What Is a UUID?
Last updated 2026-09-11
A UUID (Universally Unique Identifier) is a 128-bit value used to identify something — a database row, a session, a file — without needing a central authority to hand out unique numbers.
Understand the format
A UUID is displayed as 32 hexadecimal digits in 5 groups separated by hyphens, e.g. 550e8400-e29b-41d4-a716-446655440000.
Know the most common version
Version 4 UUIDs are generated using random or pseudo-random numbers, with a few fixed bits marking the version — no coordination between systems is required.
Use them where global uniqueness matters
Database primary keys, distributed system identifiers, API request IDs, and file names all commonly use UUIDs to avoid collisions across independent systems.
Important Considerations
- Version 4 UUIDs have 122 random bits, making accidental collisions astronomically unlikely — far less likely than, say, a hardware failure.
- Other UUID versions exist (time-based, namespace-based) for different guarantees, but version 4 is the most common for general-purpose unique IDs.
- UUIDs are longer than auto-incrementing integers, which can matter for storage and index performance at very large scale.
Frequently Asked Questions
- Can two systems generate the same UUID by accident?
- With version 4 UUIDs, the probability is so low (roughly 1 in 2^122) that it's not a practical concern for virtually any application.
- Are UUIDs and GUIDs the same thing?
- Yes, for practical purposes — GUID (Globally Unique Identifier) is Microsoft's name for the same concept as a UUID.