UUID vs ULID: Which Should You Use in 2026?
A practical comparison of UUID v4 and ULID for primary keys, distributed systems, and APIs — with concrete advice on when each one wins.
The problem with sequential integer IDs
Sequential integer IDs feel natural. They are compact, human-readable, and easy to reason about. For a simple CRUD app running on a single server they are often fine. But they carry a cluster of problems that surface quickly once a system grows or becomes externally accessible.
The most immediate issue is enumeration. If your API returns a record at /orders/1042, an attacker can trivially scrape /orders/1 through /orders/9999 and discover every resource you have ever created. You are also leaking business intelligence — anyone who creates two accounts a year apart can estimate your growth rate from the ID delta.
Coordination is the deeper architectural problem. Sequential IDs require a single authoritative counter. That counter becomes a bottleneck in distributed systems. When you have multiple database shards, multiple write regions, microservices generating records offline, or clients creating optimistic local state before syncing to a server, there is no safe way for two nodes to independently agree on what "the next integer" is without talking to each other first. In practice this forces you to either centralize writes (hurting availability) or accept conflicts (hurting correctness).
Both UUID and ULID solve the coordination problem by making identifiers large enough that independent generation is collision-resistant without any shared state. They differ in how they balance other trade-offs.
What is UUID v4?
UUID v4 is defined by RFC 4122. It is a 128-bit value where all bits except for a few version and variant markers are filled with cryptographically random data. The canonical string format is thirty-six characters including four hyphens:
xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx
Example:
f47ac10b-58cc-4372-a567-0e02b2c3d479The 4 in the third group identifies the version. The first hex digit of the fourth group is constrained to 8, 9, a, or b to encode the variant. Everything else is random.
With roughly 122 bits of randomness, the probability of two independently generated UUIDs colliding is astronomically small. Generating a billion UUIDs per second for the next hundred years would still leave the collision probability far below one in a trillion. For practical purposes, collisions are not a real concern.
UUID strengths
UUID is an open standard (RFC 4122) with native support in PostgreSQL, MySQL, SQL Server, Oracle, MongoDB, and virtually every programming language. You will never hit a library gap.
Any client, service, or shard can generate a valid UUID independently with no risk of conflict. This is the core reason to use them.
A UUID v4 reveals nothing about when a record was created, how many records exist, or what the system looks like internally. This is ideal for public-facing identifiers.
UUID has been around since the 1980s. Validation patterns, ORMs, admin UIs, and logging infrastructure all handle them without special configuration.
UUID weaknesses
Because UUID v4 values are random, new inserts scatter across the index rather than appending to the end. This causes frequent page splits in B-tree indexes and can degrade write performance significantly on high-volume tables with many indexes.
There is no way to sort UUID v4 values chronologically. If you need to page through records in creation order, you need a separate timestamp or sequence column.
At 36 characters including hyphens, UUIDs are bulky in URLs, logs, and payloads. The hyphenless form reduces this to 32 characters, but it is still large compared to alternatives.
Nobody can remember or transcribe a UUID. Support workflows that require reading IDs aloud become painful.
What is ULID?
ULID stands for Universally Unique Lexicographically Sortable Identifier. Like UUID v4, it is 128 bits and collision-resistant. Unlike UUID v4, those 128 bits are split into two parts with different purposes: a 48-bit millisecond timestamp prefix followed by 80 bits of random data.
The result is encoded using Crockford Base32 — a case-insensitive alphabet that excludes visually ambiguous characters like I, L, O, and U. The encoded string is always exactly 26 characters with no hyphens.
Structure:
ttttttttttrrrrrrrrrrrrrrrrr
^^^^^^^^^^ ^^^^^^^^^^^^^^^^
48-bit ms 80-bit random
timestamp
Example:
01J3KQZM4XCRDB6HVD8FQTKP2NBecause the timestamp occupies the most significant bits, ULIDs sort lexicographically in the same order they were created. Two ULIDs generated in the same millisecond are distinguished by their random component. The monotonic variant of ULID increments the random component within the same millisecond to guarantee strict ordering even under burst generation.
ULID strengths
ULIDs sort by creation time naturally. You can ORDER BY id to get chronological results without a separate timestamp column, and B-tree inserts append near the end of the index rather than scattering randomly.
At 26 characters with no hyphens, a ULID is significantly shorter than a UUID. It fits cleanly in URLs, log lines, and JSON payloads.
With 80 bits of randomness per millisecond, ULID remains collision-safe under any realistic generation rate.
When generating multiple ULIDs within the same millisecond, the monotonic mode increments the random component by one rather than re-rolling, ensuring strict ordering even under high throughput.
ULID weaknesses
ULID is not an IETF standard. Native database support is limited — PostgreSQL and MySQL do not have a built-in ULID type, so you store them as text or binary. Library availability varies by language and is less mature than UUID.
The embedded timestamp is not a bug — it is a feature — but it is a privacy consideration. Anyone with access to a ULID can extract an accurate creation timestamp down to the millisecond. For public-facing IDs this may be undesirable.
The monotonic variant requires in-process state to track the last-generated ULID. That state is not shared across processes, so distributed systems generating ULIDs concurrently in different processes may produce non-monotonic sequences even with the monotonic flag enabled.
Because ULID is not an RFC standard, validation rules and error messages in frameworks tend to be custom or absent, adding implementation overhead.
Format comparison side by side
Here is the same logical record represented with both identifier types, so you can see the formatting difference at a glance:
# UUID v4 (36 characters, hyphenated, fully random)
f47ac10b-58cc-4372-a567-0e02b2c3d479
# ULID (26 characters, no hyphens, time-prefixed)
01J3KQZM4XCRDB6HVD8FQTKP2N
# Multiple ULIDs generated in sequence sort correctly:
01J3KQZM4XCRDB6HVD8FQTKP2N # earliest
01J3KQZM7ABCDE1FGH2JKLMN3P # slightly later
01J3KQZM9XYZABCDEFGHJ12345 # latest
# Multiple UUID v4 values do NOT sort by creation time:
3a2bc401-...
f47ac10b-... # could be older or newer — no way to tell
01d8ef22-...Use our UUID generator tool to generate RFC 4122 compliant UUIDs instantly for development, testing, or production use.
Decision matrix: UUID vs ULID
| Requirement | Choose UUID | Choose ULID |
|---|---|---|
| Creation time privacy | Yes — fully opaque | No — timestamp is embedded |
| Universal DB support needed | Yes — native type everywhere | No — store as text/binary |
| Integrating with auth/OAuth systems | Yes — most use UUID | Less common |
| Natural sort order by time | No — use separate timestamp | Yes — built in |
| Efficient B-tree inserts | No — random scatter | Yes — near-sequential |
| Shorter IDs in URLs/logs | No — 36 chars | Yes — 26 chars |
| Time-based cursor pagination | No — needs extra column | Yes — sort by id directly |
| New greenfield system | Works well | Excellent choice |
| Existing legacy system | Easier to adopt | More migration work |
When to choose UUID
UUID v4 is the right choice when you need the creation time of a record to remain private. Because it is fully random, a UUID reveals nothing about when it was issued. This matters for user account IDs, payment references, and any public identifier where leaking a creation timestamp could be a privacy concern or give competitors insight into your activity levels.
UUID is also the pragmatic choice when you need universal database and framework support without custom configuration. If you are working with an ORM, an admin panel, a hosted database service, or a third-party auth provider, UUID v4 is the path of least resistance. Most systems speak UUID natively.
Similarly, if you are integrating with existing authentication systems — OAuth providers, JWT libraries, identity platforms — those systems almost universally use UUID as their subject identifier. Adopting ULID in that context creates a mismatch between your internal and external identifier formats.
When to choose ULID
ULID wins when natural sort order is a core system requirement. If you are building a system where records need to be paginated, browsed, or displayed in creation order, the ability to sort by the primary key directly — without maintaining a separate created_at column or secondary index — is a genuine architectural simplification. It also means your B-tree primary index stays compact and append-friendly, which can meaningfully improve write throughput on tables that would otherwise suffer from UUID-driven page fragmentation.
ULID is especially compelling on greenfield systems where you control the full stack. When you choose your own database schema, ORM configuration, and API design from scratch, adopting ULID costs little and buys you time-based sortability for free. The lack of a native database type is a minor inconvenience when you are setting up the schema yourself.
For time-based cursor pagination specifically, ULID is particularly elegant. Cursor pagination over UUID requires a composite cursor of (created_at, id) to be stable. With ULID you can use the ID itself as the cursor, simplifying both the query and the API contract.
What about UUID v7?
UUID v7 is worth a brief mention. Ratified as part of RFC 9562 in 2024, it takes the same design insight as ULID — a millisecond timestamp prefix followed by random bits — and packages it in the familiar UUID format with hyphens and a 36-character string representation.
UUID v7 is effectively the standardized answer to "I want ULID-like sortability but with UUID format compatibility." It sorts chronologically, works with existing UUID columns and tooling, and is backed by an IETF standard rather than a community specification. Database and language support is growing rapidly.
If you are starting a new project and sortability matters, UUID v7 deserves serious consideration. It gives you the ecosystem breadth of UUID with the index-locality benefits of ULID. The main caveat is that it is newer — library support is not yet as consistent as UUID v4, and some infrastructure (admin tools, hosted services) may not recognize the version marker correctly. In 2026 this is becoming less of a concern but is worth verifying for your specific stack.
Practical recommendation
For most teams in 2026, the decision comes down to two scenarios. If you are adding identifiers to an existing system, integrating with third-party auth, or need maximum database and tooling compatibility without any custom work, choose UUID v4. It is battle-tested, universally understood, and has zero adoption friction.
If you are building a new system from scratch, care about index performance at scale, and want time-based sortability built into your primary key, choose ULID or UUID v7. Both give you the sortability and B-tree efficiency that UUID v4 lacks. ULID has a slight edge in compactness and a broader existing community. UUID v7 has a slight edge in standards backing and format compatibility.
When creation-time privacy is a hard requirement for a public-facing identifier, UUID v4 remains the only clean answer — the embedded timestamp in ULID and UUID v7 is a deliberate design choice, not an optional feature that can be disabled.
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