Free ULID Generator & ULID ↔ UUID Converter

ULID Generator Guide

Why Use QuickTooly's ULID Generator?

• Timestamp visible at a glance: Each generated ULID shows its decoded creation time directly below it.
• Batch generation: Generate up to 20 ULIDs at once and copy individually or all together.
• Built-in ULID ↔ UUID converter: Paste a ULID to get a UUID, or paste a UUID to get its ULID equivalent — live, no button required.
• Private & secure: Everything runs in your browser — no server, no logs.
• 100% free: No account, no watermark, no limits.

What Is a ULID?

A ULID (Universally Unique Lexicographically Sortable Identifier) is a 128-bit identifier encoded as a 26-character string using Crockford's Base32 alphabet — for example 01ARZ3NDEKTSV4RRFFQ69G5FAV. Like UUIDs, ULIDs are globally unique without a central authority. Unlike UUID v4, ULIDs are lexicographically sortable by default, because the first 10 characters encode a millisecond-precision Unix timestamp.

ULIDs were designed to address the main shortcoming of UUID v4 for database use: random UUIDs cause index fragmentation. A ULID's time-ordered prefix keeps database B-tree indexes tightly clustered, reducing page splits and improving INSERT performance at scale — similar to UUID v7, but in a more URL-friendly, human-readable encoding.

ULID Format Breakdown

A ULID is always 26 uppercase characters split into two logical parts:

• Characters 1–10 (timestamp): Encodes the Unix time in milliseconds. The maximum representable time is ZZZZZZZZZZ = year 10889. Monotonically increases as time passes, ensuring newer ULIDs always sort after older ones.
• Characters 11–26 (randomness): 80 bits of cryptographically random data, providing collision resistance even when multiple ULIDs are generated within the same millisecond.

The Crockford Base32 alphabet (0123456789ABCDEFGHJKMNPQRSTVWXYZ) deliberately excludes the letters I, L, O, and U to avoid visual ambiguity and profanity. It is case-insensitive and URL-safe.

ULID vs UUID — Which Should You Use?

Both formats represent 128-bit unique identifiers, but differ in encoding and use case:

• ULID: 26 characters, Crockford Base32, URL-safe, case-insensitive, lexicographically sortable, human-readable timestamp prefix. Ideal for systems where sortability matters and you prefer a compact non-hyphenated ID.
• UUID v4: 36 characters with hyphens, hex encoding, purely random — no embedded timestamp. Best for session tokens and API keys where order is irrelevant.
• UUID v7: 36 characters with hyphens, hex encoding, time-ordered (RFC 9562). A direct alternative to ULID when UUID format compatibility is required (e.g., PostgreSQL uuid column type).

Rule of thumb: Use ULID when you want a compact sortable ID without UUID format constraints. Use UUID v7 when your stack already expects the standard UUID format. Use UUID v4 for secrets and tokens where ordering should not be guessable.

How ULID ↔ UUID Conversion Works

A ULID and a UUID both represent the same 128-bit value — they are just two different encodings. Converting between them is a pure re-encoding operation with no data loss:

• ULID → UUID: Decode the 26-character Crockford Base32 string to a 128-bit integer, then format it as a standard 8-4-4-4-12 hex UUID.
• UUID → ULID: Strip hyphens from the UUID, interpret the 32 hex characters as a 128-bit integer, then re-encode as 26 Crockford Base32 characters.

This means you can store a ULID-generated ID in a UUID database column and retrieve it as a ULID at any time. The round-trip is lossless and deterministic.

Common ULID Use Cases

ULIDs are popular as database primary keys in distributed systems (PostgreSQL, MySQL, MongoDB, DynamoDB), file and object names in cloud storage, log correlation IDs where chronological ordering is useful for debugging, event sourcing aggregate IDs, and any REST API resource identifier where you want the ID itself to convey creation order without a separate timestamp column.

Frequently Asked Questions