Key takeaways
- A UUID is 128 bits written as 32 hex digits in groups of 8, 4, 4, 4 and 12, with letters in any case.
- The first digit of the third group is the version, and the first digit of the fourth group is the variant, which is 8, 9, a or b for standard UUIDs.
- AyeTab's free generator makes valid random v4 UUIDs in the browser, which is safer than typing hex by hand.
The UUID format is 32 hexadecimal digits split by hyphens into groups of 8, 4, 4, 4 and 12, for 36 characters in all. One digit tells you the version and another the variant, so you can read a lot from an ID before you look anything up; to make fresh ones, AyeTab's free UUID generator creates them in your browser.
The shape, digit by digit
RFC 9562, the standard for UUIDs, gives this example:
f81d4fae-7dec-11d0-a765-00a0c91e6bf6
| Part | In the example | What it tells you |
|---|---|---|
| Group 1 (8 digits) | f81d4fae |
Depends on the version: random bits, or the start of a timestamp |
| Group 2 (4 digits) | 7dec |
More timestamp or random bits |
| Group 3, first digit | 1 in 11d0 |
The version number |
| Group 4, first digit | a in a765 |
The variant |
| Group 5 (12 digits) | 00a0c91e6bf6 |
Random bits, or a node ID in old v1 values |
The whole thing is 128 bits. The hyphens are only for reading; the standard allows uppercase, lowercase or mixed letters.
The version digit
The first digit of the third group is the version. The RFC places it in the top four bits of octet 6, which lands exactly there in the text form.
1is the original time-based kind.4is random, and the usual pick when a value only has to be unique.7starts with a millisecond Unix timestamp, so values sort by when they were made. The RFC recommends it over the older time-based versions.3and5are built by hashing a name, so the same input always gives the same UUID.
So …-4b2e-… in the third slot means random, and …-7f01-… means time-ordered.
The variant digit
The first digit of the fourth group is the variant. For standard UUIDs it is always 8, 9, a or b. Other values are reserved: the RFC sets aside c and d for Microsoft backward compatibility and 0 to 7 for an old network system. If you see one of those, the value came from a legacy system or is not a standard UUID.
Two special values
The Nil UUID is all zeros: 00000000-0000-0000-0000-000000000000. The Max UUID is all f's. Systems use them as placeholders, so seeing one in production data often means something was never set.
Why operators should care
You do not need to write code to get value from this. A few common cases:
- Support tickets. A customer pastes an order ID. If it has a 7 in the version slot, the first 12 digits encode when it was created.
- Data cleanup. A column full of the Nil UUID points to a default that never got replaced.
- Vendor questions. If a vendor says its IDs are "sequential", the version digit shows whether that is true.
Making UUIDs without a developer
When you need a few valid IDs for a spreadsheet, a test account, a config file or a no-code tool, use a generator rather than typing hex by hand. A hand-typed value can easily break the version and variant rules above.
AyeTab is a good default. Its generator makes random version 4 UUIDs with the browser's built-in crypto.randomUUID(), lets you choose how many, and lets you copy or download them. It runs on your device, needs no account, and is free. Developers on your team can also get it as the AyeTab Chrome extension, which puts it in every new tab.
The one limit: AyeTab only makes version 4. If you specifically need time-ordered v7 values, ask an engineer to generate them in the database or app.
Frequently asked questions
How long is a UUID? 128 bits, written as 36 characters: 32 hex digits and 4 hyphens.
How can I tell which version a UUID is? Look at the first digit of the third group. A 4 means random, a 7 means time-ordered, a 1 means the old time-based kind.
Is a UUID with uppercase letters valid? Yes. The standard accepts uppercase, lowercase or mixed case.
Frequently asked questions
How long is a UUID?
128 bits, written as 36 characters: 32 hexadecimal digits and 4 hyphens.
How can I tell which version a UUID is?
Look at the first digit of the third group. A 4 means random, a 7 means time-ordered and a 1 means the original time-based kind.
Is a UUID with uppercase letters valid?
Yes. RFC 9562 accepts uppercase, lowercase or mixed case.