UUID v4 vs ULID — Which Identifier Should You Use?

When you need a unique identifier, two modern options stand out: the universally supported UUID and the newer, sortable ULID. Choosing well depends on whether you care about ordering, readability and URL safety.

What is a UUID?

A UUID is a 128-bit identifier, most commonly generated as version 4 from random data. It is supported nearly everywhere and is a safe default for keys and API identifiers.

A UUID v4 looks like:

6f9c0b7e-4b3a-4c0f-9a1d-2e8f5b6c7d8e

What is a ULID?

ULID is also 128 bits, but it combines a timestamp with randomness. The result is sortable by creation time and is friendlier for URLs.

A ULID looks like:

01J9Z3A7ZQ0V4R2S8T9U1W3X5Y

Key differences

• Sortability — ULIDs sort by creation time; UUID v4 values do not.
• Readability — ULIDs are shorter and use an unambiguous alphabet.
• Ecosystem — UUID is supported natively by more databases and languages.

Generating them

JavaScript:

const uuid = crypto.randomUUID()

Python:

import uuid
print(uuid.uuid4())

PHP:

echo bin2hex(random_bytes(16));

Frequently asked questions

• Are these identifiers guaranteed unique? They rely on cryptographic randomness, so collisions are practically impossible.
• Which should I use for database keys? UUID v4 is the safest general default; ULID helps when you want time-ordered rows.
• Is anything uploaded when I generate them? No. Generation runs locally in your browser.

Need a batch of identifiers right now? Try the free UUID Generator to create UUID v4 and ULID values locally.