Fletcher-16 Checksum Calculator
Compute paired modulo-255 rolling sums over UTF-8 bytes.
Let byte order influence two rolling sums
Fletcher-16 maintains a first sum of bytes modulo 255 and a second sum of every intermediate first sum. Combining the second sum as the high byte and first sum as the low byte makes transpositions affect the result more than a plain additive checksum. This tool applies that byte-wise convention to UTF-8 text and emits four lowercase hexadecimal digits.
The five ASCII bytes in abcde produce c8f0. The low byte f0 is the final first sum, while c8 records the accumulated progression. Reordering the same characters usually changes the second sum even when the total byte sum remains unchanged, which is the principal improvement over a single eight-bit additive check.
Reserve Fletcher-16 for accidental corruption
Fletcher-16 has only 65536 possible outputs and no cryptographic collision resistance. It belongs in lightweight accidental-error detection and compatibility checks, not passwords, signatures, or hostile downloads. The rolling calculation runs in your browser. When comparing implementations, confirm whether they process bytes or 16-bit words and how they arrange the two sums in the displayed result.
Frequently Asked Questions
How is Fletcher-16 different from a simple sum?
Its second rolling sum depends on byte position, so many reordered sequences no longer collide with the same total.
Why is the modulus 255?
Each eight-bit rolling sum is reduced modulo 255 before the two are combined into 16 bits.
Is Fletcher-16 suitable for security?
No. Its small, unkeyed output is intended only for accidental-error detection.
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