How Strong Is My Password? Length, Entropy and Real Rules
Length beats symbols
Each extra character multiplies the search space, so a 16-character random password is astronomically harder to crack than an 8-character one with a symbol bolted on. Modern guidance from NIST recommends long passwords and drops forced complexity rules and periodic rotation.
What entropy means in practice
Entropy in bits measures unpredictability. Roughly: under 50 bits is weak, 60–80 bits is fine for ordinary accounts, and 100+ bits is appropriate for a password-manager master password or an encryption key. A truly random 16-character mixed-charset password lands around 100 bits.
Where passwords actually get broken
- Reuse — one breached site exposes every account sharing that password
- Predictable patterns — Summer2024!, names, keyboard walks
- Phishing, which defeats strength entirely; enable a phishing-resistant second factor
- Weak reset flows on the account's email address
Passphrases
Four to six random words are easy to type on a phone and strong when the words are chosen randomly, not by you. A phrase you invented from a favourite lyric is far weaker than it feels.
The maths behind entropy
Entropy is log2(charset_size ^ length) for a randomly generated password. A 12-character password from a 95-character printable set is about 79 bits; the same length from lowercase letters only is about 56 bits. Each additional character adds roughly 6.5 bits with a full charset, which is why length dominates every other factor.
The formula only holds when the characters are chosen randomly by a machine. A human-chosen 12-character password typically carries 25–30 bits of real entropy, because people pick words, dates and predictable substitutions.
What attackers actually do
Nobody brute-forces a login form character by character; rate limiting stops that in seconds. Real attacks run offline against a stolen password database, at billions of guesses per second on consumer GPUs, and they do not start with aaaaaaaa.
- Credential stuffing — replaying email and password pairs from earlier breaches against other sites; defeated only by never reusing passwords.
- Dictionary and rule attacks — every common word with predictable mutations such as
P@ssw0rd!andSummer2026, tried within the first seconds. - Mask attacks — exploiting the fact that most people follow "capital, word, digits, symbol" in that order.
- Phishing and session theft — no password strength helps here, which is why a second factor matters more than adding another symbol.
Why hashing choice matters more than your password
If a site stores passwords with unsalted SHA-256, an attacker tests billions of candidates per second and even good passwords fall. With bcrypt, scrypt or Argon2id at sensible parameters, the same hardware manages thousands per second and a 14-character random password becomes economically unbreakable.
As a developer, this is the highest-leverage decision you make about passwords: use Argon2id where available, bcrypt otherwise, always with a per-user salt, and never a plain fast hash.
Passphrases done properly
A diceware-style passphrase draws words at random from a known list. Four words from a 7,776-word list give about 51 bits, five words about 65, six words about 77 — comparable to a 12-character random string but far easier to type on a phone or read aloud.
The strength comes entirely from the randomness of the selection. "correct horse battery staple" is now a famous string in every cracking dictionary; a phrase you compose yourself from song lyrics or family names carries a fraction of the entropy it appears to.
A realistic password policy
Modern guidance, including NIST SP 800-63B, converges on a short list: require a minimum length of at least 12 characters, allow up to 64 and permit every character including spaces, check new passwords against a breached-password list, and drop both composition rules and scheduled expiry.
Pair that with a phishing-resistant second factor — passkeys or a hardware key rather than SMS — and the password stops being the weakest link in the account.
Generate one that is genuinely random
The Password Generator uses the browser's cryptographic random source rather than Math.random, and never sends the result anywhere. Store it in a password manager and turn on two-factor authentication.
Frequently asked questions
How often should I change passwords?
Only after a breach or suspected compromise. Forced rotation pushes people toward weaker, predictable variants.
Is a password manager safe?
Yes — the risk of one strong protected vault is far lower than the risk of reused passwords everywhere.
Are password strength meters accurate?
Only roughly. Most score composition rather than real unpredictability, so a predictable pattern can score green.
Is writing a password on paper a bad idea?
For a home account it is safer than reuse — your threat model is remote attackers, not someone in your kitchen. A password manager is still better.
Do passkeys replace passwords?
Increasingly, yes. They are phishing-resistant by design and remove the shared secret entirely where a site supports them.