FNV hash calculator — FNV-1 and FNV-1a in 32 and 64 bits

How to use the FNV hash calculator

1. Enter your key or string in Text.
2. Set Input is to Text (UTF-8), Hex bytes or Base64 bytes.
3. Choose a Variant: FNV-1a 32-bit (default), FNV-1 32-bit, FNV-1a 64-bit or FNV-1 64-bit.
4. Read the FNV (hex) value, the unsigned Decimal value and the Input size.

What it does and when to use it

FNV is a tiny non-cryptographic hash: one XOR and one multiply per byte. It is popular wherever speed and simplicity matter more than security, for example hash-table keys in C and Go, assigning users to shards or A/B test groups, bloom filters, and short cache keys. Use this page to check that your own implementation gives the same numbers, or to see which bucket a key will land in.

For a side-by-side view with djb2, MurmurHash3 and others, open the simple string hash table. The DJB2 calculator covers the other classic string hash, and the text to colour tool uses FNV-1a to pick a stable colour for a name.

How it works

The code in this page follows RFC 9923. It starts from an offset basis and, for each byte of input:

• FNV-1a: XOR the byte into the hash, then multiply by the FNV prime.
• FNV-1: multiply by the prime first, then XOR the byte.

Arithmetic wraps around at 32 or 64 bits.

The 32-bit versions use JavaScript’s 32-bit integer multiply; the 64-bit versions use BigInt so no precision is lost.

Worked examples

One letter, step by step. For a (byte 0x61), FNV-1a computes 0x811C9DC5 XOR 0x61 = 0x811C9DA4, then multiplies by 0x01000193 and keeps the low 32 bits. The tool shows FNV (hex): e40c292c, decimal 3826002220.

Reference string. foobar with FNV-1a 32-bit gives bf9cf968, decimal 3214735720, a widely used reference value for checking implementations.

64-bit FNV-1. a with FNV-1 64-bit gives af63bd4c8601b7be.

Choosing a shard. user-42 hashes to 39875499 and user-43 to 23097880. With four shards you take the decimal value modulo 4, so they go to shards 3 and 0. Neighbouring keys end up far apart, which is what you want.

Limits and tips

• Use the decimal value for modulo arithmetic; the hex value is the same number.
• If you need fewer bits, for example a 16-bit value, the FNV authors recommend XOR-folding: XOR the high half into the low half instead of just cutting bits off.
• FNV is a hash for lookups, not a checksum: it is not designed to detect transmission errors as a CRC is, and the note under the result says so.