Strong password generator: random and secure.

A strong password is one an attacker cannot guess, and the only reliable way to get that is randomness plus length. Attackers rarely type guesses into a login form. They take a leaked database of password hashes and test candidates offline at high speed, starting with words, names, dates, keyboard patterns and passwords from earlier breaches, then the same with common substitutions like @ for a and 1 for i.

That is why Summer2026! is weak despite having upper and lower case, a number and a symbol: it follows a pattern attackers try early. A randomly generated password has no pattern, so the attacker's only option is brute force across every possible combination. Reuse is the other big risk. When one site is breached, attackers try the same email and password everywhere else, so every account needs its own password.

Entropy measures, in bits, how many guesses a brute-force attack needs. A password of length L chosen at random from a pool of N characters has L × log₂(N) bits, and every extra bit doubles the work. This generator's full pool has 87 characters (26 lowercase, 26 uppercase, 10 digits and 25 symbols), so each character adds about 6.4 bits: 16 characters give about 103 bits and 20 characters about 129.

To put numbers on it, assume an attacker can test a trillion (10¹²) guesses per second offline, a generous figure for a fast hash. 40 bits falls in about a second. 60 bits takes days. 80 bits takes tens of thousands of years on average, and 128 bits is out of reach for any conceivable hardware. The strength meter here rates under 40 bits as weak, 40 to 60 fair, 60 to 80 strong and 80 or more very strong.

The formula only applies to randomly generated passwords. A human-chosen password of the same length has far less real entropy, because people choose predictably. Strictly, guaranteeing one character from every chosen set removes a tiny fraction of combinations, so the true figure is a fraction of a bit lower than the formula; at 16 or more characters that is irrelevant.

Every character comes from crypto.getRandomValues, the browser's cryptographically secure random number generator, which is seeded by the operating system. Math.random() is never used: it is fast but predictable and unsuitable for secrets.

Turning random numbers into characters has a classic trap called modulo bias: taking a random 32-bit number modulo 87 makes some characters slightly more likely than others. This generator uses rejection sampling, discarding the few values that would cause bias, so every character is exactly equally likely. It picks one character from each set you selected, fills the rest from the whole pool, then shuffles the result with a Fisher-Yates shuffle driven by the same secure source.

The symbol set is !@#$%^&*()-_=+[]{};:,.?/~, chosen to avoid quotes, backslashes and spaces that commonly break forms, shells and config files. Passwords exist only in this tab. Nothing is stored, logged or sent, and they disappear when you leave the page.

NIST Special Publication 800-63B, the US government's digital identity guideline that many organisations worldwide follow, was revised in its fourth edition in 2025. It requires passwords of at least 15 characters when a password is the only authentication factor, and at least 8 when it is used as part of multi-factor authentication. Services should accept passwords of at least 64 characters and any printable character, including spaces and Unicode.

Revision 4 also says verifiers must not impose composition rules such as 'one uppercase letter, one number and one symbol', and must not force periodic password changes unless there is evidence of compromise. Instead, they should check new passwords against lists of breached and commonly used passwords and allow password managers and pasting. In short: length and randomness beat complexity rules, which matches what the entropy maths shows.

For accounts stored in a password manager, 16 to 20 random characters from all sets is a comfortable default: well over 100 bits, and you never type it. Use 20 or more for admin, cloud and financial accounts. Wi-Fi passwords (WPA2 and WPA3 personal) typically accept 8 to 63 characters; 20 or more random characters keeps offline attacks on a captured handshake impractical.

Some sites still cap length or reject certain symbols. Untick Symbols if a site refuses them and add four or five characters to make up the entropy. Exclude look-alikes when a password will be read aloud or typed from paper: it removes I, l, 1, O, 0, o, S, 5, Z, 2, B and 8, shrinking the pool to 75 characters, so add a character or two. For API keys, database credentials and signing secrets, use 32 or more characters, or generate keys with your platform's secret tooling.

A passphrase of randomly chosen words, such as five or six words picked by dice from a 7,776-word list (about 12.9 bits per word), is easier to remember and type, which makes it a good choice for the one password you must memorise: your password manager's master password or a device login. It must be truly random; a favourite quote is not.

Passkeys go further. Built on FIDO2 and WebAuthn, they replace the password with a key pair stored on your device, so there is nothing to phish, reuse or leak from a server. Where passkeys are not offered, combine a unique generated password with multi-factor authentication. Our guide to sessions, JWTs and passkeys explains how to add them to your own app.

If you run a service, never store passwords in plain text, encrypted or with a fast hash such as SHA-256. Use a slow, salted password-hashing function: Argon2id, or scrypt, bcrypt or PBKDF2 where required. The hash generator explains why fast hashes fail here. Rate-limit login attempts, screen new passwords against breach lists, and offer passkeys and MFA.