Nil UUID

Technical specification

UUID version 4 (random) is defined in RFC 9562 (IETF, 2024), which replaced RFC 4122. A UUID v4 contains 122 random bits; the remaining 6 bits encode the version (4) and variant (10xx in bits 64–65). This tool uses the browser's crypto.randomUUID() method (W3C Web Cryptography API), which provides cryptographically strong randomness from the operating system's CSPRNG, equivalent to /dev/urandom on Linux or BCryptGenRandom on Windows.

Standard

RFC 9562: Universally Unique IDentifiers (UUIDs)

Random bits

122 bits (of 128 total; 6 bits encode version/variant)

Format

xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx

Collision probability

~50% at 2.7 × 10¹⁸ UUIDs generated

Related operations

For hashing identifiers into fixed-width values, try the hash generator. To produce strong random secrets, use the password generator. For encoding identifiers for transport, see Base64.

Frequently asked questions

When would I use the nil UUID?

The nil UUID is typically used as a default or placeholder value in UUID-typed database columns or API fields, similar to NULL or zero in other types. It signals 'no UUID has been assigned yet'.

Is the nil UUID a valid UUID?

It is a valid UUID per RFC 9562, but it is reserved as a special case. Prefer a randomly generated UUID v4 for any real identifier.

Is the nil UUID the same as NULL in a database?

The nil UUID is a real value, so a column that holds it is not NULL. Unique constraints, joins, and comparisons treat it like any other UUID, which is why some schemas use NULL for "not assigned" instead.

What is the max UUID?

RFC 9562 also defines the max UUID, ffffffff-ffff-ffff-ffff-ffffffffffff, with all 128 bits set to one. This tool produces the nil UUID and random v4 UUIDs, but not the max UUID.