CUID Generator

About CUID Generator

The CUID Generator creates Collision-Resistant Unique Identifiers — a type of structured unique ID designed by Eric Elliott for distributed systems. A CUID begins with 'c', followed by a timestamp, a counter, a client fingerprint, and random data. The structured composition makes CUIDs highly resistant to collisions even when generated simultaneously across many independent clients, without requiring coordination. CUIDs are also safe to use as HTML element IDs and URL path segments due to their lowercase alphanumeric format. This tool generates both the original CUID format and the newer CUID2 format (which improves on the original's fingerprint predictability).

Why use CUID Generator

How to use CUID Generator

1. Click Generate to create a CUID.
2. Select CUID or CUID2 from the format toggle.
3. Set a count for bulk generation.
4. Click Copy to copy the generated IDs.
5. Toggle between CUID and CUID2 to compare formats; CUID2 is the modern default and preferred for new code.
6. For CUID2, choose a length between 24 and 32 characters to balance compactness against the cryptographic security margin you need.

When to use CUID Generator

• Generating unique IDs on the client side without a server round-trip.
• Creating database primary keys safe for horizontal scaling.
• Generating unique HTML element IDs in dynamic UIs.
• Building offline-capable apps where server-issued IDs are unavailable.
• Generating IDs for offline-first applications where the device cannot reach a central counter at write time.
• Producing test data for unit and integration tests where pure-random IDs are easier to debug than UUID hex.

Examples

Tips

• Always prefer CUID2 in new projects — the original CUID's host fingerprint can leak machine details that are not always safe to expose.
• CUID2 IDs are intentionally unsorted; do not rely on them for any time-ordering, even loosely. Use ULID or UUID v7 if you need chronological order.
• Configure the CUID2 length to between 24 and 32 characters for production primary keys — shorter values are still collision-resistant but harder to debug.
• Do not pass user input through CUID generation; CUID2 hashes the inputs and re-using user data weakens uniqueness guarantees.
• When migrating from CUID v1 to CUID2, never coerce the legacy IDs to the new format — keep them side by side and add new IDs as a separate column.
• CUID2 is intentionally not strictly random; it uses a slow hash. Avoid generating millions per second on a single client without measuring CPU impact.
• Treat all CUIDs as opaque strings in your schema. Any code that parses a CUID's structure couples you to the implementation and breaks across major versions.

Frequently Asked Questions

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Glossary

CUID

Collision-resistant Unique Identifier — a structured ID format introduced by Eric Elliott in 2012 that combines timestamp, counter, fingerprint, and randomness.

CUID2

A 2022 redesign of CUID using SHA-3 hashing to remove host fingerprint leakage and predictability; default length is 24 characters.

Host fingerprint

A short hash derived from the client device or process used in CUID v1 to differentiate concurrent generators; removed in CUID2 due to leakage concerns.

Counter component

An incrementing number embedded in CUID v1 that disambiguates IDs generated in the same millisecond by the same client.

Block size (CUID2)

The number of characters in the final CUID2 string; configurable between 24 and 32 for typical use, with security increasing per character.

Entropy

Randomness drawn from the Web Crypto API and mixed into the SHA-3 input for CUID2; ensures unpredictability across calls and clients.

URL safety

Property of an ID format that uses only characters valid in URLs without percent-encoding; CUID and CUID2 are URL-safe by design.