Password Generator

The generator above builds a password in your browser and shows two things about it: how much entropy it carries, in bits, and where that lands on a five step strength scale. Nothing is transmitted. The password is not sent to a server, not logged, and not stored, and the only thing this page ever writes to your browser is your option settings, and only if you press save.

How the randomness is produced

Characters come from crypto.getRandomValues, the browser's cryptographically secure random number generator, rather than from Math.random. That distinction matters more than it sounds. Math.random is built for speed, its output is predictable from enough samples, and it was never designed to keep secrets.

Each character is also drawn with rejection sampling rather than by taking a random number modulo the set size. Modulo is the obvious approach and it is quietly biased: 2³² does not divide evenly by 68, so the first few characters in the set would come up slightly more often than the rest. The bias is small, but it is free to avoid, so the generator throws away the values that would skew the result and draws again.

What entropy actually measures

Entropy is the size of the haystack, expressed in bits. For a password chosen at random it is the length multiplied by the base-2 logarithm of the character set:

entropy = length × log₂(set size)

With the default options the set holds 68 characters, which is log₂(68) or about 6.09 bits per character. Sixteen of them gives 97.4 bits, meaning the password is one of roughly 2⁹⁷ equally likely possibilities. An attacker checking every one would find it after half of them on average.

The word random is carrying the weight in that sentence. Entropy describes the process that produced the password, not the string itself. Tr0ub4dor&3 looks like it has plenty of entropy and has very little, because it was built from a dictionary word with predictable substitutions, and that is exactly what cracking software tries first.

What each character type buys you

Character types	Set size	Bits per character	16 characters
Lower case only	26	4.70	75.2 bits
Upper and lower	52	5.70	91.2 bits
Letters and digits	62	5.95	95.3 bits
The default here	68	6.09	97.4 bits
Everything, nothing excluded	94	6.55	104.9 bits

Look at the last column rather than the middle one. Going from letters only to the full printable set adds 0.85 bits per character, while adding four more characters to a lower case password adds 18.8. Length is the cheaper lever by a wide margin, which is why a long passphrase beats a short mess of punctuation.

How long a password survives

These figures assume an offline attack against a fast hash such as MD5 or SHA-1, where a rig of GPUs tests on the order of a trillion guesses a second, and they show the average rather than the worst case.

Entropy	Average time to guess
40 bits	under a second
50 bits	9 minutes
60 bits	7 days
70 bits	19 years
80 bits	19,000 years
97 bits (this page's default)	2.5 billion years
128 bits	longer than the universe has existed, many times over

Two caveats keep this honest. Against a slow password hash such as bcrypt or Argon2, which is what a competent service uses, the same hardware manages thousands or millions of guesses a second rather than a trillion, so every row shifts enormously in your favour. And against a login form with rate limiting, brute force is not the threat at all.

Which is the real point. Passwords with this much entropy are not broken by guessing. They are lost through a breach at the service that held them, through reuse across sites, or through a convincing fake login page. The generator handles the part that arithmetic can handle; the rest is habits.

Ambiguous characters and brackets

The ambiguous option removes characters that look alike in most fonts: capital I, lower case l and the digit 1 are nearly identical in many sans-serif faces, and O against 0 is not much better. Punctuation such as the backtick, the apostrophe, the comma and the full stop is included too, since those disappear at small sizes and get eaten by autocorrect.

Brackets are separate because they cause a different problem. Some systems, particularly older ones and anything that pipes input through a shell or a query, mishandle them.

Both options cost you something, and it is worth knowing how much. Excluding both drops the set from 94 characters to 68, which at 16 characters is 97.4 bits instead of 104.9, so the haystack is about 178 times smaller. That sounds severe and is not: adding a single character brings it back to 103.5 bits. If you are transcribing a password by hand or reading it aloud, take the exclusions and add a character or two.

Why no repeated characters makes it weaker

This option is the one people misread. Forbidding repeats feels tidier and it lowers entropy every time, because each choice you make removes an option from the ones that follow.

With repeats allowed, all 68 characters are available at every position, so entropy is 16 × log₂(68) = 97.4 bits. Without repeats the set shrinks as you go, and the total is log₂(68 × 67 × 66 × ... ), which is 94.6 bits. You have thrown away nearly three bits and told an attacker something useful about the structure, which is that no character appears twice.

It is a small loss and it is a loss. Use the option when a system genuinely rejects repeats, not because it looks more random.

Making a password you can actually remember

Generated passwords belong in a password manager. For the handful you have to type from memory, the master password for the manager, your device login, build a passphrase instead: several unrelated words chosen at random, not picked by you.

Random selection is the whole trick. Four words drawn from a 7,776 word list carry 51.7 bits, and six words carry 77.5, which beats almost anything a person invents unaided. Words you choose yourself, from your own vocabulary and associations, carry a fraction of that no matter how unlikely the sentence feels.

The rules worth keeping from conventional advice are the exclusions rather than the inclusions. Keep out anything drawn from your own life: names, birthdays, pets, addresses, the company you work for and its abbreviations. Keep out anything with a predictable shape, such as dates, phone numbers and licence plates. And check anything you invented against a breach list, because a password can be original to you and still appear in a leak of a billion others.

Protecting it once you have it

Never reuse it. This is the single highest-value habit. When a site is breached, attackers take the leaked pairs and try them everywhere else, an attack so routine it has a name, credential stuffing. One unique password per account turns any breach into a single-site problem, and a password manager is what makes that practical.

Turn on two-factor authentication. A second factor keeps an account standing even when the password is already gone. An authenticator app or a hardware key beats SMS, which can be redirected by taking over the phone number.

Do not share it, and treat any password you have shared as one you now need to change. The risk is not that the other person is careless with intent; it is that your secret is now stored somewhere you cannot see.

Stop rotating passwords on a schedule. This reverses the advice most people were taught, and the reversal is official: NIST guidance now tells organisations not to force periodic changes without evidence of compromise, because in practice people respond to forced rotation by making small predictable edits to a password they already had. Change a password when there is a reason to, immediately and completely.

Keep it out of plain sight. A note on a desk, a file called passwords, or a phone note synced to a cloud account are all easier to reach than the accounts they protect. And be careful where you sign in: saved passwords on a shared or public machine outlive your session.

Reading your result

Take the bits figure rather than the word. The label is a bucket, and 60 bits and 127 bits both read as strong while being about a quintillion times apart in difficulty.

For most accounts, 16 characters from the default set is far past the point where guessing is the weak link, which is why this generator starts there rather than at the shorter defaults common elsewhere. Go longer where the account really matters, a password manager, an email account, anything financial, since email is the reset path for everything else. And if a site refuses a long password or silently truncates it, treat that as information about how carefully it is storing what you gave it.