The ULID spec is defined on ulid/spec. It has useful specs for applications (e.g. Database key), especially possess all uniqueness, randomness, extractable timestamps and sortable features. This gem aims to provide the generator, monotonic generator, parser and handy manipulation features around the ULID. Also providing ruby/rbs signature files.

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)

  • Monotonic sort order (correctly detects and handles the same millisecond)



Require Ruby 2.6 or later

This command will install the latest version into your environment

$ gem install ruby-ulid
Should be installed!

Add this line to your application/library's Gemfile is needed in basic use-case

gem 'ruby-ulid', '>= 0.1.6', '< 0.2.0'

Generator and Parser

The generated ULID is an object not just a string. It means easily get the timestamps and binary formats.

require 'ulid'

ulid = ULID.generate #=> ULID(2021-04-27 17:27:22.826 UTC: 01F4A5Y1YAQCYAYCTC7GRMJ9AA)
ulid.to_time #=> 2021-04-27 17:27:22.826 UTC
ulid.milliseconds #=> 1619544442826
ulid.to_s #=> "01F4A5Y1YAQCYAYCTC7GRMJ9AA"
ulid.timestamp #=> "01F4A5Y1YA"
ulid.randomness #=> "QCYAYCTC7GRMJ9AA"
ulid.to_i #=> 1957909092946624190749577070267409738
ulid.octets #=> [1, 121, 20, 95, 7, 202, 187, 60, 175, 51, 76, 60, 49, 73, 37, 74]

You can get the objects from exists encoded ULIDs

ulid = ULID.parse('01ARZ3NDEKTSV4RRFFQ69G5FAV') #=> ULID(2016-07-30 23:54:10.259 UTC: 01ARZ3NDEKTSV4RRFFQ69G5FAV)
ulid.to_time #=> 2016-07-30 23:54:10.259 UTC

Sortable with the timestamp

ULIDs are sortable when they are generated in different timestamp with milliseconds precision

ulids = do
ulids.uniq(&:to_time).size #=> 1000
ulids.sort == ulids #=> true

ULID.generate can take fixed Time instance. The shorthand is

time = #=> 2000-01-01 00:00:00 UTC
ULID.generate(moment: time) #=> ULID(2000-01-01 00:00:00.000 UTC: 00VHNCZB00N018DCPJA4H9379P)
ULID.generate(moment: time) #=> ULID(2000-01-01 00:00:00.000 UTC: 00VHNCZB006WQT3JTMN0T14EBP) #=> ULID(2000-01-01 00:00:00.000 UTC: 00VHNCZB002W5BGWWKN76N22H6)

ulids = do |n| + n)
ulids.sort == ulids #=> true

The basic generator prefers randomness, it does not guarantee sortable for same milliseconds ULIDs.

ulids = do
ulids.uniq(&:to_time).size #=> 35 (the size is not fixed, might be changed in environment)
ulids.sort == ulids #=> false

How to keep Sortable even if in same timestamp

If you want to prefer sortable, Use MonotonicGenerator instead. It is called as Monotonicity on the spec. (Though it starts with new random value when changed the timestamp)

monotonic_generator =
ulids = do
sample_ulids_by_the_time = ulids.uniq(&:to_time)
sample_ulids_by_the_time.size #=> 32 (the size is not fixed, might be changed in environment)

# In same milliseconds creation, it just increments the end of randomness part
ulids.take(5) #=>
# [ULID(2021-05-02 15:23:48.917 UTC: 01F4PTVCSN9ZPFKYTY2DDJVRK4),
#  ULID(2021-05-02 15:23:48.917 UTC: 01F4PTVCSN9ZPFKYTY2DDJVRK5),
#  ULID(2021-05-02 15:23:48.917 UTC: 01F4PTVCSN9ZPFKYTY2DDJVRK6),
#  ULID(2021-05-02 15:23:48.917 UTC: 01F4PTVCSN9ZPFKYTY2DDJVRK7),
#  ULID(2021-05-02 15:23:48.917 UTC: 01F4PTVCSN9ZPFKYTY2DDJVRK8)]

# When the milliseconds is updated, it starts with new randomness
sample_ulids_by_the_time.take(5) #=>
# [ULID(2021-05-02 15:23:48.917 UTC: 01F4PTVCSN9ZPFKYTY2DDJVRK4),
#  ULID(2021-05-02 15:23:48.918 UTC: 01F4PTVCSPF2KXG4ABT7CK3204),
#  ULID(2021-05-02 15:23:48.919 UTC: 01F4PTVCSQF1GERBPCQV6TCX2K),
#  ULID(2021-05-02 15:23:48.920 UTC: 01F4PTVCSRBXN2H4P1EYWZ27AK),
#  ULID(2021-05-02 15:23:48.921 UTC: 01F4PTVCSSK0ASBBZARV7013F8)]

ulids.sort == ulids #=> true

Same generator does not generate duplicated ULIDs even in multi threads environment. It is implemented with Monitor

Filtering IDs with Time

ULID can be element of the Range. If you generated the IDs in monotonic generator, ID based filtering is easy and reliable

include_end = ulid1..ulid2
exclude_end = ulid1...ulid2


When want to filter ULIDs with Time, we should consider to handle the precision. So this gem provides ULID.range to generate reasonable Range[ULID] from Range[Time]

# Both of below, The begin of `Range[ULID]` will be the minimum in the floored milliseconds of the time1
include_end = ULID.range(time1..time2) #=> The end of `Range[ULID]` will be the maximum in the floored milliseconds of the time2
exclude_end = ULID.range(time1...time2) #=> The end of `Range[ULID]` will be the minimum in the floored milliseconds of the time2

# Below patterns are acceptable
pinpointing = ULID.range(time1..time1) #=> This will match only for all IDs in `time1`
# until_the_end = ULID.range(..time1) #=> This will match only for all IDs upto `time1` (The `nil` starting `Range` can be used since Ruby 2.7)
until_the_end = ULID.range(ULID.min.to_time..time1) #=> This is same as above for Ruby 2.6
until_the_ulid_limit = ULID.range(time1..) # This will match only for all IDs from `time1` to max value of the ULID limit

# So you can use the generated range objects as below
#=> I hope the results should be actually you want!

If you want to manually handle the Time objects, ULID.floor returns new Time with truncating excess precisions in ULID spec.

time =, Rational('123456.789')).utc #=> 2000-01-01 00:00:00.123456789 UTC
ULID.floor(time) #=> 2000-01-01 00:00:00.123 UTC

Scanner for string (e.g. JSON)

For rough operations, ULID.scan might be useful.

json = <<'JSON'
  "id": "01F4GNAV5ZR6FJQ5SFQC7WDSY3",
  "author": {
    "id": "01F4GNBXW1AM2KWW52PVT3ZY9X",
    "name": "kachick"
  "title": "My awesome blog post",
  "comments": [
      "commenter": {
        "id": "01F4GNBXW1AM2KWW52PVT3ZY9X",
        "name": "kachick"
      "id": "01F4GNCXAMXQ1SGBH5XCR6ZH0M",
      "commenter": {
        "id": "01F4GND4RYYSKNAADHQ9BNXAWJ",
        "name": "pankona"

# [ULID(2021-04-30 05:51:57.119 UTC: 01F4GNAV5ZR6FJQ5SFQC7WDSY3),
#  ULID(2021-04-30 05:52:32.641 UTC: 01F4GNBXW1AM2KWW52PVT3ZY9X),
#  ULID(2021-04-30 05:52:56.707 UTC: 01F4GNCNC3CH0BCRZBPPDEKBKS),
#  ULID(2021-04-30 05:52:32.641 UTC: 01F4GNBXW1AM2KWW52PVT3ZY9X),
#  ULID(2021-04-30 05:53:04.852 UTC: 01F4GNCXAMXQ1SGBH5XCR6ZH0M),
#  ULID(2021-04-30 05:53:12.478 UTC: 01F4GND4RYYSKNAADHQ9BNXAWJ)]

ULID#patterns is a util for text based operations. The results and spec are not fixed. Should not be used except snippets/console operation

#=> returns like a fallowing Hash
  named_captures: /(?<timestamp>01F4GNBXW1)(?<randomness>AM2KWW52PVT3ZY9X)/i,
  strict_named_captures: /\A(?<timestamp>01F4GNBXW1)(?<randomness>AM2KWW52PVT3ZY9X)\z/i

Some methods to help manipulations

ULID.min and ULID.max return termination values for ULID spec.

It can take Time instance as an optional argument. Then returns min/max ID that has limit of randomness part in the time.

ULID.min #=> ULID(1970-01-01 00:00:00.000 UTC: 00000000000000000000000000)
ULID.max #=> ULID(10889-08-02 05:31:50.655 UTC: 7ZZZZZZZZZZZZZZZZZZZZZZZZZ)

time =, Rational('123456.789')).utc #=> 2000-01-01 00:00:00.123456789 UTC
ULID.min(time) #=> ULID(2000-01-01 00:00:00.123 UTC: 00VHNCZB3V0000000000000000)
ULID.max(time) #=> ULID(2000-01-01 00:00:00.123 UTC: 00VHNCZB3VZZZZZZZZZZZZZZZZ)

ULID#next and ULID#succ returns next(successor) ULID. Especially ULID#succ makes it possible Range[ULID]#each.

NOTE: But basically Range[ULID]#each should not be used, incrementing 128 bits IDs are not reasonable operation in most case

ULID.parse('01BX5ZZKBKZZZZZZZZZZZZZZZZ').next.to_s #=> "01BX5ZZKBM0000000000000000"

ULID#pred returns predecessor ULID

ULID.parse('01BX5ZZKBK0000000000000001').pred.to_s #=> "01BX5ZZKBK0000000000000000"
ULID.parse('01BX5ZZKBK0000000000000000').pred.to_s #=> "01BX5ZZKBJZZZZZZZZZZZZZZZZ"
ULID.parse('00000000000000000000000000').pred #=> nil

ULID.sample returns random ULIDs.

Basically ignores generating time.

ULID.sample #=> ULID(2545-07-26 06:51:20.085 UTC: 0GGKQ45GMNMZR6N8A8GFG0ZXST)
ULID.sample #=> ULID(5098-07-26 21:31:06.946 UTC: 2SSBNGGYA272J7BMDCG4Z6EEM5)
ULID.sample(0) #=> []
ULID.sample(1) #=> [ULID(2241-04-16 03:31:18.440 UTC: 07S52YWZ98AZ8T565MD9VRYMQH)]
#[ULID(5701-04-29 12:41:19.647 UTC: 3B2YH2DV0ZYDDATGTYSKMM1CMT),
# ULID(2816-08-01 01:21:46.612 UTC: 0R9GT6RZKMK3RG02Q2HAFVKEY2),
# ULID(10408-10-05 17:06:27.848 UTC: 7J6CPTEEC86Y24EQ4F1Y93YYN0),
# ULID(2741-09-02 16:24:18.803 UTC: 0P4Q4V34KKAJW46QW47WQB5463),
# ULID(2665-03-16 14:50:22.724 UTC: 0KYFW9DWM4CEGFNTAC6YFAVVJ6)]

You can specify a range object for the timestamp restriction, see also ULID.range.

ulid1 = ULID.parse('01F4A5Y1YAQCYAYCTC7GRMJ9AA') #=> ULID(2021-04-27 17:27:22.826 UTC: 01F4A5Y1YAQCYAYCTC7GRMJ9AA)
ulid2 = ULID.parse('01F4PTVCSN9ZPFKYTY2DDJVRK4') #=> ULID(2021-05-02 15:23:48.917 UTC: 01F4PTVCSN9ZPFKYTY2DDJVRK4)
ulids = ULID.sample(1000, period: ulid1..ulid2)
ulids.uniq.size #=> 1000
#[ULID(2021-05-02 06:57:19.954 UTC: 01F4NXW02JNB8H0J0TK48JD39X),
# ULID(2021-05-02 07:06:07.458 UTC: 01F4NYC372GVP7NS0YAYQGT4VZ),
# ULID(2021-05-01 06:16:35.791 UTC: 01F4K94P6F6P68K0H64WRDSFKW),
# ULID(2021-04-27 22:17:37.844 UTC: 01F4APHGSMFJZQTGXKZBFFBPJP),
# ULID(2021-04-28 20:17:55.357 UTC: 01F4D231MXQJXAR8G2JZHEJNH3),
# ULID(2021-04-30 07:18:54.307 UTC: 01F4GTA2332AS2VPHC4FMKC7R5),
# ULID(2021-05-02 12:26:03.480 UTC: 01F4PGNXARG554Y3HYVBDW4T9S),
# ULID(2021-04-29 09:52:15.107 UTC: 01F4EGP483ZX2747FQPWQNPPMW),
# ULID(2021-04-29 03:18:24.152 UTC: 01F4DT4Z4RA0QV8WFQGRAG63EH),
# ULID(2021-05-02 13:27:16.394 UTC: 01F4PM605ABF5SDVMEHBH8JJ9R)]
ULID.sample(10, period: ulid1.to_time..ulid2.to_time)
# [ULID(2021-04-29 06:44:41.513 UTC: 01F4E5YPD9XQ3MYXWK8ZJKY8SW),
#  ULID(2021-05-01 00:35:06.629 UTC: 01F4JNKD85SVK1EAEYSJGF53A2),
#  ULID(2021-05-02 12:45:28.408 UTC: 01F4PHSEYRG9BWBEWMRW1XE6WW),
#  ULID(2021-05-01 03:06:09.130 UTC: 01F4JY7ZBABCBMX16XH2Q4JW4W),
#  ULID(2021-04-29 21:38:58.109 UTC: 01F4FS45DX4049JEQK4W6TER6G),
#  ULID(2021-04-29 17:14:14.116 UTC: 01F4F9ZDQ449BE8BBZFEHYQWG2),
#  ULID(2021-04-30 16:18:08.205 UTC: 01F4HS5DPD1HWDVJNJ6YKJXKSK),
#  ULID(2021-04-30 10:31:33.602 UTC: 01F4H5ATF2A1CSQF0XV5NKZ288),
#  ULID(2021-04-28 16:49:06.484 UTC: 01F4CP4PDM214Q6H3KJP7DYJRR),
#  ULID(2021-04-28 15:05:06.808 UTC: 01F4CG68ZRST94T056KRZ5K9S4)]

ULID specification ambiguity around orthographical variants of the format

I'm afraid so, we should consider Current ULID spec has orthographical variants of the format possibilities.

Uses Crockford's base32 for better efficiency and readability (5 bits per character)

The original Crockford's base32 maps I, L to 1, O to 0. And accepts freestyle inserting Hyphens (-). To consider this patterns or not is different in each implementations.

Current parser/validator/matcher aims to cover subset of Crockford's base32. I have suggested it would be clarified in ulid/spec#57.

Case insensitive

I can understand it might be considered in actual use-case. But it is a controversial point, discussing in ulid/spec#3.

Be that as it may, this gem provides API for handling the nasty possibilities.

ULID.normalize and ULID.normalized?

ULID.normalize('-olarz3-noekisv4rrff-q6ig5fav--') #=> "01ARZ3N0EK1SV4RRFFQ61G5FAV"
ULID.normalized?('-olarz3-noekisv4rrff-q6ig5fav--') #=> false
ULID.normalized?('01ARZ3N0EK1SV4RRFFQ61G5FAV') #=> true

UUIDv4 converter for migration use-cases

ULID.from_uuidv4 and ULID#to_uuidv4 is the converter. The imported timestamp is meaningless. So ULID's benefit will lost.

# Currently experimental feature, so needed to load the extension.
require 'ulid/uuid'

# Basically reversible
ulid = ULID.from_uuidv4('0983d0a2-ff15-4d83-8f37-7dd945b5aa39') #=> ULID(2301-07-10 00:28:28.821 UTC: 09GF8A5ZRN9P1RYDVXV52VBAHS)
ulid.to_uuidv4 #=> "0983d0a2-ff15-4d83-8f37-7dd945b5aa39"

uuid_v4s = { SecureRandom.uuid }
uuid_v4s.uniq.size == 10000 #=> Probably `true`

ulids = { |uuid_v4| ULID.from_uuidv4(uuid_v4) } == uuid_v4s #=> **Probably** `true` except below examples.

# NOTE: Some boundary values are not reversible. See below.

ULID.min.to_uuidv4 #=> "00000000-0000-4000-8000-000000000000"
ULID.max.to_uuidv4 #=> "ffffffff-ffff-4fff-bfff-ffffffffffff"

# These importing results are same as on CPython 3.9.4
reversed_min = ULID.from_uuidv4('00000000-0000-4000-8000-000000000000') #=> ULID(1970-01-01 00:00:00.000 UTC: 00000000008008000000000000)
reversed_max = ULID.from_uuidv4('ffffffff-ffff-4fff-bfff-ffffffffffff') #=> ULID(10889-08-02 05:31:50.655 UTC: 7ZZZZZZZZZ9ZZVZZZZZZZZZZZZ)

# But they are not reversible! Need to consider this issue in
ULID.min == reversed_min #=> false
ULID.max == reversed_max #=> false

How to migrate from other gems

As far as I know, major prior arts are below

ulid gem - rafaelsales/ulid

It is just providing basic String generator only. So you can replace the code as below


NOTE: It had crucial issue for handling precision, in version before 1.3.0, when you extract timestamps from old generated ULIDs, it might be not accurate value.

  1. Sort order does not respect millisecond ordering

  2. Fixed in this PR

  3. Released in 1.3.0

ulid-ruby gem - abachman/ulid-ruby

It is providing basic generator(except monotonic generator) and parser. Major methods can be replaced as below.


NOTE: It is still having precision issue similar as ulid gem in the both generator and parser. I sent PRs.

  1. Parsed time object has more than milliseconds

  2. Fix to handle timestamp precision in parser

  3. Fix to handle timestamp precision in generator

Compare performance with them

See Benchmark.

The results are not something to be proud of.