Almost everything the average person has been told about passwords is wrong, and the wrongness traces back to one 2003 document that its own author later apologised for. The advice to force uppercase, lowercase, a number and a symbol, then rotate every 90 days, made passwords harder for humans and barely harder for machines.

You can measure any password's strength with the Password Strength Checker — it runs in your browser and nothing you type is transmitted.

Entropy, without the maths lecture

Entropy measures how many guesses an attacker needs. It is expressed in bits, and each extra bit doubles the work.

The formula is simple: bits = length x log2(size of the character set).

  • Lowercase only (26 characters): about 4.7 bits per character
  • Lower + upper (52): about 5.7 bits
  • Lower + upper + digits (62): about 5.95 bits
  • All printable ASCII (95): about 6.55 bits

Now compare. P@ssw0rd! is 9 characters from a 95-character set — roughly 59 bits on paper. correct horse battery staple is 28 lowercase characters and spaces — roughly 130 bits.

The second one is vastly stronger and far easier to remember. Length wins, and it is not close.

Why the on-paper number lies

That 59-bit estimate for P@ssw0rd! is fiction, and this is the part most strength meters get wrong.

Entropy calculations assume every character is chosen randomly. Humans do not choose randomly. We take a word, capitalise the first letter, swap a for @ and o for 0, and stick an exclamation mark on the end — because that is exactly what the complexity rule asked for.

Attackers know this. Password cracking tools apply these substitutions as standard rules. P@ssw0rd! is not a 59-bit random string; it is the word "password" with four predictable transformations, and it falls in milliseconds.

A meter that scores it highly is measuring the wrong thing. Good meters check against known-breached password lists and common patterns rather than just counting character classes.

What actually makes a password strong

Length, first and by a wide margin. Every added character multiplies the search space. Going from 8 to 16 characters is not twice as hard to crack; it is astronomically harder.

Genuine randomness. A password chosen by a machine is unpredictable in a way one chosen by a person is not — our brains reach for patterns without noticing.

Uniqueness per site. This matters more than strength. A 40-character password reused across ten sites is worth nothing once any one of those ten is breached. Credential stuffing — replaying leaked pairs against other services — is the single most common way accounts are taken over, and password strength is irrelevant to it.

Not previously breached. Billions of real passwords are in public leak sets. Attackers try those first. A password appearing in one of those lists has effectively zero strength no matter how complex it looks.

Passphrases

Four or five random words — chosen by dice or software, not by you — give excellent entropy and are memorable. With a 7,776-word list, each word contributes about 12.9 bits, so five words is roughly 64 bits of genuinely random entropy.

The catch is "random". A phrase you invent yourself is not; it comes from your vocabulary, your interests, song lyrics you know. Let a tool pick the words.

What modern guidance actually says

NIST revised its recommendations years ago, and the current advice is close to the opposite of the old rules:

  • Require length, minimum 8 and allow at least 64.
  • Do not impose composition rules. No forced symbol-and-number requirements.
  • Do not expire passwords arbitrarily. Forced rotation makes people pick weaker passwords and increment a digit. Rotate only on evidence of compromise.
  • Screen against breached password lists at the point of choosing.
  • Allow paste. Blocking paste breaks password managers, which pushes people towards passwords they can type from memory.

If you build login forms, that list is the specification worth following.

The honest answer to "how strong is strong enough"

For anything protected by a password manager: 20+ random characters, unique per site. You never type it, so length costs nothing.

For the handful you must memorise — your password manager's master password, your device login, your primary email — use a long random passphrase and turn on two-factor authentication.

Two-factor is worth more than any amount of extra password length. A strong password stops guessing; 2FA stops a password that has already leaked from being enough.

Common questions

Is a longer password better than a complex one?
Yes. Length adds entropy far faster than character variety, and it does not push people into predictable substitutions.

How many bits of entropy are enough?
Roughly 60+ for accounts that matter, 80+ for anything high-value. Above 100 the password stops being the weak point.

Should I change passwords every 90 days?
No, unless there is evidence of compromise. Forced rotation reliably produces weaker passwords.

Are password managers safe?
Far safer than the alternative, which is reuse. A single strong master password plus 2FA protects unique random passwords everywhere else.

Where to go next