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


npm install --save ulid


TypeScript, ES6+, Babel, Webpack, Rollup, etc.. environments

import { ulid } from 'ulid'

CommonJS environments

const ULID = require('ulid')

AMD (RequireJS) environments

define(['ULID'] , function (ULID) {


<script src="/path/to/ulid.js"></script>


To generate a ULID, simply run the function!

import { ulid } from 'ulid'

Seed Time

You can also input a seed time which will consistently give you the same string for the time component. This is useful for migrating to ulid.

ulid(1469918176385) // 01ARYZ6S41TSV4RRFFQ69G5FAV

Monotonic ULIDs

To generate monotonically increasing ULIDs, create a monotonic counter.

Note that the same seed time is being passed in for this example to demonstrate its behaviour when generating multiple ULIDs within the same millisecond

import { monotonicFactory } from 'ulid'
const ulid = monotonicFactory()
// Strict ordering for the same timestamp, by incrementing the least-significant random bit by 1
ulid(150000) // 000XAL6S41ACTAV9WEVGEMMVR8
ulid(150000) // 000XAL6S41ACTAV9WEVGEMMVR9
ulid(150000) // 000XAL6S41ACTAV9WEVGEMMVRA
ulid(150000) // 000XAL6S41ACTAV9WEVGEMMVRB
ulid(150000) // 000XAL6S41ACTAV9WEVGEMMVRC
// Even if a lower timestamp is passed (or generated), it will preserve sort order
ulid(100000) // 000XAL6S41ACTAV9WEVGEMMVRD

Pseudo-Random Number Generators

ulid automatically detects a suitable (cryptographically-secure) PRNG. In the browser it will use crypto.getRandomValues and on node it will use crypto.randomBytes.

Allowing the insecure Math.random

By default, ulid will not use Math.random, because that is insecure. To allow the use of Math.random, you'll have to use factory and detectPrng.

import { factory, detectPrng } from 'ulid'
const prng = detectPrng(true) // pass `true` to allow insecure
const ulid = factory(prng)
ulid() // 01BXAVRG61YJ5YSBRM51702F6M

Use your own PRNG

To use your own pseudo-random number generator, import the factory, and pass it your generator function.

import { factory } from 'ulid'
import prng from 'somewhere'
const ulid = factory(prng)
ulid() // 01BXAVRG61YJ5YSBRM51702F6M

You can also pass in a prng to the monotonicFactory function.

import { monotonicFactory } from 'ulid'
import prng from 'somewhere'
const ulid = monotonicFactory(prng)
ulid() // 01BXAVRG61YJ5YSBRM51702F6M

Implementations in other languages

Refer to ulid/spec


Refer to ulid/spec

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