Adler32 Checksum Calculator

Q: What is an Adler-32 checksum?

Adler-32 is a checksum algorithm invented by Mark Adler in 1995. It produces a 32-bit checksum from input data by computing two 16-bit sums (A and B) modulo 65521. It's faster than CRC-32 but less reliable for detecting certain error patterns, making it ideal for applications prioritizing speed over error detection precision.

Q: How is Adler-32 calculated?

Adler-32 maintains two sums: A (initialized to 1) and B (initialized to 0). For each byte, A = (A + byte) mod 65521, and B = (B + A) mod 65521. The final checksum is (B << 16) | A, combining both sums into a 32-bit value. The prime 65521 is the largest prime smaller than 2^16.

Q: What is Adler-32 used for?

Adler-32 is primarily used in zlib compression library and formats using zlib (PNG, PDF, HTTP compression). It's used for fast data integrity verification where speed matters more than cryptographic security. It should not be used for security purposes as it can be easily manipulated.

Q: What is the difference between Adler-32 and CRC-32?

Adler-32 is faster to compute but has weaker error detection than CRC-32. Adler-32 uses simple addition operations while CRC-32 uses polynomial division. CRC-32 can detect more error patterns and is better for detecting burst errors. Adler-32 is preferred when speed is critical and error detection requirements are modest.

Q: Why does Adler-32 use 65521?

65521 is the largest prime number smaller than 2^16 (65536). Using a prime modulus ensures better distribution of checksum values and reduces collision probability. The 16-bit constraint allows A and B sums to each fit in 16 bits, combining into a 32-bit result.

Calculate Adler-32 checksum of any text string with step-by-step computation trace, hexadecimal/decimal/binary output formats, and A/B component breakdown.

Free to useNo sign-up requiredUpdated Jan 2026

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Data Integrity Tool

Adler-32 Checksum Calculator

Fast checksum computation for data integrity verification. Used in zlib, PNG, and PDF formats.

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About Adler32 Checksum Calculator

Welcome to the Adler-32 Checksum Calculator, a fast and efficient tool for computing Adler-32 checksums of text strings. Adler-32 is a checksum algorithm widely used in data compression and integrity verification, offering an excellent balance between computation speed and error detection capability.

What is Adler-32?

Adler-32 is a checksum algorithm invented by Mark Adler in 1995 as part of the zlib compression library. It produces a 32-bit checksum value by computing two 16-bit sums (commonly called A and B) and combining them. The algorithm prioritizes speed over the error detection strength of CRC algorithms, making it ideal for applications where fast integrity checking is more important than detecting all possible errors.

How Adler-32 Works

The Adler-32 algorithm maintains two running sums:

Sum A: Starts at 1 and accumulates the sum of all bytes
Sum B: Starts at 0 and accumulates the sum of all intermediate A values

Both sums are computed modulo 65521, which is the largest prime number less than 2¹⁶. The final checksum combines B and A as: (B << 16) | A

The Magic Number: 65521

65521 is the largest prime smaller than 2¹⁶ (65536). Using a prime modulus ensures better distribution of checksum values and reduces the probability of collisions, where different inputs produce the same checksum.

Adler-32 vs CRC-32 Comparison

Feature	Adler-32	CRC-32
Speed	Faster (simple arithmetic)	Slower (polynomial division)
Error Detection	Good for random errors	Better for burst errors
Output Size	32 bits	32 bits
Primary Use	zlib, PNG, PDF	Network protocols, storage
Weakness	Poor for short messages	More CPU intensive

Common Applications

zlib Compression: Adler-32 is the integrity check algorithm used in zlib-compressed data streams
PNG Images: PNG files use zlib compression, which includes Adler-32 checksums
PDF Documents: Many PDF streams are zlib-compressed with Adler-32 verification
HTTP Compression: gzip/deflate content encoding uses Adler-32
Data Synchronization: rsync uses Adler-32 for fast block comparison

Algorithm Steps

Initialize: Set A = 1 and B = 0
Process each byte: For each byte D in the data:
- A = (A + D) mod 65521
- B = (B + A) mod 65521
Combine: Checksum = (B × 65536) + A = (B << 16) | A

Security Considerations

Important: Adler-32 is NOT a cryptographic hash function and should never be used for security purposes. It's designed only for error detection, not for:

Password hashing
Digital signatures
Data authentication
Tamper detection against malicious attacks

For security applications, use cryptographic hash functions like SHA-256, SHA-3, or BLAKE3.

Frequently Asked Questions

What is an Adler-32 checksum?

Adler-32 is a checksum algorithm invented by Mark Adler in 1995. It produces a 32-bit checksum from input data by computing two 16-bit sums (A and B) modulo 65521. It's faster than CRC-32 but less reliable for detecting certain error patterns, making it ideal for applications prioritizing speed over error detection precision.

How is Adler-32 calculated?

Adler-32 maintains two sums: A (initialized to 1) and B (initialized to 0). For each byte, A = (A + byte) mod 65521, and B = (B + A) mod 65521. The final checksum is (B << 16) | A, combining both sums into a 32-bit value.

What is Adler-32 used for?

Adler-32 is primarily used in the zlib compression library and formats using zlib (PNG, PDF, HTTP compression). It's used for fast data integrity verification where speed matters more than cryptographic security.

What is the difference between Adler-32 and CRC-32?

Adler-32 is faster to compute but has weaker error detection than CRC-32. Adler-32 uses simple addition operations while CRC-32 uses polynomial division. CRC-32 can detect more error patterns and is better for detecting burst errors. Adler-32 is preferred when speed is critical.

Why does Adler-32 use 65521?

65521 is the largest prime number smaller than 2¹⁶ (65536). Using a prime modulus ensures better distribution of checksum values and reduces collision probability. The 16-bit constraint allows A and B sums to each fit in 16 bits, combining into a 32-bit result.

Additional Resources

Reference this content, page, or tool as:

"Adler32 Checksum Calculator" at https://MiniWebtool.com/adler32-checksum-calculator/ from MiniWebtool, https://MiniWebtool.com/

by miniwebtool team. Updated: Jan 27, 2026

Developer API available: Run this tool from your app, automation, or agent with one JSON HTTP request. View API docs