How to Generate UUIDs v4: The Complete Developer Guide (2026)

UUIDs (Universally Unique Identifiers) are the backbone of modern systems. Whether you’re building a distributed database, implementing session management, or creating API endpoints, you need reliable unique IDs. Yet generating them correctly—securely and efficiently—is often overlooked.

Let’s dive deep into UUIDs, understand why they matter, and show you how to generate them using our free UUID Generator.

What Is a UUID?

A UUID is a 128-bit number used to identify information in computer systems. The most common version, UUID v4, generates random identifiers that are “practically unique”—meaning the probability of collision is so low that you don’t need to worry about it in any real-world application.

The standard format looks like this:

f47ac10b-58cc-4372-a567-0e02b2c3d47

This 36-character string consists of:

• 8-4-4-12 character segments separated by hyphens
• 32 hexadecimal digits (0-9, a-f)
• Total of 128 bits of information

Why UUID v4 Is Developer’s Choice

While there are 5 UUID versions (v1 through v5), UUID v4 dominates modern development:

• Random Generation: No coordination required between systems. Each device generates UUIDs independently without risking collisions.
• Security: Generated using cryptographically secure random number generators—unlike Math.random(), which is predictable.
• No Metadata: Unlike UUID v1, v4 doesn’t embed timestamps or MAC addresses, protecting privacy.
• Performance: Browser’s crypto.randomUUID() API generates UUIDs in microseconds.

Essential Use Cases for UUIDs

1. Database Primary Keys

When your application scales across multiple databases or shards, auto-incrementing IDs (1, 2, 3…) become a coordination nightmare. Two databases could generate the same ID simultaneously, causing data corruption.

UUIDs solve this completely. Each database generates IDs independently, and the probability of collision is essentially zero.

2. Session Tokens and Authentication

When users log in, you generate a session token—a string that identifies their browser session without revealing their user ID or password.

Our UUID Generator creates cryptographically secure UUIDs perfect for this. Attackers can’t predict the next session token, even if they’ve captured millions of previous ones.

3. API Request IDs

For debugging, logging, or tracking individual API calls, assign a UUID to each request:

{
  "requestId": "550e8400-e29b-41d4-a716-446655440000",
  "endpoint": "/api/users",
  "timestamp": "2026-01-17T10:30:00Z"
}

This lets you trace a specific API call across multiple services, logs, and databases.

4. Password Reset Tokens

When a user requests a password reset, email them a link containing a unique token. That token should be:

• Hard to guess: Cryptographically random
• One-time use: Expired after a short period
• Unlinkable: Can’t be used to infer other tokens

UUID v4 fits all these requirements perfectly.

How to Generate UUIDs Securely

The Problem with Math.random()

You might see tutorials suggesting this approach:

// ❌ DON'T DO THIS
function generateUuid() {
  return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
    const r = Math.random() * 16 | 0;
    const v = c === 'x' ? r : (r & 0x3 | 0x8);
    return v.toString(16);
  });
}

Why This Is Dangerous:

• Math.random() is not cryptographically secure. A determined attacker with enough captured outputs can predict future values.
• Browser implementations vary. Some have low entropy, making collisions more likely.
• No standardization across browsers and runtimes.

The Correct Approach: crypto.randomUUID()

Modern browsers provide a native API designed exactly for this purpose:

// ✅ DO THIS
const uuid = crypto.randomUUID();
// Output: "f47ac10b-58cc-4372-a567-0e02b2c3d47"

Our UUID Generator uses this API exclusively. It runs 100% client-side in your browser—your UUIDs are generated locally, never transmitted to any server.

Bulk Generation: Why It Matters

Need to generate test data? Seeding a database with mock records? Creating unique IDs for an array of items?

Generating UUIDs one at a time in a loop works, but it’s tedious. Our UUID Generator supports bulk generation—select any quantity from 1 to 100 and generate them all instantly.

JSON Output for Developers

Bulk generation is useful, but often you need UUIDs in a specific format for your code. Our UUID Generator offers two output modes:

• Plain Text: One UUID per line—perfect for copying, scripts, or logs
• JSON Array: Ready to paste directly into JavaScript, TypeScript, or configuration files

Example JSON output:

[
  "f47ac10b-58cc-4372-a567-0e02b2c3d47",
  "7c9e66f9-7291-4595-a6c5-5a6b5a5a7c5a",
  "1e4b4b8-4b8d-4b8d-8b8d-8b8d8b8d8b8d"
]

Copy this directly into your code, save as a .json file, or use it in API testing tools like Postman.

Security Considerations for UUIDs

Cryptographic Strength Matters

Not all random number generators are equal. For security-sensitive contexts (session tokens, password resets, API keys), you must use cryptographically secure algorithms.

crypto.randomUUID() meets this standard. It’s backed by:

• High-entropy random number generators (CSPRNG)
• Browser security implementations (hardware random number generation where available)
• FIPS 140-2 compliance (in supported environments)

UUID vs Hash

Sometimes developers confuse UUIDs with hashes (like MD5 or SHA-256). Key differences:

Feature	UUID v4	Hash (SHA-256)
Purpose	Unique identification	Data integrity/fingerprinting
Input	Random (no input)	Any data (deterministic)
Collision	Extremely unlikely	Impossible with same input
Reversibility	Cannot reverse to original	Cannot reverse to original

If you need both unique ID and data integrity, use both: generate a UUID, then compute a hash of associated data.

Performance: UUIDs vs Auto-Increment

Common concern: “Aren’t UUIDs slower than 1, 2, 3...?”

Reality Check:

• Database Indexing: Modern databases handle UUID indexing efficiently. PostgreSQL, MySQL, and MongoDB all optimize for UUID storage.
• Insert Speed: Yes, UUIDs are slightly slower to insert than auto-increment integers. But in real-world applications, this difference is measured in microseconds—not seconds.
• Network Transmission: 36 characters vs. 1-10 digits for integers adds negligible latency.

If you’re experiencing performance issues with UUIDs, it’s usually not the UUIDs themselves—but how you’re using them (non-clustered indexes, improper column types, wrong index strategy).

Common Mistakes to Avoid

1. Using UUID v1 for Privacy-Sensitive Apps

UUID v1 embeds your MAC address and timestamp. If an attacker captures one UUID, they can infer your identity and when you generated it.

Always use v4 for user-facing applications.

2. Removing Hyphens “For Performance”

Some developers strip hyphens to save 4 bytes (36 characters → 32 characters). This is premature optimization.

The standard format with hyphens is:

• More readable (easier to copy-paste manually)
• Standard-compliant (spec requires hyphens)
• Identifiable as UUID (regex ^[0-9a-f]{8}-[0-9a-f]{4}-... matches)

Save 4 bytes elsewhere—like using Protocol Buffers or MessagePack for data transmission.

3. Generating in Backend Instead of Frontend

Sometimes you want to generate UUIDs on the server (e.g., for record creation). But for many use cases (session tokens, password resets, client-side tracking), generating in the browser reduces server load and improves privacy.

Our UUID Generator handles both scenarios perfectly.

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Frequently Asked Questions

Q: How many UUIDs can I generate at once?

A: Use our UUID Generator and select any quantity from 1 to 100. All are generated instantly in your browser, and you can copy them as plain text or as a JSON array for direct integration.

Q: What’s the collision probability of UUID v4?

A: The chance of two UUID v4s colliding is approximately 1 in 2^122, or about 1 in 5.3 × 10^36. For all practical purposes, consider it zero. You’d need to generate billions of UUIDs per second for thousands of years before a likely collision.

Q: Why use UUIDs instead of auto-incrementing IDs?

A: UUIDs don’t require coordination between systems. They prevent ID conflicts across distributed databases, shards, or microservices. They also don’t reveal business information (like “we have 1 million users” which auto-increment IDs do). Our UUID Generator makes creating them trivial.

Q: Can I predict the next UUID generated?

A: No. Our UUID Generator uses crypto.randomUUID(), a cryptographically secure API. This means generated values are unpredictable, making them suitable for security-sensitive uses like session tokens and password resets.

Q: What’s the difference between UUID v4 and UUID v5?

A: UUID v4 is randomly generated, while UUID v5 is derived from a namespace and a name (deterministic). Use v4 for most cases where you just need unique IDs. Use v5 when you need the same input to always produce the same UUID (like generating consistent IDs for given email addresses).

Q: Are UUIDs URL-safe?

A: Standard UUIDs contain hyphens (f47ac10b-...) which are technically URL-safe. However, some developers prefer to remove hyphens. Our UUID Generator outputs standard format with hyphens—you can strip them if needed for your specific use case.

Q: How long does it take to generate a UUID?

A: Instantly. Our UUID Generator runs 100% in your browser using native crypto.randomUUID() API, which generates a UUID in microseconds. Even generating 100 UUIDs at once happens in milliseconds.

Q: Why do UUIDs have hyphens?

A: Hyphens are part of the UUID 4122 standard. They make UUIDs more readable and help identify them as UUIDs (vs. random 32-character strings). The format is 8-4-4-12, representing the 5 distinct time/version, clock sequence, and node segments.

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Start Generating Secure UUIDs Today

Whether you’re building a new feature, seeding test data, or implementing authentication, UUIDs are the modern, secure way to create unique identifiers.

Our UUID Generator is free, runs entirely in your browser (no data leaves your device), and supports bulk generation for any scale.

Try it now:

• Generate Single UUID - For immediate use
• Generate 100 UUIDs at Once - For batch processing
• Copy as JSON Array - For direct code integration

Stop worrying about ID collisions. Start using cryptographically secure UUIDs with our UUID Generator.

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