What is ULID?

ULID (Universally Unique Lexicographically Sortable Identifier) is a 128-bit identifier format designed to overcome limitations in UUIDs. It consists of:

Timestamp: 48 bits of millisecond precision Unix time
Randomness: 80 bits of cryptographically-secure random data
Encoding: Base32 representation (26 characters)

Format example: 01ARZ3NDEKTSV4RRFFQ69G5FAV

How ULID works

Generate 48 bits for timestamp (Unix time in milliseconds)
Generate 80 bits of cryptographically-secure random data
Combine timestamp (most significant) with randomness (least significant)
Encode the entire 128-bit value using Crockford's Base32 alphabet
Result is a 26-character string (first 10 chars are timestamp, last 16 are randomness)

Advantages & Considerations

Advantages	Considerations
Lexicographically sortable URL-safe character set No special encoding needed Case-insensitive No privacy concerns	Newer, less widely used Limited library support No UUID compatibility Cannot go back before 1970

ULID Structure

Component	Bits	Characters	Description
Timestamp	48 bits	10 chars	Unix timestamp (ms) encoded in Base32
Randomness	80 bits	16 chars	Random bytes encoded in Base32

ULID vs. UUID Comparison

Feature	ULID	UUID v1	UUID v4	UUID v7
Size	128 bits	128 bits	128 bits	128 bits
String length	26 chars	36 chars	36 chars	36 chars
Time-based	✅	✅	❌	✅
Natural sorting	✅	❌	❌	✅
URL safe	✅	❌	❌	❌
MAC address exposure	❌	✅	❌	❌
Random component	✅	❌	✅	✅
Canonical standard	❌	✅	✅	⚠️ Draft

Common Use Cases

Database primary keys: Performance-optimized indexes
Distributed systems: Coordination-free generation
APIs: Clean, URL-safe identifiers
Time-series data: Native chronological ordering
Log systems: Sortable event identifiers
Modern applications: Where UUID limitations are problematic

Why Choose ULID?

Sortability: Lexicographically sorts by creation time
URL safety: No URL encoding needed (unlike UUIDs with hyphens)
Compactness: 26 characters vs 36 for UUID
Case insensitivity: Reduces user input errors
Monotonicity: Optional monotonic sorting within same millisecond
No special characters: Simple, alphanumeric-only format

Frequently Asked Questions

How does ULID compare to UUID v7?

Both use Unix timestamps for sortability and have random components. The key differences are encoding (Base32 vs hex), format (26 characters vs 36), and standardization (UUID v7 is in draft RFC specification).

Can I extract the creation time from a ULID?

Yes. The first 10 characters represent a millisecond-precision Unix timestamp that can be decoded and converted to a standard datetime.

Are ULIDs compatible with UUID database fields?

No. ULIDs use a different format and would need to be stored as strings or binary data rather than in native UUID database types.

What is monotonicity in ULIDs?

Some ULID implementations offer a monotonic option that ensures sequential sort order even within the same millisecond by incrementing the random component for ULIDs created in the same timestamp.

What character set does ULID use?

ULID uses Crockford's Base32 encoding (0-9, A-Z excluding I, L, O, U) which is designed to minimize confusion and transcription errors when IDs are read or entered manually.

Resources

Crockford's Base32 Alphabet

Encoding Value	Character	Notes
0-9	0-9	Decimal digits
10-31	A-Z	Excluding I, L, O, U for readability

Characters: 0123456789ABCDEFGHJKMNPQRSTVWXYZ

This encoding is case-insensitive for decoding (but ULIDs are typically generated in uppercase).

ULID Generator - Create Sortable Universal Identifiers

Try these examples: