What is the difference between a ULID and a UUID?

Both are 128-bit unique IDs, but a ULID is sortable and shorter on the wire. A UUIDv4 is fully random, so two IDs created a second apart have no order — you need a separate created_at column to sort by time. A ULID puts a millisecond timestamp in its high bits, so 01ARZ3NDEK... created earlier always sorts before 01ARZ3NF... created later. ULIDs are 26 characters in Crockford Base32 versus 36 for a hyphenated UUID, and they are URL-safe with no dashes.

Why can ULIDs be sorted by time with a plain string sort?

Because the timestamp lives in the leading characters and is encoded big-endian. The first 10 of the 26 characters are the 48-bit millisecond time, most-significant bits first, in Crockford Base32. Lexicographic ordering of those characters matches numeric ordering of the timestamp, so ORDER BY id and ORDER BY created_at give the same result. The 16 random characters only break ties between IDs minted in the very same millisecond.

How are the 26 characters split between time and randomness?

10 characters of timestamp and 16 of randomness. Crockford Base32 stores 5 bits per character: 10 chars × 5 = 50 bits hold the 48-bit timestamp (the first character only uses 3 of its 5 bits, which is why a valid ULID never starts above 7), and 16 chars × 5 = 80 bits of randomness. 48 + 80 = 128 bits total, the same payload size as a UUID, just encoded more compactly.

What does monotonic mode do and when should I use it?

Monotonic mode guarantees strictly ascending ULIDs even when you generate several in the same millisecond. Without it, two IDs sharing a timestamp differ only in their random tail, so their relative order is random. With it, the generator reuses the timestamp and increments the random component by one, so id[1] > id[0] always holds. Use it when you insert many rows per millisecond and rely on the ID for ordering, e.g. an event log or an outbox table.

Can a ULID be reversed into its creation time, and is that a privacy risk?

Yes — the first 10 characters decode straight back to the millisecond it was minted, which the decoder on this page shows. That is a feature for debugging and sorting, but be aware: exposing a ULID in a public URL leaks roughly when the record was created, and adjacent monotonic ULIDs can hint at how fast you are creating records. The 80 random bits are not guessable, so the ID itself is not predictable, but if creation timing is sensitive, do not surface ULIDs publicly.

Pick a primary key that sorts by insertion time

You are choosing IDs for a new Postgres table and want the latest rows without an extra ORDER BY created_at. Switch from UUIDv4 to ULID: a B-tree index on the ID column stays roughly time-ordered, so inserts append near the right edge instead of scattering, and "give me the 50 newest" is just ORDER BY id DESC LIMIT 50. Generate a few here to eyeball the format before you commit to the column type.

Build an append-only event log or outbox

An outbox table or event stream needs a key that is unique AND keeps events in the order they happened. Turn on monotonic mode and every ULID is strictly greater than the last, even at thousands of inserts per second within one millisecond. A consumer reads ORDER BY id and replays events in exactly the order the producer wrote them.

Decode a ULID from a log line to find when it happened

A support ticket quotes a record ID like 01HN8X... and you need to know when it was created without querying the database. Paste it into the decoder and read the exact millisecond plus the UTC time. No schema access, no created_at lookup — the timestamp is baked into the ID itself.

Seed test fixtures with realistic, ordered IDs

Writing integration tests that assert ordering? Generate a batch of monotonic ULIDs, drop them into your fixtures, and your assertions on "newest first" hold deterministically. Copy all and paste straight into a seed file or a JSON fixture.

ULID Generator — Sortable Unique IDs + Timestamp Decoder

Sortable, URL-safe unique IDs — batch generate, monotonic mode, decode the timestamp back out — 100% in your browser

Runs locally
Category Developer & DevOps
Best for Formatting, validating, shrinking, or inspecting code-adjacent text.

A ULID is 26 Crockford Base32 characters: the first 10 are a millisecond timestamp, the last 16 are randomness. Sort the strings and you sort by time.

How many

Monotonic

Lowercase

Strictly ascending, even within the same millisecond.

Generated ULIDs

Click Generate to mint ULIDs.

Decode a ULID

The decoded timestamp shows up here.

What this tool does

Generate ULIDs in the browser: 26-character, lexicographically sortable, URL-safe unique identifiers that beat random UUIDv4 when you want a primary key you can also sort by time. A ULID packs a 48-bit millisecond timestamp into its first 10 Crockford Base32 characters and 80 bits of randomness into the last 16, so a plain string sort of your IDs is also a sort by creation time — no separate created_at column required. Generate 1 to 100 at once, flip on monotonic mode for strictly ascending order even when many IDs land in the same millisecond, lowercase them, copy one or copy all. The built-in decoder takes any ULID and reads the timestamp back out as milliseconds plus a human-readable UTC and local time, and it validates the Crockford alphabet (no I, L, O, U) so a mistyped ID is caught immediately. Randomness comes from crypto.getRandomValues; nothing is uploaded.

Tool details

Input: Text + Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result + Copy + Download; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 9 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Developer & DevOps · Developer; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How ULID Generator fits into your work

Use it in the small gaps between coding, reviewing, debugging, and shipping.

Developer jobs

Formatting, validating, shrinking, or inspecting code-adjacent text.
Preparing snippets for documentation, tickets, commits, or handoff.
Checking a small payload quickly without switching tools.

Developer checks

Run irreversible transforms like minify or obfuscate on a copy.
Keep secrets out of pasted snippets unless the tool explicitly stays local.
Use your normal tests or linter before shipping transformed code.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Pick a primary key that sorts by insertion time
You are choosing IDs for a new Postgres table and want the latest rows without an extra ORDER BY created_at. Switch from UUIDv4 to ULID: a B-tree index on the ID column stays roughly time-ordered, so inserts append near the right edge instead of scattering, and "give me the 50 newest" is just ORDER BY id DESC LIMIT 50. Generate a few here to eyeball the format before you commit to the column type.
Build an append-only event log or outbox
An outbox table or event stream needs a key that is unique AND keeps events in the order they happened. Turn on monotonic mode and every ULID is strictly greater than the last, even at thousands of inserts per second within one millisecond. A consumer reads ORDER BY id and replays events in exactly the order the producer wrote them.
Decode a ULID from a log line to find when it happened
A support ticket quotes a record ID like 01HN8X... and you need to know when it was created without querying the database. Paste it into the decoder and read the exact millisecond plus the UTC time. No schema access, no created_at lookup — the timestamp is baked into the ID itself.
Seed test fixtures with realistic, ordered IDs
Writing integration tests that assert ordering? Generate a batch of monotonic ULIDs, drop them into your fixtures, and your assertions on "newest first" hold deterministically. Copy all and paste straight into a seed file or a JSON fixture.

Common pitfalls

Treating a ULID's randomness as a security token. The 80 random bits make collisions virtually impossible, but the timestamp half is fully readable. A ULID is an identifier, not a secret — never use one as a session token, password-reset code, or anything that must be unguessable in full.
Forgetting to enable monotonic mode for high-rate inserts. If you mint many IDs in the same millisecond without it, their order is random and your "sort by ID" ordering silently breaks for bursts. Turn monotonic on whenever insertion order must equal ID order.
Hand-typing or regex-validating ULIDs with the wrong alphabet. Crockford Base32 excludes I, L, O and U, so a validator that allows the full A–Z will accept malformed IDs. Decode or validate against the exact 32-character set, which is what this tool does.

Privacy

Every ULID is built in your browser tab. The 80 random bits come from crypto.getRandomValues — the same cryptographic source the platform uses for keys — never Math.random, and they never leave the page. The decoder runs entirely client-side too, so a ULID you paste to inspect its timestamp is not logged or transmitted. Only your settings (count and the monotonic / lowercase toggles) ride in the shareable URL; the generated IDs themselves are re-randomized on every click and stay local.

FAQ

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ULID Generator — Sortable Unique IDs + Timestamp Decoder

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How ULID Generator fits into your work

Developer jobs

Developer checks

Good next steps

Real-world use cases

Pick a primary key that sorts by insertion time

Build an append-only event log or outbox

Decode a ULID from a log line to find when it happened

Seed test fixtures with realistic, ordered IDs

Common pitfalls

Privacy

FAQ

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