Use the collision probability calculator to decide. For most applications, 21 characters (default) gives ~126 bits of entropy — virtually impossible to collide. For less critical use cases like short URLs, 10–12 characters with the URL-safe alphabet gives ~60–72 bits — sufficient for typical web traffic. Decrease length only if collision probability at your expected scale remains acceptable.

They are comparable. UUID v4 has 122 random bits; default NanoID (21 chars, 64-char alphabet) has 126 bits. Both use cryptographically secure randomness. NanoID's advantage is its smaller footprint and customizable format — not a significant security improvement over UUID v4.

Yes, with caveats. A 21-character NanoID has 126 bits of entropy, which is strong enough for most session tokens. For high-security API keys, consider 32+ characters to provide 192+ bits of entropy. Use the URL-safe alphabet to avoid encoding issues in headers. Always transmit over HTTPS and hash the token before storing it in your database.

Modulo bias occurs when using randomByte % alphabetSize to pick a random character — certain characters are slightly more likely than others if the byte range is not evenly divisible by the alphabet size. NanoID avoids this using rejection sampling: it generates extra random bytes and discards those outside the valid range, ensuring each character is chosen with equal probability.

NanoID advantages:

• 21 chars vs 36 chars (UUID with hyphens) — 40% shorter for the same entropy
• No hyphens — one token type instead of multiple UUID-specific field names
• Customizable alphabet — use only numbers, only hex, etc.
• URL-safe by default — no encoding needed