UUID vs ULID — Which Should You Use?

Every application needs unique identifiers. Whether you're identifying database rows, tracking distributed events, or generating public-facing resource IDs, the format you choose has real consequences for performance, readability, and correctness. The two most common options are UUID and ULID — and they differ in more ways than most developers realise. In this guide we'll cover the full anatomy of both formats, walk through code examples in JavaScript and Python, dig into database B-tree fragmentation, and give you a clear framework for deciding which to reach for in any given situation. What Is a UUID? UUID stands for Universally Unique Identifier. Defined in RFC 4122 (and later updated in RFC 9562), a UUID is a 128-bit value typically rendered as 32 hexadecimal characters split into five hyphen-separated groups: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx └──────┘ └──┘ └──┘ └──┘ └──────────┘ 8 hex 4 4 4 12 hex The M nibble encodes the version (1–8) and N encodes the variant. The total human