When should you use exactly 21 characters?
Use this preset for accounts and applications that publish this exact maximum or required length. Do not shorten or extend the result manually after generation.
Compatibility and security: A 21-character random password can be strong when it is unique and generated from a broad alphabet, but the destination's current rules remain authoritative.
Character-set compatibility
Some systems count punctuation but reject selected symbols, spaces, or non-ASCII characters. Disable only the unsupported group, generate a completely new value, and verify that the destination preserves all 21 characters.
Storage and reuse
Save the accepted result in a trusted password manager or secret store. Never reuse it on another account, and do not paste a generated account password into a public strength-testing service.