Adler-32 Checksum Calculator

Adler-32 checksum calculator

Adler-32 is a checksum invented by Mark Adler in 1995 for the zlib compression library (RFC 1950). It is simpler and faster to compute than CRC-32 while still catching most accidental errors. This Adler-32 calculator shows the 32-bit value of any text or file as 8 hex characters.

Example

Input:

The quick brown fox jumps over the lazy dog

Adler-32:

5bdc0fda

The classic textbook example is “Wikipedia”, whose Adler-32 is 11e60398.

How Adler-32 is calculated

1. Start with A = 1 and B = 0.
2. For each byte: A = (A + byte) mod 65521 and B = (B + A) mod 65521.
3. The checksum is B × 65536 + A, written as 8 hex digits.

65521 is the largest prime below 2¹⁶. Because A starts at 1, the checksum of empty input is 00000001, not zero.

Strengths and weaknesses

• Fast. Only additions and occasional modulo operations — no tables or polynomial arithmetic.
• Weak for short inputs. With only a few hundred bytes, the sums do not wrap around the modulus, so many values are never used and error detection is noticeably weaker than CRC-32.
• Not cryptographic. Like any checksum, it is easy to forge deliberately. Use a cryptographic hash or HMAC to detect tampering.

Zlib vs gzip trailers

Zlib and gzip compress data with the same DEFLATE algorithm but check it differently: a zlib stream ends with an Adler-32 of the uncompressed data, while a gzip file stores a CRC-32 and the original length. Mark Adler chose the faster Adler-32 for zlib because it is embedded in other formats where speed mattered most.

Where Adler-32 is used

• The 4-byte trailer of every zlib stream, including data inside PNG images and PDF files, checks the uncompressed data.
• rsync’s rolling “weak” checksum is an Adler-32-style sum that can be updated in constant time as a window slides through a file.
• Quick integrity checks in games, embedded systems and network protocols where speed is key.

In code: Python zlib.adler32(data), Java java.util.zip.Adler32, PHP hash('adler32', $s). To create a zlib stream and see its Adler-32 trailer, try Zlib Compress; for stronger error detection, use CRC-32. The calculation runs in your browser.

Computing Adler-32 in code

Most standard libraries already include Adler-32, so you rarely need to write the loop yourself. Each of these prints 11e60398 for the text “Wikipedia”:

# Python
import zlib
print(format(zlib.adler32(b"Wikipedia"), "08x"))

// PHP
echo hash('adler32', 'Wikipedia');

// Java
Adler32 a = new Adler32();
a.update("Wikipedia".getBytes(StandardCharsets.UTF_8));
System.out.printf("%08x%n", a.getValue());

// Go
fmt.Printf("%08x\n", adler32.Checksum([]byte("Wikipedia")))

Why your value might not match

• A trailing newline. echo "Wikipedia" adds \n, which changes the result to 158803a2. Use printf or paste the text without a final line break.
• A different text encoding. The same word encoded as UTF-16 gives 23cc0398. This tool hashes UTF-8 bytes.
• Signed integers. Some languages print the checksum as a negative number; format it as unsigned hex before comparing.