Comparisons Development

UUID v4 vs UUID v7 vs ULID

Random UUIDs, time-ordered UUIDs and ULIDs compared: bit layout, text length, sorting, database index behaviour and when to use each, with example values decoded and checked.

Last reviewed:

AdSense Placeholder
Slot: header_reference_page
On this page

All three produce 128-bit identifiers you can generate without asking a central server. They differ in how the bits are used. Generate and inspect them with the UUID Generator and the UUID & ULID Decoder; the background is in How UUIDs Work.

Example values

  • Example UUID v4: f47ac10b-58cc-4372-a567-0e02b2c3d479
  • Example UUID v7: 01a17fce-7800-7abc-8123-456789abcdef (its first 48 bits are the Unix time in milliseconds: 2026-10-28 00:00:00 UTC)
  • Example ULID: 01M5ZWWY0004HMASW9NF6YY093 (the first 10 characters are the same time)

At a glance

UUID v4 UUID v7 ULID
Bits 122 random + 6 fixed 48 time + 74 random-ish + 6 fixed 48 time + 80 random
Text form 36 characters, hex with hyphens 36 characters, hex with hyphens 26 characters, Crockford Base32
Sorted by creation time No Yes Yes (as text, too)
Reveals creation time No Yes, to the millisecond Yes, to the millisecond
Standard RFC 9562 RFC 9562 Community specification
Index locality in a database Poor (random inserts) Good Good

What it means in practice

  • Random v4 keys scatter inserts across a B-tree index, which causes page splits and cache misses on large tables. Time-ordered v7 and ULID append near the end.
  • Time-ordered IDs leak when a record was made. Do not use them where that is sensitive, such as password-reset tokens (use v4 or a secure random token for those).
  • Within the same millisecond, v7 and ULID need extra care to stay monotonic; libraries add a counter or increment the random part.
  • Both formats are case-insensitive in text, but ULID avoids the letters I, L, O and U so that it is easier to read aloud.

Which to choose

  • New database primary keys: UUID v7.
  • Identifiers that must reveal nothing: UUID v4.
  • Short, sortable, URL-friendly IDs where the 26-character form is welcome: ULID.

Try these tools

UUID Generator

Generate cryptographically random RFC 4122 v4 UUIDs online, one at a time or in bulk.

UUID & ULID Decoder

Decode UUIDs (versions 1 to 8) and ULIDs online.

See also

  • Guide How UUIDs Work
    The structure of a UUID (version and variant bits), what each version means.
  • Glossary UUID
    A UUID (universally unique identifier) is a 128-bit value, written as 36 characters.
  • Glossary Unix timestamp
    A Unix timestamp is the number of seconds since 00:00:00 UTC on 1 January 1970, the "Unix epoch".
  • Glossary Base32
    Base32 is an encoding that represents binary data using 32 safe characters, the letters A to Z and the digits 2 to 7.
  • Glossary Hash
    A hash is the fixed-length fingerprint a hash function computes from any input.

Frequently Asked Questions

For database primary keys, yes in most cases: v7 values created later sort later, so inserts land at the end of an index instead of at random places. Use v4 when the ID must reveal nothing, not even when it was created.

No, but both are 128 bits. A ULID is written as 26 Crockford Base32 characters and has its own layout, so it is not a valid UUID string, although it can be converted to one.

Back to the comparisons
AdSense Placeholder
Slot: footer_leaderboard