UUID Generator

About the UUID Generator

Software constantly needs names for things that must never clash: a customer record, an uploaded file, a message in a queue, a browser session, a row created offline on a phone. The classic way to hand out identifiers is a counter in one database, but a counter needs a single authority that everyone has to ask. A UUID, short for universally unique identifier, removes that bottleneck. Any machine can invent one on its own, at any time, and the odds that two machines ever pick the same value are so small that nobody checks.

This tool produces version 4 UUIDs, the random variety defined in RFC 4122. Out of the 128 bits in a UUID, 6 are fixed to mark the version and variant, leaving 122 bits chosen by your browser's cryptographic random generator. They are displayed as 32 hexadecimal digits in groups of 8-4-4-4-12, such as 3fa73e40-e967-46bf-9d62-988d4e4ee1a1. You can make up to 100 at a time, remove the hyphens, switch to uppercase, copy one, or copy the whole list.

Developers use these values for database keys, correlation IDs in logs, test fixtures, file names, and idempotency keys for payment requests. Because the list is created in your browser, nothing is sent to a server. A version 4 UUID is an identifier, not a secret and not a sortable timestamp, and the sections below explain the practical consequences of each fact so you pick the right tool for each job.

When to Use It

How to Use It, Step by Step

1. Look at the "How many" box, which starts at 5. Type a whole number from 1 to 100. Values above 100 are reduced to 100, and anything that is blank, zero, or not a number gives a single UUID.
2. Tick or clear the Hyphens box. It is ticked by default, which gives the standard 36-character form. Clear it for a 32-character string with no separators, which some databases and file systems prefer.
3. Tick or clear Uppercase. It is cleared by default, giving lowercase hex digits, which is the form most libraries and standards emit. Uppercase is common in some Microsoft tools.
4. Press Generate to create a fresh batch. A first batch of five is created automatically when the page loads, so there will always be something to copy right away.
5. Changing the Hyphens or Uppercase options after generating re-formats the same list immediately; it does not create new identifiers. Only the Generate button makes new ones, and that replaces the previous list.
6. Press the small copy icon at the end of a row to copy that single UUID, or press "Copy all" to copy the whole list with one value per line, in the format currently selected.
7. Paste the values where you need them and keep a note of which ones you used, because there is no history: once you press Generate again, the earlier list is gone.

Understanding Your Results

Each row is a version 4 UUID. You can verify this by eye. In the standard hyphenated form, the first digit of the third group is always 4, which marks the version, and the first digit of the fourth group is always one of 8, 9, a, or b, which marks the RFC 4122 variant. Everything else is random. With hyphens off, the same facts hold for characters 13 and 17 of the 32-character string. If a value fails these checks, it did not come from a standard version 4 generator.

Formatting options change only how a value is displayed, not what it is. Uppercase and lowercase forms are the same identifier, and hyphens are only punctuation, so 3fa73e40-e967-46bf-9d62-988d4e4ee1a1 and 3FA73E40E96746BF9D62988D4E4EE1A1 are equal. When storing or comparing, normalize to one form, or better, store the 16 raw bytes your database offers, and compare case-insensitively.

Uniqueness is statistical, not guaranteed, but the numbers are extreme. With 122 random bits there are about 5.3 x 10^36 possible values. Generating 100 UUIDs in a batch gives a collision probability around 10^-33, far below the chance of a hardware fault. The real risk is not collisions but misuse: a weak random source, copying one value into many places, or assuming that a UUID is secret or ordered when it is neither.

More Worked Examples

Tips for Better Results

• If you need more than 100 values, press Generate, copy the batch, and repeat. Each batch is independent, so pasting several together is safe.
• Store UUIDs in your database's native uuid type, if it has one, instead of as 36-character text. This halves the storage and speeds up comparisons.
• When using random UUIDs as primary keys in a large indexed table, expect scattered inserts. If insert speed matters at scale, consider a time-ordered scheme such as UUID version 7, or add a separate sequential column for ordering.
• Keep hyphens in anything that humans will read or paste into URLs. They make a long value much easier to check by eye, and most libraries expect the hyphenated form.
• Do not parse meaning out of a version 4 UUID. It carries no time, machine, or user information, so use a separate created-at column for that.
• Test your parsers with both cases and both forms. Real-world systems send uppercase, lowercase, hyphenated, and bare values, and your code should accept all of them.

Common Mistakes to Avoid

• Using a UUID as a password, session token, or API secret by default. It is unpredictable, but it is designed to be an identifier that is often logged and displayed; use a purpose-built token generator for secrets.
• Expecting UUIDs to sort by creation time. Version 4 values are fully random, so their alphabetical order has no relation to when they were made.
• Comparing UUID strings without normalizing case or hyphens. 3FA7... and 3fa7... are the same identifier, but a naive text comparison treats them as different.
• Regenerating when you meant to reformat. Pressing Generate replaces the list with new values; use the Hyphens and Uppercase boxes if you only want a different appearance of the same IDs.
• Assuming you can recreate a lost value later. Nothing is stored, and the earlier list disappears on the next Generate or page reload, so copy what you need first.

Assumptions and Limits

• Values come from the browser's cryptographic random generator through crypto.randomUUID when it is available, or from random bytes with the version and variant bits set by hand when it is not. Both yield valid version 4 UUIDs.
• Uniqueness is probabilistic. For practical purposes collisions do not happen with a healthy random source, but no tool can offer an absolute guarantee, so systems that depend on uniqueness should still enforce it, for example with a database constraint.
• The tool produces version 4 only. It does not generate time-based version 1 or version 7 UUIDs, namespace-based version 3 or 5, or sequential GUIDs.
• Nothing is stored or transmitted. That also means there is no history or undo, and it is your responsibility to save any values you want to keep.

Key Terms

UUID

A universally unique identifier, a 128-bit value usually written as 32 hexadecimal digits in groups of 8-4-4-4-12, designed to be unique without central coordination.

GUID

Globally unique identifier, Microsoft's name for the same kind of 128-bit value. In practice, GUID and UUID are interchangeable.

Version 4

The UUID version that fills nearly all of its bits with random data. The only fixed parts are the version digit 4 and the variant bits.

Collision

Two independently generated identifiers that happen to be identical. With 122 random bits it is astronomically unlikely.

Idempotency key

A unique value sent with a request so the server can recognize a retry and avoid doing the same action twice.