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Universally Unique Lexicographically Sortable Identifier

UUID can be suboptimal for many uses-cases because:

  • It isn't the most character efficient way of encoding 128 bits of randomness
  • UUID v1/v2 is impractical in many environments, as it requires access to a unique, stable MAC address
  • UUID v3/v5 requires a unique seed and produces randomly distributed IDs, which can cause fragmentation in many data structures
  • UUID v4 provides no other information than randomness which can cause fragmentation in many data structures

Instead, herein is proposed ULID:

  • 128-bit compatibility with UUID
  • 1.21e+24 unique ULIDs per millisecond
  • Lexicographically sortable!
  • Canonically encoded as a 26 character string, as opposed to the 36 character UUID
  • Uses Crockford's base32 for better efficiency and readability (5 bits per character)
  • Case insensitive
  • No special characters (URL safe)



npm install --save ulid


import ulid from 'ulid'
// You can also input a seed time which will consistently give you the same time component
// This is useful for migrating to ulid
ulid(1469918176385) // 01ARYZ6S41TSV4RRFFQ69G5FAV

Implementations in other languages

From the community!

Language Author Binary Implementation
C++ suyash
Clojure theikkila
Objective-C ricardopereira
Crystal SuperPaintman
Dart isoos
Delphi matinusso
D enckse
Erlang savonarola
Elixir merongivian
F# lucasschejtman
Go imdario
Go oklog
Haskell steven777400
Java huxi
Java azam
Java Lewiscowles1986
Julia ararslan
Lua Tieske
.NET RobThree
.NET fvilers
Nim adelq
Perl 5 bk
PHP Lewiscowles1986
PHP robinvdvleuten
PowerShell PetterBomban
Python mdipierro
Python ahawker
Python mdomke
Ruby rafaelsales
Rust mmacedoeu
Rust dylanhart
SQL-Microsoft rmalayter
Swift simonwhitehouse
Tcl dbohdan


Below is the current specification of ULID as implemented in this repository.

Note: the binary format has not been implemented in JavaScript as of yet.

 01AN4Z07BY      79KA1307SR9X4MV3

|----------|    |----------------|
 Timestamp          Randomness
   48bits             80bits



  • 48 bit integer
  • UNIX-time in milliseconds
  • Won't run out of space till the year 10895 AD.


  • 80 bits
  • Cryptographically secure source of randomness, if possible


The left-most character must be sorted first, and the right-most character sorted last (lexical order). The default ASCII character set must be used. Within the same millisecond, sort order is not guaranteed

Canonical String Representation


t is Timestamp (10 characters)
r is Randomness (16 characters)


Crockford's Base32 is used as shown. This alphabet excludes the letters I, L, O, and U to avoid confusion and abuse.


Overflow Errors when Parsing Base32 Strings

Technically, a 26 character Base32 encoded string can contain 130 bits of information, whereas a ULID must only contain 128 bits. Therefore, the largest valid ULID encoded in Base32 is 7ZZZZZZZZZZZZZZZZZZZZZZZZZ, which corresponds to an epoch time of 281474976710655.

Any attempt to decode or encode a ULID larger than this should be rejected by all implementations, to prevent overflow bugs.

Binary Layout and Byte Order

The components are encoded as 16 octets. Each component is encoded with the Most Significant Byte first (network byte order).

0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|                      32_bit_uint_time_high                    |
|     16_bit_uint_time_low      |       16_bit_uint_random      |
|                       32_bit_uint_random                      |
|                       32_bit_uint_random                      |

Prior Art

Partly inspired by:

Test Suite

npm test


npm run perf
336,331,131 op/s » encodeTime
102,041,736 op/s » encodeRandom
17,408 op/s » generate

Suites:  1
Benches: 3
Elapsed: 7,285.75 ms