because it only does additions, but it is weaker for very short messages and detects fewer error patterns. It is used in zlib and PNG where speed matters and data is rarely tiny.

Q: Is Adler-32 a cryptographic checksum?

No. Adler-32 detects accidental corruption only and is trivial to forge. Never use it for security, passwords, or integrity against deliberate tampering — use SHA-256 or BLAKE3 instead.

This tool computes the Adler-32 checksum of any text or raw hex bytes and shows the combined 32-bit value plus its two component sums, updating live. Adler-32 is the integrity check used inside the zlib (DEFLATE) stream format and PNG, chosen because it is very cheap to compute.

How it works

Adler-32 (RFC 1950) maintains two 16-bit running sums modulo 65521, the largest prime below 2^16. Sum A is initialised to 1 and accumulates each input byte; sum B is initialised to 0 and accumulates the current value of A after every byte, so it weights earlier bytes more heavily. After all bytes are processed, the checksum is (B << 16) | A — B in the high 16 bits and A in the low 16 bits. Using a prime modulus spreads the values more evenly than a power-of-two modulus would.

Example and notes

Input	Adler-32 (hex)
(empty)	00000001
`123456789`	091e01de

So the check string 123456789 produces 0x091E01DE, and an empty input produces 0x00000001 because A starts at 1 and B remains 0. These are reliable reference values.

Adler-32 is faster than CRC-32 but weaker on short data and is not cryptographic. Use it for quick accidental-corruption detection, never for security. Everything is computed locally in your browser; nothing is uploaded.