What is the Fletcher-16 checksum?

Fletcher-16 is a position-dependent checksum that computes two running sums modulo 255 over a byte stream, then packs them into a 16-bit value. It detects all single-bit errors and most burst errors at very low cost.

Why does Fletcher use modulus 255 instead of 256?

Using 255 (an odd modulus) makes the checksum sensitive to the order and value of bytes, which a simple modulo-256 sum is not. This is what gives Fletcher its stronger error-detection properties.

How is the final value assembled?

The high byte is sum2 and the low byte is sum1, so the checksum equals (sum2 << 8) | sum1. The tool shows both component bytes so you can verify the packing.

Can I checksum raw bytes instead of text?

Yes. Switch to Hex bytes mode and enter pairs of hex digits separated by spaces or commas, for example 61 62 63. The tool requires an even number of hex digits.

Is Fletcher-16 a secure hash?

No. Fletcher-16 is an error-detection checksum, not a cryptographic hash. It is fast and good at catching accidental corruption but must never be used for security, passwords or signatures.

Fletcher-16 Checksum Calculator

Q: Can I checksum raw bytes instead of text?

Yes. Switch to Hex bytes mode and enter pairs of hex digits separated by spaces or commas, for example 61 62 63. The tool requires an even number of hex digits.

Q: Is Fletcher-16 a secure hash?

No. Fletcher-16 is an error-detection checksum, not a cryptographic hash. It is fast and good at catching accidental corruption but must never be used for security, passwords or signatures.

The Fletcher-16 checksum is a lightweight error-detection algorithm devised by John G. Fletcher. It is far stronger than a plain modular byte sum because it is position-sensitive, yet it is cheap enough to run on small embedded devices. This calculator runs the exact algorithm in your browser over either UTF-8 text or raw hex bytes.

How it works

Fletcher-16 keeps two running sums, both reduced modulo 255:

Start with sum1 = 0 and sum2 = 0.
For each byte b: set sum1 = (sum1 + b) mod 255, then sum2 = (sum2 + sum1) mod 255.
The final checksum is (sum2 << 8) | sum1 — sum2 is the high byte and sum1 is the low byte.

Because sum2 accumulates every intermediate value of sum1, the checksum depends on the position of each byte, not just the total. Swapping two bytes changes the result, which a simple sum would miss.

Example

For the ASCII string abcde (bytes 97 98 99 100 101) the running sums settle at sum1 = 240 and sum2 = 200, giving a checksum of 0xC8F0. Try it in the tool to see the same value.

Notes

The odd modulus 255 is deliberate — it prevents the running sums from ever locking into a value that would hide a byte swap. Fletcher-16 is an integrity check only and offers no protection against deliberate tampering. Everything here is computed locally; your data never leaves the browser.