A universally unique identifier is a 128-bit value, written as 32 hexadecimal digits in five hyphen-separated groups (8-4-4-4-12). RFC 9562 defines it, and the same value is called a GUID on Windows. Because there are so many possible values, systems can create IDs independently, with no central counter, and expect them not to repeat.

What is the difference between UUID v4 and v7?

Version 4 is 122 random bits, so UUIDs have no order and a database index fills in random places. Version 7 puts a 48-bit Unix millisecond timestamp first, followed by 74 random bits, so newer UUIDs sort after older ones and inserts land at the end of an index. The cost is that a v7 UUID reveals when it was created, to the millisecond.

Are the UUIDs random enough, and are they unique?

They come from your browser’s cryptographically secure generator (crypto.randomUUID, or crypto.getRandomValues where that is missing). With 122 random bits, you would need about 2.7 quintillion version 4 UUIDs for a 50% chance of any two matching; generating a billion per second, that takes about 86 years. For a one in a billion chance of a repeat you could still make about 103 trillion of them. A collision is possible in theory but not something to plan around. Uniqueness is only as good as the random source, which is why generators should never use Math.random.

Is a UUID safe to use as a secret or password?

No. A UUID is built to be unique, not secret. A version 7 UUID shows its creation time, version 4 has a fixed shape, and UUIDs often appear in URLs and logs. Use a token from a password or secret generator for anything that grants access.

How does the inspector find the version and variant?

The version is the first hex digit of the third group (the 4 in 4320). The variant is the top bits of the first digit of the fourth group: 9bac starts with 9, binary 1001, which is the RFC 9562 variant. Nil (all zeros) and Max (all ones) are reported as such, and a UUID whose variant is not RFC 9562 has no version to report.

How do I read the timestamp inside a UUID?

For version 7 the first 12 hex digits are Unix milliseconds: 017F22E279B0 is 1645557742000, which is 2022-02-22T19:22:22.000Z. Versions 1 and 6 count 100-nanosecond ticks since 1582-10-15 instead; the version 1 example C232AB00-9414-11EC-B3C8-9F6BDECED846 decodes to 2022-02-22T19:22:22.0000000Z. The inspector does both and shows the time in UTC. Versions 3, 4, 5 and 8 contain no time.

Do v7 UUIDs made in the same millisecond stay in order?

Yes. After the first UUID in a millisecond the generator adds one to the random part for each further UUID, so a batch is strictly increasing (for example 018bcfe5-6800-7000-8000-000000000000 then 018bcfe5-6800-7000-8000-000000000001). Presses of the generate button on this page stay in order too, even if the clock steps back.

Which format should I use: hyphens, uppercase or braces?

The standard text form is lowercase with hyphens, and it is the form RFC 9562 specifies for output. Parsers accept either case. Use no hyphens for compact keys and file names, uppercase or braces only where a system such as the Windows registry or a .NET GUID expects them. The table above shows each form.

Are my UUIDs sent anywhere?

No. They are made in your browser and the inspector reads only what you type; nothing is sent or stored. The page counts only that the generate, copy or inspect buttons were used, never a UUID.

UUID Generator

A UUID is a 128-bit identifier written as 32 hex digits, such as 919108f7-52d1-4320-9bac-f847db4148a8. Version 4 is random (122 random bits) and version 7 starts with a Unix millisecond timestamp so UUIDs sort by creation time. Choose a version and a count up to 1,000 below to generate them on this device, then copy them all. The inspector reads any UUID you paste: the RFC 9562 example 017F22E2-79B0-7CC3-98C4-DC0C0C07398F is 7: Unix time, ordered, in the RFC 9562 variant, created at 2022-02-22T19:22:22.000Z.

Updated 2026-10-07Runs in your browser: nothing you enter is uploadedHow this tool works

Generate UUIDs

Example UUID from RFC 9562Made on your device

919108f7-52d1-4320-9bac-f847db4148a8

This example is printed in the page itself, so do not use it. With JavaScript on, the page makes fresh UUIDs on this device.

Inspect a UUID

Paste a UUID

Valid UUID: version 7 (unix time, ordered)

Standard form: 017f22e2-79b0-7cc3-98c4-dc0c0c07398f
Version: 7: Unix time, ordered
Variant: RFC 9562 (bits 10xx)
Embedded timestamp (UTC): 2022-02-22T19:22:22.000Z
Unix time (milliseconds): 1645557742000

UUIDs are made with your browser’s Web Crypto random number generator and are never sent, saved or logged. A UUID identifies something; it is not a secret, so do not use one as a password or access token.

UUID versions

The version is the first digit of the third group. Versions 1 to 8 are those RFC 9562 lists; version 2 predates it.
Version	Name	What it holds	Embedded timestamp
1	Gregorian time	60-bit count of 100-nanosecond ticks since 1582-10-15, a clock sequence and a node ID	Yes
2	DCE Security	Like version 1, with a POSIX user or group ID replacing part of the time; not defined by RFC 9562	No
3	Name-based, MD5	MD5 hash of a namespace UUID and a name	No
4	Random	122 random bits	No
5	Name-based, SHA-1	SHA-1 hash of a namespace UUID and a name	No
6	Reordered Gregorian time	Version 1 with the time fields reordered so UUIDs sort by time	Yes
7	Unix time, ordered	48-bit Unix millisecond timestamp followed by 74 random bits	Yes
8	Custom	122 bits whose layout the application defines	No

UUID variants

The variant is in the top bits of the first digit of the fourth group: 8, 9, a or b for the RFC 9562 variant.
Leading bits	Variant	Meaning
0xxx	NCS backward compatibility	Reserved for the old Apollo Network Computing System.
10xx	RFC 9562	The variant this standard defines, used by every UUID the tool generates.
110x	Microsoft (legacy GUID)	Reserved for early Microsoft Windows GUIDs.
111x	Reserved for future definition	Includes the Max UUID, all ones.

Example UUIDs from RFC 9562, inspected

Each row is the inspector’s own output for a UUID printed in the standard.
UUID	Source	Version	Variant	Timestamp (UTC)
c232ab00-9414-11ec-b3c8-9f6bdeced846	RFC 9562 §5.1	1: Gregorian time	RFC 9562	2022-02-22T19:22:22.0000000Z
919108f7-52d1-4320-9bac-f847db4148a8	RFC 9562 §5.4	4: Random	RFC 9562	None
017f22e2-79b0-7cc3-98c4-dc0c0c07398f	RFC 9562 §5.7	7: Unix time, ordered	RFC 9562	2022-02-22T19:22:22.000Z
00000000-0000-0000-0000-000000000000	RFC 9562 §5.9	Nil UUID	NCS backward compatibility	None
ffffffff-ffff-ffff-ffff-ffffffffffff	RFC 9562 §5.10	Max UUID	Reserved for future definition	None

Where the 128 bits go

Both versions spend 6 bits on the version and variant, leaving 122 for the rest.
Field	Version 4	Version 7
Unix timestamp (milliseconds)	0 bits	48 bits
Version	4 bits (0100)	4 bits (0111)
Variant	2 bits (10)	2 bits (10)
Random	122 bits	74 bits
Total	128 bits	128 bits

The same UUID in common formats

The RFC 9562 version 7 example written the ways other systems expect it. Every one reads back as the same UUID (7: Unix time, ordered).
Format	Where you meet it	Written as
Lowercase with hyphens	RFC 9562, JSON, most databases and APIs	017f22e2-79b0-7cc3-98c4-dc0c0c07398f
Uppercase with hyphens	Some Windows and .NET output	017F22E2-79B0-7CC3-98C4-DC0C0C07398F
No hyphens	Compact IDs and file names	017f22e279b07cc398c4dc0c0c07398f
Braces	Windows registry and COM GUIDs	{017F22E2-79B0-7CC3-98C4-DC0C0C07398F}
URN	XML, RDF and other places that need a URN	urn:uuid:017f22e2-79b0-7cc3-98c4-dc0c0c07398f

How to generate UUIDs

Pick a version. Choose version 4 for a fully random identifier, or version 7 when you want UUIDs that sort by creation time, for example as database primary keys.
Choose how many. Enter a count from 1 to 1,000. The list updates as you change it.
Set the format. Tick uppercase, no hyphens or braces to match what your system expects, for example {919108F752D143209BACF847DB4148A8} for all three together.
Copy all. Press Copy all to put the list, one UUID per line, on your clipboard. Generate new makes a fresh list.
Inspect a UUID. Paste any UUID into the inspector to see its version, its variant and, for versions 1, 6 and 7, the time inside it.

Frequently asked questions

What is a UUID?: A universally unique identifier is a 128-bit value, written as 32 hexadecimal digits in five hyphen-separated groups (8-4-4-4-12). RFC 9562 defines it, and the same value is called a GUID on Windows. Because there are so many possible values, systems can create IDs independently, with no central counter, and expect them not to repeat.
What is the difference between UUID v4 and v7?: Version 4 is 122 random bits, so UUIDs have no order and a database index fills in random places. Version 7 puts a 48-bit Unix millisecond timestamp first, followed by 74 random bits, so newer UUIDs sort after older ones and inserts land at the end of an index. The cost is that a v7 UUID reveals when it was created, to the millisecond.
Are the UUIDs random enough, and are they unique?: They come from your browser’s cryptographically secure generator (crypto.randomUUID, or crypto.getRandomValues where that is missing). With 122 random bits, you would need about 2.7 quintillion version 4 UUIDs for a 50% chance of any two matching; generating a billion per second, that takes about 86 years. For a one in a billion chance of a repeat you could still make about 103 trillion of them. A collision is possible in theory but not something to plan around. Uniqueness is only as good as the random source, which is why generators should never use Math.random.
Is a UUID safe to use as a secret or password?: No. A UUID is built to be unique, not secret. A version 7 UUID shows its creation time, version 4 has a fixed shape, and UUIDs often appear in URLs and logs. Use a token from a password or secret generator for anything that grants access.
How does the inspector find the version and variant?: The version is the first hex digit of the third group (the 4 in 4320). The variant is the top bits of the first digit of the fourth group: 9bac starts with 9, binary 1001, which is the RFC 9562 variant. Nil (all zeros) and Max (all ones) are reported as such, and a UUID whose variant is not RFC 9562 has no version to report.
How do I read the timestamp inside a UUID?: For version 7 the first 12 hex digits are Unix milliseconds: 017F22E279B0 is 1645557742000, which is 2022-02-22T19:22:22.000Z. Versions 1 and 6 count 100-nanosecond ticks since 1582-10-15 instead; the version 1 example C232AB00-9414-11EC-B3C8-9F6BDECED846 decodes to 2022-02-22T19:22:22.0000000Z. The inspector does both and shows the time in UTC. Versions 3, 4, 5 and 8 contain no time.
Do v7 UUIDs made in the same millisecond stay in order?: Yes. After the first UUID in a millisecond the generator adds one to the random part for each further UUID, so a batch is strictly increasing (for example 018bcfe5-6800-7000-8000-000000000000 then 018bcfe5-6800-7000-8000-000000000001). Presses of the generate button on this page stay in order too, even if the clock steps back.
Which format should I use: hyphens, uppercase or braces?: The standard text form is lowercase with hyphens, and it is the form RFC 9562 specifies for output. Parsers accept either case. Use no hyphens for compact keys and file names, uppercase or braces only where a system such as the Windows registry or a .NET GUID expects them. The table above shows each form.
Are my UUIDs sent anywhere?: No. They are made in your browser and the inspector reads only what you type; nothing is sent or stored. The page counts only that the generate, copy or inspect buttons were used, never a UUID.

How it works

A version 4 UUID is crypto.randomUUID(), or 16 bytes from crypto.getRandomValues with the version bits set to 0100 and the variant bits to 10, which leaves 122 random bits. A version 7 UUID is the current time in Unix milliseconds as 48 bits, the version bits 0111, the variant bits 10 and 74 more bits taken from the same generator. Within one millisecond the random part starts from a random 73-bit value and counts up by one for each further UUID (the monotonic random method of RFC 9562 §6.2), and a clock that steps backwards keeps the last timestamp, so output is always in increasing order. The inspector accepts upper or lower case, optional hyphens, braces and a urn:uuid: prefix, reads the version and variant bits, and for versions 1 and 6 converts the 60-bit tick count from 1582-10-15 to UTC, and for version 7 the 48-bit millisecond count from 1970. The collision figures use the birthday approximation n ≈ √(2 · 2^122 · ln(1/(1 − p))). The example above is fixed, so the page looks the same whenever it is built.

Sources

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