What is a UUID?

A UUID (Universally Unique Identifier) is a 128-bit identifier formatted as a 36-character string of hexadecimal digits separated by hyphens (e.g., 550e8400-e29b-41d4-a716-446655440000). UUIDs are designed to be unique across space and time without requiring a central authority to issue them.

UUID versions

There are several UUID versions, each using a different generation strategy:

• v1: Based on timestamp and MAC address. Guarantees uniqueness but leaks the machine’s network address.
• v4: Generated from random or pseudo-random numbers. The most commonly used version — 122 random bits make collisions astronomically unlikely.
• v5: Derived from a namespace and a name using SHA-1 hashing. Deterministic — the same input always produces the same UUID.
• v7: A newer format (RFC 9562) that combines a Unix timestamp with random bits, making UUIDs sortable by creation time while remaining unique.

The format

A UUID follows the pattern xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx, where M indicates the version and N indicates the variant. The 8-4-4-4-12 grouping is purely cosmetic — the underlying value is just 128 bits.

550e8400-e29b-41d4-a716-446655440000
        │    │
        │    └─ version (4 = random)
        └─ variant bits

Why developers use UUIDs

UUIDs solve the problem of generating unique identifiers in distributed systems where you can’t rely on a single database sequence. They’re standard as primary keys in databases, correlation IDs in microservices, session tokens, and resource identifiers in REST APIs.

Compared to auto-incrementing integers, UUIDs don’t reveal how many records exist and can be generated on any node without coordination. The downside is size — 36 characters vs. a compact integer — and their randomness makes B-tree indexing less efficient (v7 addresses this by being time-sorted).

Practical examples

In JavaScript: crypto.randomUUID() generates a v4 UUID natively. In Python: uuid.uuid4(). In PostgreSQL: gen_random_uuid().

Generate UUIDs instantly with the UUID Generator. If you need shorter, URL-friendly identifiers, check the NanoID Generator — it produces compact IDs with comparable collision resistance.

Collision probability

A v4 UUID has 2^122 possible values. You’d need to generate about 2.7 quintillion UUIDs to have a 50% chance of a single collision. In practice, you will never see one.