Fletcher & Adler Checksum Calculator

What Is a Fletcher & Adler Checksum Calculator?

A Fletcher & Adler Checksum Calculator computes lightweight arithmetic checksums designed to detect accidental changes in a sequence of bytes or words. Unlike CRC algorithms, Fletcher and Adler checksums are based on running modular sums rather than polynomial division.

This calculator supports three useful variants: Fletcher-16, Fletcher-32, and Adler-32. You can enter ordinary text or exact hexadecimal bytes and calculate the resulting checksum directly in your browser.

These algorithms are useful for protocol analysis, data integrity testing, embedded development, compression formats, file handling, networking research, and debugging software that implements rolling or arithmetic checksums.

How to Use the Fletcher & Adler Checksum Calculator

What Is Fletcher-16?

Fletcher-16 maintains two 8-bit running sums. The first sum accumulates each input byte modulo 255. The second sum accumulates the changing first sum, also modulo 255.

Because the second sum depends on the sequence of intermediate first sums, Fletcher checksums provide sensitivity to both byte values and their positions.

Fletcher-16 Worked Example

For the ASCII text abcde, the bytes are:

You can load this exact test by clicking the Fletcher-16 example button above.

What Is Fletcher-32?

Fletcher-32 applies the same two-running-sum concept to 16-bit words. Each word contributes to a first accumulator modulo 65535, while a second accumulator tracks the running first sum.

This calculator interprets adjacent input bytes as big-endian 16-bit words. If the input contains an odd number of bytes, a zero byte is appended only for the checksum arithmetic so the final byte can form a complete word.

RFC 1146 discusses Fletcher checksum algorithms in the context of alternate TCP checksum methods. :contentReference[oaicite:1]{index=1}

What Is Adler-32?

Adler-32 is another two-sum checksum, but it differs from Fletcher in important ways. Adler-32 initializes the first accumulator to 1, uses the prime modulus 65521, and processes individual bytes.

RFC 1950 specifies Adler-32 for the zlib data format and explains why the prime modulus 65521 and initial value 1 are used. :contentReference[oaicite:2]{index=2}

Adler-32 Worked Example

A common demonstration uses the text Wikipedia.

The calculator’s default input uses this example so you can immediately verify that Adler-32 calculation works as expected.

Fletcher vs Adler Checksums

RFC 1950 specifically contrasts Adler-32’s prime modulus 65521 with the Fletcher approach and notes that Adler-32 initializes its first sum to 1. :contentReference[oaicite:3]{index=3}

Why Does Adler-32 Use 65521?

The Adler-32 modulus is 65521, the largest prime number smaller than 65536. Using a prime modulus helps avoid certain classes of undetected changes that can occur with a non-prime modulus.

RFC 1950 notes this distinction directly when comparing Adler-32 with Fletcher-style checksums. :contentReference[oaicite:4]{index=4}

Why Does Adler-32 Start at 1?

Adler-32 initializes its first running sum to 1 instead of zero. This makes the message length influence the second accumulator even when bytes contain zero values.

RFC 1950 explains that this initialization avoids the behavior where a sequence containing only zero bytes would otherwise produce the same all-zero Fletcher-style result regardless of length. :contentReference[oaicite:5]{index=5}

Text Input vs Hexadecimal Input

Fletcher-32 and Odd-Length Data

Fletcher-32 works on 16-bit quantities. Therefore a byte sequence with an odd number of bytes cannot naturally form complete 16-bit words.

This calculator pairs the final unmatched byte with a zero byte for checksum calculation. The results section clearly indicates when that padding was required.

Fletcher and Adler vs CRC

Fletcher and Adler checksums are arithmetic checksums based on modular addition. CRC algorithms use polynomial division over binary fields.

CRC algorithms generally have stronger and more formally characterized error detection properties for many communications applications, while Fletcher and Adler algorithms can be simpler and computationally inexpensive.

The zlib manual describes Adler-32 as a checksum available alongside CRC-32 and notes its use by zlib streams. :contentReference[oaicite:6]{index=6}

Where Fletcher and Adler Checksums Are Useful

Common Fletcher & Adler Checksum Mistakes

Fletcher & Adler Checksum Calculator FAQs