Adler-32 is a checksum algorithm designed by Mark Adler. It produces a 32-bit integer from two 16-bit running sums computed over the input bytes. It is notably used inside the zlib compression format (RFC 1950).

Is my input sent to a server?

No. The entire computation runs in JavaScript inside your browser. Your text never leaves your device.

How is Adler-32 different from CRC-32?

Adler-32 is simpler and faster to compute in software because it uses only addition and modulo operations, whereas CRC-32 relies on bitwise polynomial division. CRC-32 offers slightly better error-detection properties.

Can Adler-32 be used as a cryptographic hash?

No. Adler-32 is a checksum, not a cryptographic hash. It is trivially reversible and susceptible to collisions. Use SHA-256 or SHA-3 when security is required.

Why does the empty string give 1?

The Adler-32 specification initializes A = 1 and B = 0. With no input bytes, the result is (0 << 16) | 1 = 1, which is the defined output for an empty message.

What is the modulus value used?

Both running sums are reduced modulo 65521, which is the largest prime smaller than 2^16. This choice maximizes the range of B and improves the checksum distribution.

ADLER32 Hash Generator

Name: ADLER32 Hash Generator
Availability: InStock
Author: Nham Vu

Type or paste any text to instantly compute its Adler-32 checksum in decimal and hexadecimal.

Input Text

0 bytes · 0 characters

Adler-32 Checksum

Hexadecimal (uppercase)

Hexadecimal (lowercase)

Decimal

Binary (32 bits)

A (low 16 bits)

B (high 16 bits)

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Summary

Type or paste any text to instantly compute its Adler-32 checksum in decimal and hexadecimal.

How it works

Type or paste your text into the input area.
The tool initializes A = 1 and B = 0 (the Adler-32 starting values).
Each byte of the input updates A = (A + byte) mod 65521 and B = (B + A) mod 65521.
The final checksum is (B << 16) | A, displayed as a 32-bit value.
Results appear instantly in decimal, hexadecimal (upper and lower case), and binary.
Click any result field to copy that value to your clipboard.

Use cases

Verify the integrity of a small text payload or configuration value.
Learn how the Adler-32 algorithm works step by step.
Generate expected checksums for unit tests involving data integrity.
Cross-check Adler-32 output when debugging compression libraries (zlib uses it).
Quick sanity-check that two strings are identical without storing either.
Educational exploration of checksum algorithms for developers.

Frequently Asked Questions

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Last updated: 2026-05-23 · Reviewed by Nham Vu