What is the difference between UUID v4 and v7?

v4 is 122 bits of randomness — great for uniqueness but unordered, which fragments database indexes. v7 puts a 48-bit Unix-millisecond timestamp in the high bits, so v7 UUIDs sort in creation order while staying globally unique. Use v7 for new database primary keys.

What are v5 and v3 namespace UUIDs?

v5 (SHA-1) and v3 (MD5) are deterministic: the same namespace + name always produces the same UUID. They are ideal for generating stable IDs from URLs, file paths, or other natural keys. Prefer v5 over v3 because SHA-1 is stronger than MD5.

Can I decode a UUID to read its timestamp?

Yes for v1 and v7. The Decode tab extracts the embedded timestamp and shows the version, variant, and a color-coded bit layout. v4, v5, and v3 carry no recoverable timestamp.

Is this UUID generator secure and private?

v4 and v7 random bits come from the browser's cryptographically secure crypto.getRandomValues. Everything runs locally — no UUID, name, or namespace ever leaves your device.

UUID Generator — v4, v7, v1, v5 & v3

Version	Source	Ordered?	Best for
v1	Timestamp + node (MAC)	Yes (not lexically)	Legacy systems
v3	MD5(namespace + name)	No	Deterministic IDs (legacy)
v4	122 random bits	No	General-purpose unique IDs
v5	SHA-1(namespace + name)	No	Deterministic IDs (preferred)
v7	Unix-ms timestamp + random	Yes (lexically)	Database primary keys

Namespace	UUID
DNS	6ba7b810-9dad-11d1-80b4-00c04fd430c8
URL	6ba7b811-9dad-11d1-80b4-00c04fd430c8
OID	6ba7b812-9dad-11d1-80b4-00c04fd430c8
X.500	6ba7b814-9dad-11d1-80b4-00c04fd430c8

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Walk-through

How to Use This Calculator

3 steps

1

Pick a UUID version

Choose the version that fits your use. v4 (random) is the safe default for most IDs. v7 (time-ordered) is the modern choice for database primary keys because the rows sort by creation time. v1 is the legacy timestamp format. v5 and v3 are deterministic — the same namespace and name always produce the same UUID — which is ideal for stable IDs derived from a URL, file path, or other natural key.

2

Set the count and formatting

Enter how many UUIDs you need (1 to 1000) for the random and timestamp versions; namespace versions always return one value. Then toggle the format: UPPERCASE, with or without hyphens, wrapped in braces { } for Microsoft GUID style, or prefixed with urn:uuid: for the RFC URN form. The output regenerates instantly. For v5/v3, pick a namespace (DNS, URL, OID, X.500, or a custom UUID) and type the name to hash.

3

Copy, export, or decode

Use Copy All to grab the whole list, Download .txt for a plain file, or CSV for a spreadsheet with an index column. Switch to the Decode / Validate tab and paste any UUID to confirm it is valid, read its version and variant, see the bit layout, and — for v1 and v7 — extract the embedded creation timestamp.

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Reference

Formula & Methodology

3 formulas

Version 4 (random)

128 bits random → set bits 48–51 = 0100 (version 4), bits 64–65 = 10 (variant)

16 cryptographically random bytes are generated, then 6 bits are overwritten to mark the version (4) and the RFC 4122 variant. That leaves 122 bits of entropy — the source of v4's uniqueness.

Version 7 (time-ordered)

uuid = unix_ts_ms(48 bits) ‖ ver(4) ‖ rand_a(12) ‖ var(2) ‖ rand_b(62)

The most-significant 48 bits hold the Unix timestamp in milliseconds, so v7 UUIDs sort lexicographically in creation order. The remaining 74 bits (minus the version and variant markers) are random, preserving uniqueness within the same millisecond.

Version 5 / 3 (namespace)

hash = SHA1(namespace_bytes ‖ name_bytes) → take first 16 bytes → set version + variant

The 16 namespace bytes are concatenated with the UTF-8 bytes of the name and hashed — SHA-1 for v5, MD5 for v3. The first 16 bytes of the digest become the UUID after the version (5 or 3) and variant bits are set. Because hashing is deterministic, identical inputs always yield the same UUID.

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Glossary

Key Terms Explained

6 terms

UUID / GUID A Universally Unique Identifier: a 128-bit value written as 32 hexadecimal digits in five hyphen-separated groups (8-4-4-4-12). GUID is Microsoft's name for the same thing.

Version The generation scheme, stored in the 13th hex digit (e.g. the '4' in …-4xxx-…). Versions 1, 3, 4, 5, and 7 each derive their bits differently — from time, a name hash, or randomness.

Variant Two or three bits at the start of the 17th hex digit that identify the UUID layout family. The standard variant (10xx) is 'RFC 4122'; other values mark legacy NCS or Microsoft layouts.

Namespace A fixed UUID that scopes a name in v3/v5 generation. RFC 4122 defines four (DNS, URL, OID, X.500); you can also supply a custom namespace UUID.

Entropy / collision The number of random bits backing a UUID's uniqueness. v4 has 122 bits, making an accidental collision astronomically unlikely under normal generation volumes.

Nil / Max UUID Special all-zero (00000000-0000-0000-0000-000000000000) and all-one (ffffffff-…-ffffffffffff) values used as sentinels or range bounds. They carry no version.

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Scenarios

Real-World Examples

3 worked examples

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App developer seeding test data

Needs a batch of random IDs for fixtures

Version v4 (random) Count 25 Format lowercase, hyphenated

v4 is the right call for throwaway test IDs — no ordering needed, just 25 unique values. Each carries 122 random bits, so collisions across a test suite are effectively impossible. Copy All drops them straight into a fixtures file.

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Backend engineer choosing a primary key

Wants UUID keys that don't fragment the index

Version v7 (time-ordered) Count 10 Format lowercase, hyphenated

Because each v7 UUID begins with a millisecond timestamp, the ten values sort in the order they were created — new rows append near each other in a B-tree index instead of scattering like v4. Pasting one into the Decode tab shows the exact creation time, confirming the ordering.

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Platform team generating stable IDs

Needs the same ID every time for a given domain

Version v5 (namespace) Namespace DNS Name example.com

v5 hashes the DNS namespace with the name 'example.com', always producing cfbff0d1-9375-5685-968c-48ce8b15ae17. Re-running it anywhere yields the identical UUID, so two services can derive the same key for the same domain without coordinating — no database lookup required.

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Deep Dive

Understanding UUID Versions

3 chapters · 4 min read

A UUID is a 128-bit identifier you can generate anywhere, with no central registry, and still trust to be unique. But not all UUIDs are built the same way. This guide explains the five versions you'll actually encounter, why v7 is winning for database keys, how namespace UUIDs stay deterministic, and just how unlikely a collision really is.

v4 vs v7 — random vs time-ordered

Version 4 is 122 bits of pure randomness, which makes it the simplest and most widely supported UUID. Its weakness shows up at scale: because the values are unordered, inserting v4 keys into a database index scatters writes across the whole B-tree, causing page splits and cache misses.

Version 7 fixes this by placing a 48-bit Unix-millisecond timestamp in the high bits. The result still has plenty of randomness for uniqueness, but consecutive UUIDs share a leading prefix, so new rows append near each other. That keeps indexes compact and inserts fast — which is why v7 is increasingly the default for new primary keys.

Namespace UUIDs (v5 and v3)

Versions 5 and 3 are deterministic. You give them a namespace UUID and a name; they concatenate the bytes and hash them — SHA-1 for v5, MD5 for v3 — then carve a UUID out of the digest. The same inputs always produce the same UUID, which is perfect for deriving a stable ID from something you already have, like a URL or a file path.

Prefer v5 over v3: SHA-1 is stronger than MD5, and v3 exists mainly for compatibility with older systems. RFC 4122 ships four standard namespaces (DNS, URL, OID, X.500), and you can define your own by supplying any UUID as the namespace.

How likely is a collision?

With 122 random bits, version 4 draws from about 5.3 × 10³⁶ possible values. Using the birthday-problem approximation, you would need to generate roughly 2.71 × 10¹⁸ (2.71 quintillion) UUIDs before reaching even a 50% chance of a single collision — about a billion UUIDs every second for 85 years.

v7 carries fewer random bits (74) but scopes them to a single millisecond, so the practical collision risk only matters if you generate enormous volumes within the same millisecond on the same machine. For virtually every application, both versions can be treated as collision-free.

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Questions

Frequently Asked Questions

7 questions

What does this UUID generator compute?+

It produces RFC 4122 / RFC 9562 UUIDs in five versions — v4 (random), v7 (time-ordered), v1 (timestamp + node), and v5/v3 (deterministic namespace hashes) — and can decode any existing UUID to reveal its version, variant, and embedded timestamp. Random bits come from the browser's cryptographically secure generator.

Which version should I use?+

Use v4 for general-purpose unique IDs. Use v7 for database primary keys, where time-ordering keeps indexes efficient. Use v5 when you need the same ID every time for a given name (for example, deriving an ID from a URL). Use v1 or v3 only when an existing system requires them.

Are UUIDs guaranteed to be unique?+

Not mathematically guaranteed, but the probability of a collision is so small it can be ignored in practice. A v4 UUID has 122 random bits; you would need to generate about 2.71 quintillion of them to reach a 50% chance of any two matching.

Can I decode a UUID to find when it was created?+

Yes, for v1 and v7. Paste the UUID into the Decode / Validate tab and the embedded timestamp is extracted and shown in UTC. v4, v5, and v3 contain no recoverable timestamp — v4 is random, and v5/v3 are hashes.

Is a GUID the same as a UUID?+

Yes. GUID (Globally Unique Identifier) is Microsoft's term for the identical 128-bit identifier defined by RFC 4122. They are interchangeable, though Microsoft tooling often wraps GUIDs in braces — which you can reproduce here with the Braces { } format option.

How do I share my settings?+

Your version, count, and formatting choices are encoded in the page URL, so copying the address shares your exact configuration with a colleague. The generated UUIDs themselves are never stored or shared — only the settings, since the UUIDs would be different (and random) on every visit anyway.

Why generate v5/v3 only one at a time?+

Namespace UUIDs are deterministic: a given namespace and name always hash to the same value. Generating a 'batch' would just repeat that single UUID, so the tool returns one result and ignores the count for v5 and v3. Change the name to get a different UUID.

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UUID Generator

UUID Versions at a Glance

Predefined Namespaces (RFC 4122)

Code Snippets

How to Use This Calculator

Pick a UUID version

Set the count and formatting

Copy, export, or decode

Formula & Methodology

Key Terms Explained

Real-World Examples

App developer seeding test data

Backend engineer choosing a primary key

Platform team generating stable IDs

Understanding UUID Versions

v4 vs v7 — random vs time-ordered

Namespace UUIDs (v5 and v3)

How likely is a collision?

Frequently Asked Questions

Get a branded PDF of your results

UUID Generator

UUID Versions at a Glance

Predefined Namespaces (RFC 4122)

Code Snippets

How to Use This Calculator

Pick a UUID version

Set the count and formatting

Copy, export, or decode

Formula & Methodology

Key Terms Explained

Real-World Examples

Understanding UUID Versions

v4 vs v7 — random vs time-ordered

Namespace UUIDs (v5 and v3)

How likely is a collision?

Frequently Asked Questions

Get a branded PDF of your results

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