What is the FNV-1a hash?

FNV-1a is a fast non-cryptographic hash by Fowler, Noll and Vo. For each input byte it XORs the byte into the hash and then multiplies by a fixed FNV prime, all modulo 2^32 or 2^64.

How does FNV-1a differ from FNV-1?

FNV-1 multiplies first and then XORs each byte, while FNV-1a XORs first and then multiplies. The order swap in FNV-1a gives slightly better avalanche behaviour and is the more commonly used variant.

What are the FNV constants?

The 32-bit variant uses offset basis 2166136261 and prime 16777619. The 64-bit variant uses offset basis 14695981039346656037 and prime 1099511628211.

Why does the 64-bit value use BigInt arithmetic?

JavaScript numbers cannot represent 64-bit integers exactly, so the tool computes the 64-bit hash with BigInt masked to 2^64 to keep every bit correct.

Is FNV-1a safe for passwords?

No. FNV-1a is non-cryptographic and easily reversible or collidable on purpose. Use it for hash tables and integrity hints only, never for passwords, tokens or signatures.

FNV-1a Hash Calculator

FNV-1a is one of the simplest fast hash functions in wide use, valued for hash tables, bloom filters and quick content keying. This calculator computes both the 32-bit and 64-bit FNV-1a hashes of your text directly in the browser, with copy buttons for each result.

How it works

FNV-1a processes the input one byte at a time:

Initialise the hash to the FNV offset basis (2166136261 for 32-bit, 14695981039346656037 for 64-bit).
For each byte: first XOR the byte into the hash, then multiply the hash by the FNV prime, keeping the result modulo 2^32 or 2^64.
The final hash is the accumulated value.

The defining feature of the “a” variant is that the XOR happens before the multiply, which improves how input changes spread across the output bits.

Example

Hashing the string hello gives 0x4f9f2cab (32-bit) and 0xa430d84680aabd0b (64-bit). The tool reproduces both exactly.

Notes

The 64-bit hash is computed with BigInt so all 64 bits stay accurate; the 32-bit hash uses an emulated 32-bit multiply for correct overflow. FNV-1a has no cryptographic strength — never rely on it for security. Everything is computed locally and nothing leaves your browser.