Developer & Tech

ULID Generator

What this does

Generate lexicographically sortable ULIDs; Crockford base32 timestamps plus 80 bits of monotonic randomness per the spec.

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Runs in your browser

Calculator inputs

How to use it

Using the ulid generator

  1. 01

    Set the count

    Up to fifty per run for fixtures or seed data.

  2. 02

    Copy the newline-separated list

    They sort correctly as plain strings; no special comparison needed.

  3. 03

    Verify via the timestamp row

    The first ULID decodes back to its creation millisecond in UTC.

Good to know

ULID versus UUIDv4

Both give 128-bit uniqueness, but ULIDs embed creation time up front. Database indexes stay append-friendly, logs interleave chronologically under sort, and debugging gains a free “when was this created?”; advantages UUIDv7 pursues with a different layout.

Why Crockford base32

Human transcription drops characters that look alike. Removing I, L, O and U (and accepting lowercase input) makes ULIDs safe to read aloud or retype; a small ergonomic win that compounds across on-call shifts.

How it's calculated

The math behind this calculator

ULID = 10 chars (48-bit ms timestamp) + 16 chars (80-bit randomness), Crockford base32

Every identifier packs the current epoch millisecond into its leading ten characters using Crockford base32 (alphabet 0123456789ABCDEFGHJKMNPQRSTVWXYZ; no I, L, O or U, avoiding transcription mistakes), followed by sixteen characters of cryptographically-sourced randomness.

Within the same millisecond, randomness is forced monotonically increasing, so sorting strings sorts them by creation order exactly. The first ULID’s embedded timestamp is decoded back to a human-readable date so you can verify the structure.

Assumptions & limitations

  • 48-bit timestamps cover dates until year 10895 CE.
  • Monotonicity is enforced within a generation batch, not across processes.
  • Charset excludes I/L/O/U per the Crockford base32 convention.

Worked example

Five identifiers come out sharing a timestamp prefix (same generation burst) with distinct random tails; and string order equals creation order.

FAQ

Frequently asked questions

Are ULIDs URL-safe?
Yes; the entire alphabet is uppercase alphanumerics with no symbols, so they drop into URLs, filenames and CSVs without escaping.
How do I extract the timestamp myself?
Decode the first ten characters as Crockford base32; the resulting integer is epoch milliseconds. This page shows that decoded value for the first ULID automatically.
What if two services generate ULIDs in the same millisecond?
Cross-process monotonicity is not guaranteed. Collisions remain astronomically unlikely thanks to 80 random bits, but strict global ordering needs single-writer design.
Can existing UUID columns migrate to ULIDs?
Commonly yes: both fit in binary(16) or fixed-width character columns; many stores keep both formats side by side during migration windows.

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